1

Book of Abstracts

Plenary Talks – page 2

Oral Contributions – page 7

Poster Contributions – page 54

List of presenting authors – page 108

http://superfox.como.polimi.it

2

Plenary Talks

3

Phase diagrams of Fe based superconductors

B. Büchner

Institut für Festkörperforschung, IFW Dresden

Institut für Festkörperphysik, TU Dresden

Using a broad spectrum of experimental techniques, such as NMR, µSR, ARPES, magnetometry,

thermodynamics, x-ray diffraction, and transport measurements, we have studied the interplay between

magnetism and superconductivity in several classes of iron pnictide superconductors. In LaO1-xFxAsFe and

other 1111 type materials an intimate interplay between magnetism and electronic properties is evident.

Moreover, measurements of the electrical field gradient by NQR yield clear-cut evidence for nanoscale order

of charges and/or orbitals which are reminiscent to the famous stripe order found in cuprates. The phase

diagram of Co doped NaFeAs is qualitatively very similar to that of the 1111 and 122 type pnictides. In

contrast, LiFeAs, a second member of the 111 type pnictide shows quite different behaviour. From our

measurements on pristine material as well as hole and electron doped compounds we do not find any

evidence for strong antiferromagnetic correlations. Instead, measurements of NQR, µSR, magnetisation and

magnetic resonance reveal a weak ferromagnetic order in hole doped LiFeAs. Based on our determination of

the phase diagram and the results from spectroscopic studies the possible relationship between this unusual

ferromagnetic state and superconductivity in stoichiometric LiFeAs is discussed.

4

Filamentary superconductivity in Sr_2RuO_4-Ru eutectics - features of topology

and symmetry in a chiral p-wave superconductor

M. Sigrist

Institute for Theoretical Physics, ETH Zurich, Zurich, Switzerland

Strong evidence for chiral p-wave pairing in Sr2RuO4, a pairing state analogous to the A-phase of superfluid

He-3, has accumulated over the past decade or so. In the eutectic Sr2RuO4-Ru system, characterized by

micrometer-size Ru-metal inclusions, inhomogeneous superconductivity is found at a temperature roughly

twice the bulk transition temperature. The nature of this superconducting phase - called 3-Kelvin phase - is

very likely filamentary, due to the nucleation at the interface between Sr2RuO4 and the Ru-inclusions. The

path from the first nucleation to bulk superconductivity requires a further symmetry breaking transition for

this chiral p-wave superconductor, which at the same time involves a change of the topology of the

filamentary phase. The realization of such a transition is consistent with experiments of the critical current

and quasiparticle tunneling in the 3-Kelvin phase. Recent experiments measuring the Josephson effect

between a Pb-film via Ru-inclusions to Sr2RuO4 show an anomalous temperature dependence of the

Josephson critical current, with a rather sharp drop at the bulk Tc of Sr2RuO4. This may be explained

assuming that the Josephson coupling of s-wave superconductivity (Pb - Ru) to the chiral p-wave state leads

to phase frustration which limits the critical current. In this presentation I will give an overview on the

present status of the phenomenological understanding of this complex superconductor.

5

Strain-engineered properties of functional oxide thin films: from bulk to free

surfaces

Josep Fontcubertaa, I. Fina

a, D. Pesquera

a, X. Martí

a,b, V. Skumryev

c, L. Fàbrega

a, F. Sánchez

a and

G. Herranza

a Institut de Ciència de Materials de Barcelona (ICMAB-CSIC). Campus UAB, Bellaterra, 08193. Catalonia. Spain

b Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5,

121 16 Prague 2, Czech Republic

c Institució Catalana de Recerca i Estudis Avançats (ICREA) and Departament de Física, Universitat Autònoma de

Barcelona, 08193 Bellaterra, Catalonia Spain

Magnetic and dielectric properties of AMO3 (M is a dn-metal) oxide perovskites are strongly dependent on

the nature of the A-O, M-O and A-O-M bonds, the occupancy of electronic levels, the corresponding

bandwidth and the topology of the distinct and sometimes competing, Mi-O-Mj exchange interactions.

As a result, perovskites display an extremely rich variety of properties, ranging from ferroelectricity,

antiferromagnetism or ferromagnetism, insulating or metallic behavior or ionic conductivity, etc, that boosts

up their relevance in many different areas of science and technology.

It is now known that properties can be severely modified, and emerging properties have been discovered, by

suitable AMO3 confinement in sandwich epitaxial heterostructures or by appropriate strain-engineering.

Here, we shall illustrate this progress by reviewing some examples of using epitaxial strain to modify: (a)

bulk properties in multiferroic AMnO3 thin films, allowing to obtain distinct antiferromagnetic magnetic

orders and subsequently different dielectric ground states, including the magnetically switchable ferroelectric

polarization and (b) we will also show that epitaxial clamping across some interfaces in

magnetic/ferroelectric heterostructures provides an additional way to fine tuning the dielectric response. We

will also show (c) that exploiting orbital symmetry breaking by epitaxial strain and the inherent symmetry

breaking at free surfaces, the electron occupancy of surface 3d-orbitals can be tailored at wish. Some

implications of these findings will be discussed.

6

Novel Two-dimensional Electron Systems at Oxide Interfaces

J. Mannhart

Max Planck Institute for Solid State Research

Stuttgart, Germany

Two-dimensional electron gases based on conventional semiconductors such as Si or GaAs play a

pivotal role in fundamental science and technology. The high mobilities achieved enabled the discovery of

the integer and fractional quantum Hall effects and are exploited in high-electron-mobility transistors.

Recent work has shown that two-dimensional electron systems can also exist at oxide interfaces [1].

These electron systems are characterized by properties that fundamentally differ from those of

semiconductor interfaces. In the presentation I will provide an overview of our studies of these surprising

electronic systems (see, e.g., [2,3]) and explore the potential of electron liquids at oxide interfaces for the use

in nanoscale electronic devices.

[1] A. Ohtomo et al., Nature 419, 378 (2002)

[2] N. Reyren et al., Science 317, 1196 (2007)

[3] J. Mannhart and D.G. Schlom, Science 327, 1607 (2010)

7

Oral Contributions

8

Iron-based superconductor: Improving global critical current in the FeSeTe

polycrystalline samples

A.Sala

1,2, A.Palenzona

1,3, C. Bernini

3, V. Braccini

3, M.R.Cimberle

4, C.Ferdeghini

3, G. Lamura

3,

A.Martinelli3, I. Pallecchi

3, G. Romano

3, M.Tropeano

1,3, R.Fittipaldi

5, A.Vecchione

5 and

M. Putti2,3

1 Chemistry and Industrial Chemistry Department, University of Genova, via Dodecaneso 31, I-16146 Genova, Italy 2

Physics Department, University of Genova, via Dodecaneso 33, I-16146 Genova, Italy

3 CNR-SPIN-Genova corso Perrone 24, 16152, Genova, Italy

4 CNR-IMEM Physics Department, University of Genova, via Dodecaneso 33, I-16146 Genova, Italy

5 CNR-SPIN-Salerno and Physics Department University of Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA),

Italy

Since the discovery of the superconductivity in layered iron-based compounds by the Hosono’s

group, a lot of different families has been discovered, all characterized by layers of iron atoms. This

discovery was interesting because the magnetism has been considered an antagonist of superconductivity,

oppositely in these compounds it seems to be a fundamental requirements. These kinds of compounds are

very interesting for the understanding of the interplay between magnetisms and superconductivity.

At the same times they can also promising for applications. Indeed they have characteristics in common with

other technological superconductors like high temperature superconducting cuprates (HTS) and MgB2.

The iron-based superconductors show an extraordinary high upper critical field and strong pinning, in

addition critical current is rather independent of the field like in HTS.

At the same time the critical current density shows low anisotropy in respect to the crystalline directions, as

in the case of MgB2, which allows the current flow along the c-axis.

These characteristics make the iron-based superconductors good candidates for applications in the

generations of high magnetic fields.

However, polycrystalline materials currently available still exhibit electromagnetic granularity, like the HTS,

which suppresses superconducting the current flow over long lengths.

Our work is focused on polycrystalline samples of iron chalcogenides Fe(Te,Se) that in the optimally doped

compound FeSe0.5Te0.5 show a critical temperature of 15 K. This family presents the simplest

crystallographic structure and does not contain poisonous elements like arsenic.

We have optimized a novel method to prepare polycrystalline FeSe0.5Te0.5 samples using a melting process

and a subsequent annealing treatment. Compared to the standard sintering method, usually used to obtain a

bulk sample, we can achieve much denser and homogeneous samples. Our samples show large grains well

connected to each other with few inhomogeneities, moreover they show optimal critical temperature with a

sharp drop of the resistive and magnetic transitions, large hysteresis loops and high upper critical fields are

observed. The global critical current densities of our optimized samples are the highest compared to the

values reported in literature for bulk samples of the same family.

In addition the dependence of the critical current from the external magnetic field result very weak, such that

at µ0H = 7 T it is decreased only of about 40%.

9

Transient reflectivity change measurements on FeSe0.5Te0.5 thin films

Giovanni Piero Pepe1, Carmela Bonavolontà

1, Loredana Parlato

1, Corrado De Lisio

1, Giuseppe

Peluso1, Antonio Barone

1, Emilio Bellingeri

2, Ilaria Pallecchi

2, Marina Putti

2, and Carlo Ferdeghini

2

1CNR-SPIN UOS Napoli and University of Napoli “Federico II”, Dip. Scienze Fisiche, via Cinthia, 80126 Napoli, Italy

2CNR-SPIN-UOS Genova and University of Genova, Corso Perrone 24, 16152 Genova, Italy

The Iron Selenides (FeSe), with Tellurium (Te) substitution are the latest discovered family of Fe-based

pnictides superconductors. These compounds share some common properties with high-Tc cuprates but have

also some differences. A time-resolved spectroscopy is considered an efficient and useful technique to study

the electronic excitations in superconductors due to its capability to distinguish between scattering processes

characterized by different relaxation times. Femtosecond spectroscopy has been used to investigate the

quasi-particle relaxation times in a nearly optimally doped chalcogenide superconductor FeSe0.5Te0.5 with

strain enhanced critical temperature (Tc 18K). Results indicate the existence of temperature dependence of

quasi-particle recombination time in the superconducting state characterized by a temperature-dependent

energy gap (T) at T<Tc. The fitting procedure gives 0=3.0±0.7meV for the superconducting energy gap,

which corresponds to a 2/kBTc = 3.91 ratio. Moreover, a clear feature around T=50K is also evidenced both

by fast spectroscopy and Hall measurements which can be signatures of a “pseudogap-like” behavior in the

normal state.

10

Point contact Andreev reflection spectroscopy of FeSe0.44Te0.56 single crystals,

understanding electron-boson coupling

K.A. Yates a

, A.D. Caplin, L.F. Cohen a

, A.Y. Ganinb, M.J.Rosseinsky

b

a Physics Department, Blackett Laboratory, Imperial College London, London, UK, SW7 2AZ

b Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD UK

Since the discovery of the iron based superconductors, point contact Andreev reflection spectroscopy

(PCAR) has proved to be an important tool for elucidating the gap structure in these materials [1]. However,

many published spectra show features at voltages higher than the superconducting gap energy that in some

cases are attributed to secondary gaps. It has recently been suggested that one of these features can be

identified with electron-boson coupling [2,3]. Here we measure single crystals of FeSe0.44Te0.56 (Tc = 14.3K)

by PCAR using a variety of superconducting and normal tips. Our data suggests that several features may be

associated with this electron-boson coupling. To investigate the nature of the excitation we present data

taken systematically as a function of temperature and magnetic field. The results suggest that PCAR may be

capable of providing complementary information to that obtained from inelastic neutron scattering and may

indeed help elucidate the characteristics of this excitation.

[1] D Daghero and RS Gonnelli, Supercond Sci Technol., 23, 043001 (2010)

[2] M Tortello et al., Phys Rev Lett., 105, 237002 (2010)

[3] Y Fasano et al., Phys Rev Lett, 105, 167005 (2010)

11

Orbital selective physics in Iron-based Superconductors

G. Giovannettia, N. Lanatà

b, L. de’ Medici

c M. Capone

a,

a CNR-IOM and Scuola Internazionale Superiore di Studi Avanzati (SISSA), Trieste, Italy

b University of Gothenburg, SE-412 96 Gothenburg, Sweden

c Laboratoire de Physi ue des Solides, CN S-UM 8 02, Universit e Paris-Sud, Orsay 91405, France

One of the most distinctive features of the iron-based superconductors is the multi-band nature of the bandstructure. Essentially all the five bands arising from iron d-orbitals contribute to the valence bands

leading to the well-known pocket Fermi surface.

A multiband Fermi surface leads to remarkable properties, like the possibility of an orbital-selective physics,

in which different orbitals present a different level of correlation, and to a central role of the Hund’s

interaction.

Using the Gutzwiller method and a slave-spin formalism, we show that the FeSe and FeTe present strong

correlation effects which are mainly controlled by the Hund’s coupling J rather than the Hubbard scale U.

Moreover, the correlation effects are remarkably “orbital selective”, with the xy orbital significantly more

localized than the other bands [1,2]. The orbital selective behavior establishes even if the width of the

different bands is close because of a Hund’s driven band decoupling mechanism [3]

We discuss the signatures of this physics in various experiments, including recent angle-resolved

photoemission, which shows that the xy orbital has a significantly smaller superconducting gap than the

other orbitals [4].

[1] N. Lanatà, H. Strand, G. Giovannetti, L. de’ Medici and M. Capone, in preparation

[2] L. de' Medici, S.R. Hassan and M. Capone, Journal of Superconductivity and Magnetism 22, 535 (2009)

[3] L. de' Medici, S.R. Hassan, M. Capone, and X. Dai, Phys. Rev. Lett. 102, 126401 (2009); L. de’ Medici,

Phys. Rev. B 83, 205112 (2011)

[4] S. Borisenko, private communication (2012)

12

Probing the FLL motions in iron-pnictide superconductors by high magnetic

field NMR

L. Bossonia,b

, P. Carrettab, M.Horvatic

c, A. Taler

d, P. C. Canfield

d

a Dipartimento di Fisica “E. Amaldi”, Università di oma T E-CNISM, Italy

bDipartimento di Fisica, Università di Pavia, Italy

cLaboratoire National des Champs Magnétiques Intenses, LNCMI-CNRS, Grenoble, France

dAmes Lab and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA

Since the discovery of high temperature superconductivity in the Fe-based pnictides, the structural and

dynamical properties of the flux lines lattice (FLL) have been the subject of intense research activity, and the

investigation of the mixed phase in these materials is important both for the fundamental and the applicative

aspects.

We present a 75

As NMR study of a single crystal iron-based superconductor, belonging to the 122 family:

Ba(Fe0.93Rh0.07)2As2. The sample shows some similarities with the Co-doped compounds but, unlike the

majority of those samples [1], this Rh-doped sample shows a clear peak in the spin-lattice relaxation rate

1/T1, below the irreversibility temperature. Since

this feature is characteristic of the superconducting

phase, and it is field-orientation dependent, we

argued that this effect is typical of the FLL

dynamics taking place in the liquid-FLL phase,

where the vortices are rather uncorrelated 2D

objects. Furthermore the peak cannot be ascribed

to the opening the superconducting gap (Hebel-

Slichter peak). As far as the FLL phase diagram is

concerned, the thermally activated response of the

2D pancakes can be very different depending on

the field and temperature conditions. In particular,

increasing the field intensity one expects a more

“movable” FLL (Fig. 1). In order to carry out a field-

dependent study on the FLL dynamics we employed the

high fields available at LNCMI, and lower fields available

at Pavia NMR lab.

Another marker of the vortices dynamics is the spin-echo decay time T2. A strong peak in 1/T2 was seen for

H || c, and also in the Co-doped compounds by Oh et al. [2]. The peak is strongly suppressed in the

transverse geometry, evidencing the presence of an anisotropic dynamics. This effect can be explained in

terms of vortices which, stuck by the layer boundaries, in the transverse configuration.

[1] F. Ning, K. Ahailan, T. Imai, A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, and D. Mandrus, Jour.

Phys. Soc. of Jpn. 77, 103705 (2008).

[2] S. Oh, A. M. Mounce, S. Mukhopadhyay, W. P. Halperin, A. B. Vorontsov, S. L. Bud’ko, P. C.

Canfield, Y. Furukawa, A. P. Reyes, and P. L. Kuhns, Phys. Rev. B 83, 214501 (2011).

Fig. 1: Pinning activation energy obtained

by fitting the peak in 1/T1.

13

AC susceptibility investigation of vortex dynamics in nearly-optimally doped

REFeAsO1−xFx superconductors (RE = La, Ce, Sm)

G. Prando,a,b,c

P. Carretta,a R. De Renzi,

d S. Sanna,

a

H.-J. Grafe,c S. Wurmehl,

c B. Büchner

c

a Department of Physics “A. Volta,” University of Pavia-CNISM, I-27100 Pavia, Italy

b Department of Physics “E. Amaldi,” University of Roma Tre-CNISM, I-00146 Roma, Italy

c Leibniz-Institut für Festkörper- und Werkstoffforschung (IFW) Dresden, D-01171 Dresden, Germany

d Department of

Physics, University of Parma-CNISM, I-43124 Parma, Italy

The recently discovered iron-containing pnictide REFeAsO1−xFx high-Tc superconductors (RE: rare-earth

ion) seem to be quite promising in view of possible technological applications in spite of the present

difficulties in their chemical synthesis. Besides the relatively high values of Tc, in fact, the robustness of SC

has been evidenced also by focussing on the high values of both the upper critical magnetic fields Hc2 and of

the intrinsic critical current densities from measurements on single-crystals.1

In this respect, the study of the properties of the intrinsic irreversibility line in the mixed phase of the

magnetic field – temperature phase diagram (H −T PD) of such materials is of the utmost importance in

order to bring further the knowledge about their possible applications. In fact, the irreversibility line

separates a “li uid” region where dissipative processes occur due to the motion of flux lines from a “glassy”

one where pinning mechanisms are highly effective in anchoring vortices, so preventing their motion. Such

effects are typically investigated by means of magnetic ac susceptometry. In this way, not only one directly

picks the irreversibility line out in the H − T PD but interesting information on thermally-activated depinning

pro-cesses can be easily obtained by accessing the typical correlation time of the flux line motion. Such

measurements were first performed on a powder sample of SmFeAsO0.8F0.2 in nearly-optimally-doped

conditions,2 where a small li uid region could be located in the H − T PD and estimates of the depinning

energy barriers as well as of the critical current densities were obtained in perfect quantitative agreement

with those reported from transport measurements on single crystals.3

It should be stressed that a strongly marked dependence on the RE ion has been reported relatively to the

features of the superconducting phase from both global and local points of view. The optimal-doping Tcopt

,

for instance, is known to double its value after a full La/Sm substitution. In this respect, an investigation of

the dependence of the irreversibility line on the RE ion was performed in order to focus on possible intrinsic

effects of the RE ions on the pin-ning mechanisms. Ac susceptibility and static magnetization measurements

were performed in the nearly-optimally doped LaFeAsO0.9F0.1 and CeFeAsO0.92F0.08 superconductors and the

H − T PD was drawn for the three materials.4 Interestingly, results display a sizeable reduc-tion of the liquid

phase upon increasing Tc in the range of applied fields (H 5 T). This result clearly confirms that

SmFeAsO0.8F0.2 is the most interesting compound among the investi-gated ones in view of possible

applications. The field-dependence of the intra-grain depinning energy, moreover, exhibits a common trend

for all the samples with a typical crossover field value (2500 Oe Hcr 5000 Oe) separating regions where

single and collective depinning processes are at work. These results suggest that a similar underlying

mechanism is shared among the different samples independently on the precise RE ion.

1P. J. W. Moll et al., Nature Mater. 9 628 (2010)

2G. Prando et al., Phys. Rev. B 83 174514 (2011)

3H.-S. Lee et al., Phys. Rev. B 82 104523 (2010)

4G. Prando et al., arXiv:1201.0044v1 (2012)

14

Phase-slip phenomena in proximitized NbN/NiCu superconducting nanostripes

G. P. Pepe1, L. Parlato

1, V. Pagliarulo

1, H. Myoren

3, A. Esmaeli

2, H. Sedghi

2, R. Cristiano

4, M.

Ejrnaes4, and Roman Sobolewski

5

1 CNR-SPIN and Dipartimento Scienze Fisiche, Università di Napoli Federico II, Fac. Ingegneria, P.le Tecchio 80, I-

80125 Napoli, Italy

2 Department of Physics, Faculty of Science, Urmia University, P.O.B. 165, 11 km Sero Road, Urmia, Iran

3 Graduate School of Science and Engineering, Saitama University, 338–8570 Saitama, Japan

4 CNR-Istituto di Cibernetica “E. Caianiello”, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy

5 Department of Electrical and Computer Engineering and the Laboratory for Laser Energetics, University of Rochester,

Rochester, New York 14627-0231, USA

Many scientific studies have been devoted to low-dimensional superconducting systems, such as nanowires,

nanostripes, nanostructured multilayers, thin films, because they show many interesting properties not

observable in bulk samples. In this work, transport properties of superconducting NbN/NiCu

superconductor/ferromagnet nanowires with different relative thickness are presented down to T=4.2 K. We

observe several voltage steps in the current-voltage characteristics and investigate the temperature

dependence of some of these voltage steps that it’s possible to ascribed to active phase-slip phenomena.

Experimental results have been analyzed using the time-dependent Ginzburg-Landau theory, and the energy

relaxation time has been estimated leading to E=1.1 ± 0.2 ps. This value is an agreement with that obtained

by ultrafast transient optical reflectivity experiments on similar superconducting/ferromagnet bilayers where

fast=1.2 ± 0.3 ps was estimated, giving an overall consistency of the adopted physical picture.

15

SQUID-based nanodevices for nanomagnetic applications

E. Espositoa, C. Granata

a, B. Ruggiero

a, R. Russo

a, M. Russo

a, A. Vettoliere

a, C. Cannas, D.Peddis

a CNR-Consiglio Nazionale delle Ricerche, Istituto di Cibernetica, 80078, Pozzuoli (Napoli), Italy

b Dipartimento di Scienze Chimiche, Università di Cagliari, Cagliari, Italy

Nanosized Superconducting Quantum Interference Devices (nano-SQUIDs) offer the possibility to

investigate small spin populations and the magnetization of nanoparticles opening new horizons in the

interesting word of nanomagnetism[1-2]. Furthermore, the nanoSQUIDs can be employed in stimulating

applications such as the spintronics, the single photon detection, the readout in quantum computing, the

nanoelectronics including memory, the quantum metrology and the scanning SQUID microscopy with a very

high spatial resolution[3] .

We will report our recent results about the design, the fabrication and the characterization of Dayem bridges

based nano-SQUIDs having a loop area ranging from 4 m2 to 0.04 m

2. The devices have been patterned by

Electron Beam Lithography in a 20-25 nm thick Nb layer [2,4]. The smallest sensor has shown a spectral

density of the magnetic flux noise of 1.5 0/Hz1/2

corresponding to a spin sensitivity of about 60 spin/Hz1/2

(in units of Bohr magneton). Preliminary measurements on Fe3O4 nanoparticles having a size between 4 and

8 nm will be discussed [5].

Figure 1: left) Scanning electron micrograph of a nanoSQUID having a hole side length of 0.75 m. The

lower inset shows the nanoparticles on the sensor. right) Magnetic field dependence of the magnetization for

a small cluster of (Fe3O4-SiO2) core/shell nanoparticles at T = 4.2 K [5].

[1] A.G.P. Troeman, H. Derking, B. Boerger, J. Pleikies, D. Veldhuis, H. Hilgenkamp, Nano Lett. 7, 2152

(2007)

[2]C.Granata, E.Esposito, A. Vettoliere, L.Petti, M.Russo, Nanotechnology 19, 275501 (2008)C. P. Foley

and H. Hilgenkamp, Supercond. Sci. Technol. 22, 064001(2009).

[3]C. Granata, A. Vettoliere, R. Russo, E. Esposito, M. Russo and B. Ruggiero, Appl. Phys. Lett. 94, 062503

(2009).

[4]R. Russo, C. Granata, P. Walke, A. Vettoliere, E. Esposito and M. Russo, J. Nanopart. Res., DOI:

10.1007/s11051-011-0330-2 (2011);

[5]E. Esposito, C. Granata, A. Vettoliere, R. Russo, D. Peddis and M. Russo, J. Nanosci. Nanotechno, (in

press).

16

A tuneable antiferromagnetic-only-based tunnel junction

D. Pettia, E. Albisetti

a, H. Reichlova

b,c, X. Marti

b,c, J. Gaumez

d, M.M. Ruiz

e, A. Lopendia

e,

V. Novakb, K. Olejnik

b, J. Wunderlich

b, T. Jungwirth

b, R. Bertacco

a

a LNESS center, Physics Department, Politecnico di Milano, Como, Italy

bInstitute of Physics ASCR, Praha 6, Czech Republic

cFaculty of Mathematics and Physics, Charles University in Prague, Prague 2, Czech Republic

dDepartment Fisica Aplicada III, Universidad Complutense de Madrid. Madrid, Spain; Oak Ridge National Laboratory,

Oak Ridge TN 37831, USA eDepartament de Física, Universitat Autònoma de Barcelona Bellaterra (Spain)

Spintronics is nowadays an expanding field with numerous fronts in both fundamental physics and

applications, still with ferromagnets as an undismissable item. The introduction of insulating spacing layers

in the magnetic tunnel junctions opened the door to a plethora of novel and larger effects, but also to the

reduction to one only or none the ferromagnetic layers in the devices. For instance, in 2004, the term tunnel

anisotropic magnetoresistance (TAMR) was coined after the experiments showing the tunable tunnelling

from ferromagnetic (Ga,Mn)As into a non magnetic metal [1]. It demonstrated that the tuning of density of

states only at one side of the barrier was enough to produce measurable changes in the tunnel resistance.

Recent theoretical and experimental works suggested that the magnetic anisotropy phenomena, for instance

ruling the TAMR response, will be present equally in antiferromagnets as in ferromagnetic materials thus

opening the door to the removal of the ferromagnets -and their inherent stray fields- from the spintronic

devices. In this work, tunnel junctions have been fabricated on top of atomically flat (001)-oriented SrTiO3

substrates. The multilayer stack, which contains no ferromagnetic layers, comprises antiferromagnetic IrMn

and non-magnetic Ta electrodes sandwiching a 2 nm thick MgO insulating barrier. Control experiments

indicated that the Neel temperature of the intentionally thin 2 nm thick IrMn is 120 K. Transport experiments

revealed that the set-in of antiferromagnetic ordering plays a role in electron tunneling, also coexisting with

the strain effects induced by the structural transitions of the SrTiO3 substrate below 100 K. We show that the

magnetic field-cooling procedure and the strain induced by the phase transitions are suitable tools to

manipulate the otherwise elusive antiferromagnetic moments in spintronic devices without net magnetic

moment, paving the way towards to the realization of tunable spintronic devices only comprising

antiferromagnetic layers.

Fig.1: Change of tunnel resistance as a function of the field-cooling for different magnitudes. The kink at 100

K matches the cubic-to-tetragonal transition of STO evidencing that strain-induced changes can modulate the

magnetic-induced increase of tunnelling resistance. Inset: TEM image of the structure

[1] A. D. Giddings et al. Phys. Rev. Lett. 94, 127202 (2005)

0 50 100 150 200 250 300 350-50

0

50

100

150

200

250

Temperature (K)

[TM

R(H

)-T

MR

(H=

0)]

/TM

R(H

=0

)*1

00

0.3 T (perp)

3.0 T (perp)

17

Thermally isolated La0.7Sr0.3MnO3 suspended bridges on silicon substrate for

uncooled bolometers

S. Liu1, B. Guillet

1, A. Aryan

1, C. Adamo

2, C. Fur

1, J.-M. Routoure

1, F. Lemarié

3, D.G Schlom

2,4,

L. Méchin1

1 GREYC, UMR6072, CNRS-ENSICAEN-Université de Caen Basse Normandie,

6 Bd Maréchal Juin, Caen 14050, France 2 Department of Materials Science and Engineering, Cornell University

Ithaca, NY 14853-1501, USA 3 CIMAP, UMR 6252, CNRS-CEA-ENSICAEN-Université de Caen Basse-Normandie,

6 Bd Maréchal Juin, Caen 14050, France 4 Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA

Among manganites, La0.7Sr0.3MnO3 (LSMO) is a promising material for the realization of uncooled

mid-infrared bolometers because it shows a strong resistance variation as a function of temperature at room

temperature.

In this work, LSMO thin films of high epitaxial quality were deposited on SrTiO3 (STO) buffered

silicon substrates by MBE (Molecular Beam Epitaxy) [1]. We successfully fabricated LSMO/STO suspended

bridges using reactive ion etching (RIE) of the silicon substrate (figure 1). The electro-thermal and electrical

properties between 300 K to 3 0 K of 4 μm wide 0 – 200 μm long LSMO/STO suspended bridges have

been investigated.

The electrical resistivity of suspended bridges measured as a function of temperature was very close

to the characteristics of the non-suspended LSMO thin film, thus demonstrating that the fabrication process

did not degrade the quality of LSMO. The temperature coefficient of resistance R (defined as 1/R × dR/dT)

ranged from 0.012 to 0.021K-1

, the thermal conductance G was very low (of the order of 10-7

W.K-1

). The

optical sensitivity of such bolometers was up to 5 orders of magnitude higher compared with non suspended

films due to the reduced thermal conductance (figure 2). In addition small time constant of 1 ms and low

noise level (similar to non-suspended LSMO film) were measured [2].

In conclusion, we demonstrated that the fabrication process did not damage the electrical transport

and noise properties, which enable the fabrication of uncooled LSMO bolometers with very low thermal

conductance thus leading to state-of-the-art performances (Noise Equivalent Power NEP <1 pW.Hz-0.5

at

300K). Our technology is also promising for the realization of many other MEMS (Micro-Electro-

Mechanical-Systems) devices based on LSMO or other functional oxides.

[1] L. Méchin et al, Physica status solidi (a), http://dx.doi.org/10.1002/pssa.201127712.

[2] S. Wu et al., this conference.

Figure 2: sensitivity and dR/dT of the suspended bridge

versus temperature

Figure 1: Scanning Electrical Microscope

photograph of a suspended bridge.

18

Conductive AFM study of nanoscale memory devices with ultralow-power

resistive switching operation

S. Brivioa, G. Tallarida

a, D. Perego

b, D. Deleruyelle

c, S. Franz

b, C. Muller

c and S. Spiga

a

a

Laboratorio MDM, IMM-CNR, via C. Olivetti 2 20864 Agrate Brianza (MB) Italy b Dipartimento di Chimica, Materiali ed Ing. Chimica “G.Natta”, Politecnico di Milano, Via Mancinelli, 20131 Milano

c IM2NP, UMR CNRS 7334, Aix-Marseille Université, IMT Technopôle de Château-Gombert, 13451 Marseille Cedex

20, France

Resistive switching metal-oxide-metal (MOM) heterostructures, displaying two

or more conductive states accessible by electric pulses, have been gaining huge

interest in the non-volatile memory community due to their potentiality mainly

in terms of low-power operation and downscaling [1]. In order to investigate

the effect of downscaling on resistive switching, nanowires (NWs) constitutes

have been employed [2]. Furthermore nanowires featuring an inner MOM

structuration offer a larger versatility, implementing a memory cell as a whole,

together with its electrodes, while allowing the tuning of the oxide thickness, as

well.

To this aim, segmented 3 μm Au/20 nm NiO/17 μm Au NWs arrays have been

produced by electrodeposition of the metallic constituents in a free standing

anodized alumina matrix with pore diameter of 50 nm and inter-pores distances

of 100 nm, followed by thermal annealing for the oxidation and crystallization

of the Ni segment.

The array of nanoscaled MOM memory units has been characterized by conductive atomic force microscopy

(CAFM) technique by contacting the top gold electrode of single Au/NiO/Au NWs vertically ordered in the

alumina matrix, see figure 1. After an electroforming procedure, necessary to break a highly insulating virgin

state, complete unipolar resistive switching cycles have been measured demonstrating that the operation of a

single NW does not affect the surrounding ones. Additionally, resistive switching occurs over multiple

resistance levels in a ultralow-power DC regime, i.e. few nW per switching operation.

Furthermore the scaling trend of the maximum operation current with the set level resistance will be

discussed as a prove of the filamentary fuse-antifuse nature of the resistive switching phenomenon.

This work was partially supported by Fondazione Cariplo (MORE Project n°2009- 2711) and by CNR through the Short Term Mobility Program.

[1] J. Hutchby et al., Assessment of Potential & Maturity of selected Emerging Research Memory Technologies Workshop & ERD/ERM working group meeting (2010).

[2] S. I. Kim, et al. Appl. Phys. Lett. 93, 033503 (2008)

19

Magnetic excitations and charge density modulation in YBCO and NBCO

studied with Cu L3 RIXS

G. Ghiringhelli a, M. Le Tacon

b, M. Minola

a, S. Blanco-Canosa

b, C. Mazzoli

a, N.B. Brookes

c,

G.M. De Luca d, A. Frano

b,e, C.T. Lin

b, M. Moretti Sala

c, M. Salluzzo

d, G.A. Sawatzky

f, E.

Schierle e, R. Sutarto

f, E. Weschke

e, B. Keimer

b, L. Braicovich

a

a CNR/SPIN, CNISM and Dipartimento di Fisica, Politecnico di Milano, Italy

b Max Planck Institute for Solid State

Research, Stuttgart, Germany c

European Synchrotron Radiation Facility, Grenoble, France d CNR/SPIN, Dipartimento Scienze Fisiche, Iniversità di Napoli Federico II, Italy

e Helmholtz-Zentrum Berlin für

Materialien und Energie, Berlin, Germany f

Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada We have studied a large series of YBa2Cu3Ox (YBCO) crystals and Nd1.2Ba1.8Cu3Oy (NBCO) films using

high resolution resonant inelastic scattering (RIXS) at the Cu L3 absorption edge.

This spectroscopic technique, which we have recently developed at the ESRF and SLS [1,2], has allowed us

to observe two important phenomena in the 123 high Tc superconductor (HTcS) family.

First we have determined the dispersion of spin excitations over most of the first Brillouin zone along the

antinodal direction (100). From undoped to underdoped to optimally doped compounds we have found that

the energy and spectral intensity of mangnons/paramagnons are both preserved. This fact complements what

had been previously found by inelastic neutron scattering, confined inside a small region surrounding the ( )

antiferromagnetic Bragg point (fig. 1). The robustness of the antiferromagnetic short range order up to

optimal doping supports the role of superexchange interaction as Cooper pairing force in HTcS [3].

Secondly we have observed a weak incommensurate Bragg peak in underdoped YBCO and NBCO at wave

vector (0.31,0,0) and (0,0.31,0), constant versus doping. This peak has maximum intensity at Tc and for hole

doping p≈0.11. We assign this peak to a charge density modulation of period 3 lattice units in both a and b

directions [4]. The temperature and doping dependence indicates that charge density modulation is in

competition with superconductivity, in analogy with the case of (LaSr)2CuO4 and (LaBa)2CuO4 families for

period 4 static stripes.

Figure 1 (from ref. [5]): Comparison of magnetic excitation dispersion for YBCO6.6 as observed with neutrons and resonant x-ray scattering.

[1] G. Ghiringhelli et al, Rev, Sci. Instrum. 77, 113108 (2006)

[2] L. Braicovich et al, Phys. Rev. Lett. 104, 077002 (2010). [3] M. Le Tacon et al, Nature Phys. 7, 725 (2011) [4] G. Ghiringhelli et al, in preparation

20

Cooper pair and quasiparticle tunneling in infinite layer Sr1-xLaxCuO2 (SLCO) thin film grain boundary junctions and planar SLCO/Au/Nb tunnel junctions

Tomaschko

a, S. Scharinger

a, V. Leca

a,b, J. Nagel, M. Kemmler

a, T. Selistrovski

a,

Kleiner a, D. Koelle

a

a Physicalisches Institut – Experimentalphysik II and Center for Collective Quantum Phenomena in LISA

+, Universität

Tübingen, Tübingen, Germany b National Institute for Research and Development in Microtechnologies, Molecular Nanotechnology Laboratory,

Bucharest, Romania The electron-doped infinite-layer (IL) compounds Sr1-xLnxCuO2 (Ln = La, Pr, Nd, Sm, Gd) have the highest transition temperature Tc among electron-doped cuprate superconductors and the simplest crystal structure of any cuprate superconductor: Its CuO2 planes are only separated by a single Sr1-xLnx layer. This makes the IL compounds particularly interesting for fundamental research on cuprate superconductivity. However, as the fabrication of high-quality superconducting IL samples is challenging, these compounds have been less examined than other cuprate superconductors. We used pulsed laser deposition (PLD) to grow epitaxial Sr1-xLaxCuO2 (SLCO; x~0.15) thin films on SrTiO3 substrates with BaTiO3 buffer layers [1]. After successful implementation of a PLD process for the fabrication of superconducting SLCO thin films, we fabricated devices for the investigation of quasiparticle (QP) an Cooper pair (CP) tunneling. Here, we report on the electric transport properties of two types of SLCO devices: (i) thin film planar tunnel junctions with SLCO as bottom electrode, a 5-nm-thick Au interlayer and Nb as top electrode [2], and (ii) SLCO grain boundary junctions (GBJs) on BTO-buffered STO bicrystal substrates with 24° misorientation angle [3]. Experimental data on electric transport for the SLCO/Au/Nb junctions provide information on the interface and surface properties of our SLCO thin films. No CP tunneling is observed; however, nonlinear current-voltage characteristics give evidence for QP tunneling across a thin insulating SLCO barrier at the SLCO/Au interface, with a single gap value ~1.4 meV, originating from superconducting Nb. The absence of a superconducting SLCO gap in the QP conductance curves indicates a thin normal conducting SLCO layer below the insulating SLCO barrier, which is consistent with X-ray photoelectron and Auger-electron spectroscopy studies on our SLCO thin films [2]. On the SLCO GBJs we examined both CP and QP tunneling by electric transport measurements at temperatures down to 4.2K. CP tunneling revealed an extraordinary high critical current density for electron-doped cuprates of jc > 10

3 A/cm

2 at 4.2 K. Thermally activated phase slippage was observed as a dissipative

mechanism close to the transition temperature. Out-of-plane magnetic fields H revealed a remarkably regular Fraunhofer-like jc(H) pattern as well as Fiske and flux-flow resonances, both yielding a Swihart velocity of 3.1 × 10

6 m/s. Furthermore, we examined the superconducting gap by means of QP tunneling spectroscopy.

The gap was found to be V shaped with an extrapolated zero-temperature energy gap 0 ≈ 2.4 meV. No zero-bias conductance peak was observed.

[1] J. Tomaschko, V. Leca, T. Selistrovski, S. Diebold, J. Jochum, R. Kleiner, D. Koelle, Phys.

Rev. B 85, 024519 (2012)

[2] J. Tomaschko, C. Raisch, V. Leca, T. Chassé, R. Kleiner, D. Koelle, Phys.

Rev. B 84, 064521 (2011)

[3] J. Tomaschko, V. Leca, T. Selistrovski, R. Kleiner, D. Koelle, Phys.

Rev. B 84, 214507 (2011)

21

Magnetic excitations in spin liquids measured via Resonant Inelastic X-ray

Scattering

Filomena Forte

a,Mario Cuoco

a,1,2 Canio Noce

a,1,2 and Jeroen van den Brink

b

aCNR-SPIN UOS Salerno, I-84084 Fisciano (SA) and Dipartimento di Fisica “E. . Caianiello”, Università di Salerno,

I-84084 Fisciano (SA), Italy bInstitute for Theoretical Solid State Physics, IFW-Dresden, D01171 Dresden, Germany

Resonant Inelastic X-ray Scattering (RIXS) has been established recently as a powerful experimental

technique to map out momentum dispersion of elementary charge, spin and

orbital excitations in complex oxides, promoted by considerably improved energy resolution and detector

sensitivity. From a theoretical perspective, it is fundamental to understand to which correlation functions it is

sensitive. In magnetic RIXS, the scattering amplitude can be expressed in terms of momentum dependent

dynamic two-spin and/or four-spin correlation functions, depending on the experimental set up. In this

framework, the dispersion of magnons and bi-magnons in several cuprates at Cu L-edge and K-edge has

been theoretically predicted. In spin liquid systems, the increase in quantum fluctuations as a due to charge

doping in a localized spin system, or by the frustration of magnetic interactions, leads to a melting of the

long-range ordering and to exotic excitations with fractional quantum numbers on top of this ground state.

We will present exact numerical investigations of indirect RIXS and direct RIXS on small Heisenberg and t–

J one-dimensional quantum systems, showing that the main magnetic excitations allowed theoretically are

two-spinon excitations in the indirect and S = 0 direct RIXS processes and two-spinon excitations in S = 1

direct processes. The magnetic features at S = 0 and S = 1 are found to soften and broaden as a function of

the doping concentration, while spectra show a remarkable robustness, both in their overall dispersion and

intensity.

22

Resonant generation of coherent phonons in a superconductor

by ultrafast optical pump pulses

Adolfo Avellaa,b,c

, Dirk Manskeb, and Andreas P. Schnyder

b

a Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

b Dipartimento di Fisica “E. . Caianiello” - Unità CNISM di Salerno

Università degli Studi di Salerno, I-84084 Fisciano (SA), Italy c CNR-SPIN, UoS di Salerno, I-84084 Fisciano (SA), Italy

We study the generation of coherent phonons in a superconductor by ultrafast optical pump pulses [1]. The

nonequilibrium dynamics of the coupled Bogoliubov quasiparticle-phonon system after excitation with the

pump pulse is analyzed by means of the density-matrix formalism with the phonons treated at a full quantum

kinetic level. For ultrashort excitation pulses, the superconductor exhibits a nonadiabatic behavior in which

the superconducting order parameter oscillates. We find that in this nonadiabatic regime the generation of

co- herent phonons is resonantly enhanced when the frequency of the order-parameter oscillation is tuned to

the phonon energy, a condition that can be achieved in experiments by varying the integrated pump pulse

intensity.

[1] A.P. Schnyder, D. Manske, and A. Avella, Phys. Rev. B 84, 214513 (2011).

Figure 1 (a) and (b) Temporal evolution of the lattice

displacement U 0, t( ) and its spectral distribution,

respectively. (c) Temporal evolution of the gap D t( ) .

A0

2tp is a measure of pump laser-pulse intensity. t

p is

pump laser-pulse duration.

Figure 2 (a) and (b) Temporal evolution of the lattice

displacement U 0, t( ) and its spectral distribution,

respectively, for tp= 0.5ps. (c) Dependence of

limiting value of the gap D¥= lim

t®¥D t( ) on pump

laser-pulse intensity A0

2tp.

23

Hall effect in the presence of vertex corrections in a multiband model:

application to pnictides

L. Fanfarillo a,

L. Benfatto a

, E. Cappelluti a,b

, C. Castellani a

a Institute for Complex Systems (ISC), CNR, U.O.S. Sapienza, and Department of Physics, Sapienza University of

Rome, Piazzale Aldo Moro 2, I-00185 Rome, Italy b Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, Cantoblanco, E-28049 Madrid, Spain

The presence of several sheets of the Fermi surface, in pnictides superconductors, makes the multiband

character of superconductivity an unavoidable ingredient of pnictides, that must be accounted for by any

theoretical approach.

Moreover, since the calculated electron-phonon coupling cannot account for the high values of Tc, it has

been suggested that the pairing glue is provided by spin fluctuations exchanged between electrons in

different bands.

Recently the outcomes of an intermediate-coupling Eliashberg approach to iron-pnictides have been

investigated in [1,2].

It has been shown that in these systems new physics comes out from two basic ingredients of a minimal low-

energy model, i.e. the multiband nature of the Fermi surface and the interband character of the pairing. In

particular it has been shown that the Fermi-surface shrinking measured by de Haas van Alphen experiments

and the gap hierarchy observed in photoemission can be quantitatively accounted for by our multiband

approach.

From the experimental point of view also the transverse conductivity in the presence of a magnetic field

(Hall coefficient) shows significant anomalies.

The experimental findings in this respect are quite peculiar: despite the fact that on the basis of LDA band

structure one would expect a strong cancellation between the hole- and electron-bands contribution to the

transverse current, the Hall coefficient is anomalously large, with a predominant hole/electron character in

hole/electron doped systems[3,4]. This lead some authors to suggest the existence of a strong anisotropy

between the scattering time of carriers having different character: however, such an interpretation does not

have presently any support on explicit theoretical calculations for realistic models.

In this work [5] we investigate the role of the momentum dependence of the spin fluctuation spectrum, which

introduces vertex corrections to the quasiparticle current. We showed that in a multiband system such vertex

corrections have a much more peculiar role than in a single-band system, explaining the puzzling

experimental data on the Hall effect.

In particular, when hole and electron pockets have different sizes, as it happens in electron or hole-doped 122

systems, and when interband

interaction dominate, vertex corrections lead to a predominant hole or electron character of the transport

(depending on the Fermi surface sizes). This effect in turn depends on the strength of the spin-fluctuations

mediated interaction, that is affected by several parameters as for example doping and temperature.

[1] L. Ortenzi, E. Cappelluti, L. Benfatto, L. Pietronero, Phys. Rev. Lett. 103, 046404 (2009).

[2] L. Benfatto and E. Cappelluti, Phys. Rev. B 83, 104516 (2011).

[3] F. Rullier-Albenque, D. Colson, A. Forget, and H. Alloul, Phys. Rev. Lett. 103, 057001 (2009);

[4] L. Fang, H. Luo, P. Cheng, Z. Wang, Y. Jia1, G. Mu1, B. Shen, I.I. Mazin, L. Shan, C. Ren,

and H-H. Wen, Phys. Rev. B 80, 140508(R) (2009).

[5] L. Fanfarillo. L. Benfatto, E. Cappelluti, C. Castellani, in preparation.

24

Exchange bias in manganite bilayer films

A. Yu. Petrova, G. Kurij

b, L. Alvarez Miño

c, R. Werner

b, M. Cantoni

d, P. Parisse

e, R. Bertacco

d, R.

Kleinerb, D. Koelle

b and B. A. Davidson

a

a CNR-IOM TASC National Laboratory, Area Science Park-Basovizza, 34149 Trieste, Italy

b Physikalisches Institut - Experimentalphysik II, Universität Tübingen, 72076 Tübingen, Germany

c Universidad Nacional de Colombia, Sede Manizales, Cra 27 # 64-60 Manizales, Colombia

d L-NESS, Dipartimento di Fisica del Politecnico di Milano, via Anzani 52, 22100 Como, Italy

e Sincrotrone Trieste S.C.p.A. S.S. 14 Km 163,5, Area Science Park-Basovizza, 34149 Trieste, Italy.

We have studied exchange bias in ferromagnet/antiferromagnet (F/AF) bilayers of La1-xSrxMnO3 grown by

molecular beam epitaxy, with AF doping between x=0.55–1 and fixed F doping x=0.35. The observed

exchange bias is characterized by a large increase in coercive field accompanied by a smaller shift of the

magnetization hysteresis loop. The largest values of coercive field (~600 Oe) and loop shift (~150 Oe) were

obtained for AF doping x=0.65 at low temperature. The loop shift disappears for all AF dopings above

~100K, while the increased coercive fields persist above 300K, well above the maximum Neél temperature

(250K) for bulk AF phases in this doping range. Consequently, these exchange bias characteristics can

produce large coercivity contrast for manganite magnetic tunnel junctions, both at low temperature where

large tunneling magnetoresistance has been measured [1,2] as well as high-temperature operation above

320K.

[1] R. Werner, A. Yu. Petrov, L. Alvarez Miño, R. Kleiner, D. Koelle and B. A. Davidson, Appl. Phys. Lett.

98, 162505 (2011).

[2] R. Werner, M. Weiler, A. Yu. Petrov, B. A. Davidson, R. Gross, R. Kleiner, S. T. B. Goennenwein and

D. Koelle, Appl. Phys. Lett. 99, 182513 (2011).

25

Electronic and magneto transport properties of the Fe1.5Ti0.5O3-δ magnetic

conductive oxide

E. Chikoidzea*

, Y. Dumonta, T.Tchelidze

b, E. Popova

a, B. Berini

a and N. Keller

a

aGroupe d’Etudes de la Matière Condensée (GEMaC), CN S - Université de Versailles, Versailles, France

bFaculty of natural and exact sciences, tbilisi state university, Georgia

On the goal of an alternative way to realize an above 300K ferromagnetic semiconductor for intrinsic

spin injection, the solid solution hematite-ilmenite Fe2-xTixO3 appeared to be a good candidate: Curie

temperature TC>400 K, intrinsically conductive due to Fe3+

/Fe2+

mixed valences [1-4]. Furthermore, it is a

wide band-gap (3.2 eV) and transparent in the visible range; ab-initio calculations predict a high spin

polarization for x=0.5 composition [5].

In the presented work, we study in details the conductivity mechanism and magneto-transport

electronic properties of Fe1.5Ti0.5O3-δ thin films deposited by pulsed laser deposition (PLD) on Al2O3(0001)

substrates, in the 100K-400K range with magnetic fields up to 9T. The zero-field transport properties are

governed by the oxygen stoichiometry with a variable range hopping mechanism. Magnetoresistance

presents significant value, and has different behavior depending on the magnetic field orientation.

[1] Y. Ishikawa et al., Jpn. J. Phys. Soc. 12, 1083 (1957); Y. Ishikawa, ibid. item 13, 37 (1958).

[2] H. Hojo, et al., Appl. Phys. Lett. 89, 082509 (2006).

[3] H. Ndilimabaka, et al., J. Appl. Phys. 103, 07D137 (2008)

[4] E. Popova, et al., J. Appl. Phys. 103, 093909 (2008)

[5] A. Bandyopadhyay, et al., Phys. Rev. B 69, 174429 (2000)

26

Polaronic transport in the metallic phase of epitaxial La0.7Sr0.3MnO3 thin

films

P. Graziosi a,b

, A. Gambardellab, M. Prezioso

b, I. Bergenti

b, A. Riminucci

b, D.Pullini

c, D. Busquets-

Mataixa, V. Dediu

b

a Instituto de Tecnología de Materiales, Universidad Politécnica de Valencia, Camino de Vera s/n, 46022, Valencia,

Spain b CNR - ISMN, Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati, v. Gobetti 101,

40129, Bologna, Italy c Centro Ricerche Fiat, 10043, Orbassano (TO), Italy

Transport measurements of epitaxial La0.7Sr0.3MnO3 (LSMO) thin films were analysed in the framework of

polarons in polar crystals [1] at the both sides of the metal-insulator transition (MIT). LSMO thin films are

grown on matching substrates SrTiO3 (100) and NdGaO3 (110) by Channel Spark Ablation [2] and are

measured in the temperature range 25 - 400 K.

The resisivity is proportional to wb∙n(1+n) where n is the phonon distribution and wb is the polaronic

bandwith which narrows as exp(-coth(h/kT)) up to the MIT; the approximation of one dominant optical

phonon mode is used for to fit the experimental curve. An excellent agreement with the theory was found

also in the temperature range below the MIT, where the films show metallic behaviour, and the soft mode

frequency obtained by numerical fits of the transport measurements matches with the soft optical phonons

associated with a rattling mode of manganese ions in the oxygen octahedrals [3].

The polaron band transport describes the transport in the whole range of temperature below the MIT, up to

room temperature, while an electron-electron scattering term must be added below 80 K. The numerical fits

demonstrate the inadequacy of the T4.5

or T5 usually used to describe the transport in LSMO at the metallic

side below MIT. The polaronic transport across the MIT is understood in the Charge Carriers Density

Collaps (CCDC) picture [4].

Finally a correlation between the growth mode dimension (2D or 3D) and the transport parameters is found.

Figure 1: R(T) of a 9 nm thick LSMO/STO film, fits at the two sides of the MIT, and their combination

according to the CCDC model; inset: AFM image of the surface, the rms roughness is 0.2 nm.

[1] G. L. Sewell, Phyl. Mag. 3 1361 (1958); J. T. Devreese, A. S. Alexandrov, Rep. Prog. Phys. 72

066501 (2009).

[2] P. Graziosi et al. , submitted.

[3] J.-S. Zhou, J. B. Goodenough et al. Phys. Rev. B 74, 014422 (2006).

[4] A. S. Alexandrov, A. M. Bratkovsky, and V. V. Kabanov, Phys. Rev. Lett. 96, 117003(2006).

27

Towards a control over the electronic properties and magnetism in formally 3d0

compounds: the case of rutile TiO2

G. Drera a,b

, M.C. Mozzati c, P. Galinetto

c, Y.A. Diaz-Fernandez

c, L. Malavasi

c, F. Bondino

d, M.

Malvestuto e, G. Salvinelli

a,b, L. Sangaletti

a,b

a

Università Cattolica del Sacro Cuore, Brescia, Italy b Interdisciplinary Laboratories for Advanced Materials Physics, Brescia, Italy

c Università di Pavia, Pavia,

Italy d

IOM-CNR Laboratorio TASC, Basovizza, Italy e

Sincrotrone Trieste, Basovizza, Italy

TiO2 is currently studied for its useful properties in many applications like high-k gate insulators, gas

sensing, photocatalysis, and as a closed-shell (formally d0) magnetic system. In particular the mechanism

underlying d0 ferromagnetism (FM) are still under debate and present a challenge both from a theoretical and

experimental point of view. Evidences suggest that doping impurities, lattice defect or oxygen vacancies are

involved in such behaviour. The present investigation is rooted in a study of rutile single crystals doped with

TM impurities such as Cr, Mn, Fe, Co, Ni and Cu [1]. We found that room temperature (RT) FM ordering is

already present in the undoped crystals, and it is enhanced upon TM doping. From this starting point we

resorted to investigate the correlation between the electronic and magnetic properties of pure rutile TiO2

grown in the form of thin films prepared by rf-sputtering. The rf-sputtering deposition of thin films allowed

us to vary the oxygen stoichiometry by changing the O content in the Ar/O2 mixture used to sputter the TiO2

target. We show that control over the magnetic properties can be reached by post-growth techniques such as

vacuum annealing or annealing in O2 atmosphere. Furthermore, we show that the enhancement of saturation

magnetization can be achieved by doping the pure TiO2 rutile thin films with nitrogen, rather than with TM

ions. We contrast and compare the magnetic and local electronic properties of undoped and N-doped TiO2

thin films grown by RF sputtering in an Ar-N2 controlled atmosphere. Both undoped and N-doped films

display FM at room temperature (RT). In particular, N-doping results in a 5-fold increase of saturation

magnetization as compared to undoped TiO2. By using a combination of GGA-PBE density functional theory

calculations and element specific spectroscopic techniques, such as X-ray absorption and resonant

photoemission (RESPES) at the Ti L-edge, we have been able to identify new electronic in-gap states related

to N-doping [2]. Both acceptor and donor levels appear in the rutile gap and add to electronic states due to

oxygen vacancies already present in the undoped sample. On this basis, we discuss the possibility to relate

the observed FM to an increase of the spin polarization of in-gap states with respect to the case of undoped

TiO2 rutile.

[1] L. Sangaletti, M.C. Mozzati, P. Galinetto, C.B. Azzoni, A. Speghini, M. Bettinelli, C. Calestani,

Journal of Physics: Cond.Matter, 18, 7643 (2006)

[2] G. Drera, M.C. Mozzati, P. Galinetto, Y. Diaz-Fernandez, L. Malavasi, F. Bondino, M. Malvestuto,

and L. Sangaletti, Appl. Phys. Lett. 97, 012506 (2010)

28

High Tc superconductivity in cuprate/titanate superlattices:

the role of the interfaces

D. Di Castroa, C. Aruta

b, M. Minola

c, D. Innocenti

a, A. Tebano

a, M. Salvato

d, W. Prellier

e, O. I.

Lebedeve, I. Ottaviani

d, N. B. Brookes

f, M. Moretti Sala

f, C. Mazzoli

c, P.G. Medaglia

g, G.

Ghiringhellic, L. Braicovich

c, M. Cirillo

d, G. Balestrino

a

a CNR-SPIN and Dip. di Ing. Informatica Sistemi e Produzione, Università di Roma Tor Vergata, Italy

b CNR-SPIN, Dipartimento di Scienze Fisiche, Napoli, Italy

c CNISM and Dipartimento di Fisica, Politecnico di Milano, Italy

d Dipartimento di Fisica and MINAS Lab., Università di Roma Tor Vergata, Italy

e Laboratoire CRISMAT, UMR 6508, CNRS-ENSICAEN, France

f European Synchrotron Radiation Facility, Grenoble, France

g CNR-SPIN and Dipartimento di Ingegneria Industriale, Università di Roma Tor Vergata, Italy

Advanced layer by layer deposition techniques allowed in recent years to synthesize high Tc superconducting

films by the engineering of heterostructures (HSs) and superlattices (SLs) based on an insulating and a

metallic (non-superconducting) cuprate, such as CaCuO2/BaCuO2 [1] and La2CuO4/La2-xSrxCuO4 [2]. In

these systems the metallic cuprate acts as charge reservoir, injecting carriers in the CuO2 planes of the

insulating cuprate, thus artificially reproducing the mechanism which naturally occurs in cuprate high Tc

superconductors. We followed a different approach by choosing two insulating oxides, namely CaCuO2, an

antiferromagnetic insulator with infinite layer structure, and SrTiO3, a wide gap semiconductor with

perovskite structure, as building blocks for the engineering of (CCO)n/(STO)m SLs. If grown in highly

oxidizing atmosphere, these SLs are superconducting with maximum Tc = 40 K [3].

Significant experimental evidence points towards the confinement of superconductivity within few unit cells

at the CaCuO2/SrTiO3 interface and the important role of additional oxygen atoms entering the interfaces

during growth in oxygen rich environment.

One of the advantages of these cuprate/perovskite systems [3] with respect to the cuprate/cuprate HSs [1,2] is

that the copper belongs only to one of the constituent blocks. Therefore, resonant spectroscopy technique

could be an important tool to disentangle the role of each layer block. Thus, in addition to conventional

characterization techniques, such as XRD, transport, TEM, we used soft x-ray absorption spectroscopy

(XAS) at Cu and Ti L-edges to investigate the doping mechanism and the role of the interfaces. Clear

evidence of an increase in the concentration of holes delocalized on the Cu3d-O2p band was obtained in the

case of superconducting SLs by XAS at Cu L-edge. On the contrary, no relevant changes in the XAS spectra

at the Ti L-edge were observed between superconducting and non superconducting samples, indicating that

STO does not contribute to transport. A detailed analysis of the XAS features helped to individuate the

important role played by the interfaces.

Additionally, we used Resonant Inelastic X-Ray Scattering (RIXS) in order to study electronic and magnetic

excitations in the CCO/STO SLs and to further investigate the role of the interfaces. This study reveals that

dispersing magnons are present in the SLs as in the insulating single phase film of CCO [4]. A study of the

dd excitations as a function of CCO unit cells in the SL allowed pointing out the interface effect [5].

The overall experimental data points toward an interfacial reconstruction by oxygen redistribution that, in

case of excess oxygen, generates holes in the CuO2 planes. Therefore, the interfaces in these SLs behave as a

charge reservoir for the CuO2 planes allowing superconductivity in the CCO layers.

[1] G.Balestrino et al. Phys. Rev.B 58, R8925 (1998)

[2] A. Gozar et al., Nature 455, 782-785 (2008)

[3] D. Di Castro et al., cond-mat arXiv:1107.2239

[4] M. Moretti Sala et al., New Journal of Physics 13, 043026 (2011)

[5] M. Minola et al, submitted for publication

29

Barium doped SrCu2O2 as a new p-type transparent conducting oxide material:

first principles modelling and experimental studies

Mircea Modreanu1, Michael Nolan

1, Guy Garry

2, Bernard Servet

2, Guido Huyberechts

3, Ekaterina

Chikoidze4

1Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland; E-mail: mircea.modreanu@tyndall.ie

2 Thales Research & Technology France, Route Départementale 128, F-91767 Palaiseau cedex, France ;

3 FLAMAC, 9052 Zwijnaarde, Belgium

4 Institute of Electronic Structure & Laser, Foundation for Research and Technology –Hellas, P.O. Box 1527, Heraklion

71110, Crete, Greece

5 University of Versailles CNRS (GEMAC), 1, Place A. Briand, Meudon 92195, France

Transparent conductive oxides (TCOs) are materials displaying the remarkable combination of high

electrical conductivity and optical transparency. Most of the currently known transparent conductive oxides

exhibit only n-type electrical conductivity and due to the lack of p-type transparent conductive oxides, there

are almost no applications based on transparent p-n oxide junctions. In this paper, we report on experimental

studies of optical, microstructural and electrical properties of Ba doped SrCu2O2 thin films (BaSCO) grown

on various substrates by Pulsed Laser Deposition (PLD) using a KrF excimer laser (λ = 248 nm). PLD has

been performed under reproducible conditions, base pressure of 10-7–5×10-8 mbar and O2 pressure of 10-5

mbar, to produce uniform BaSCO layers with the stoichiometric phase. However, the optimal temperature is

strongly dependent on the nature of the substrate, as demonstrated by XRD and AFM characterizations of the

crystalline phase and microstructure. The optical properties have been inferred from spectroscopic

ellipsometry and reflectance/transmission spectrophotometry measurements over a large spectral range from

ultraviolet to far infrared. BaSCO offers extended transparency both in UV and infrared range while

retaining good electrical properties. The correlation between PLD deposition kinetics and optical,

microstructural and electrical properties is discussed.

30

Non-conducting interfaces of LaAlO3/SrTiO3 made by sputter deposition and

the role of stoichiometry

Ishrat. M. Dildara*

, D. Boltjea, M. Hesselberth

a, Q. Xu

b, H. Zandbergen

b, S. Harkema

c, and J. Aarts

a

a Kamerlingh Onnes Laboratorium, Neils Bohrweg 2, 2333CA, Leiden Institute of Physics (LION), Leiden University,

the Netherlands. b National Centre for High Resolution Microscopy, Kavli Institute for Nanoscience, Delft Technical University,

Lorentzweg 1, 2628 CJ Delft, the Netherlands.

c Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE

Enschede, the Netherlands.

We have investigated the interface properties of LaAlO3 grown on TiO2-terminated SrTiO3 (001) fabricated

by RF sputtering in a high-pressure oxygen atmosphere. The LaAlO3 films are smooth with flat surfaces and

have the correct tetragonal phase. Transmission electron microscopy shows atomically sharp, coherent and

continuous interfaces while EELS measurements show some slight intermixing. However, we find these

interfaces to be non-conducting. For films grown by molecular beam epitaxy, it was found that the elemental

ratio of La to Al has to be below 0.97 in order to have a conducting interface [1]. In our case this ratio,

determined by TEM / EDX, is found to be 1.07 (taking a LAO crystal as a reference). This suggests that

intrinsic electronic reconstruction is not the main phenomenon even in conducting interfaces fabricated by

pulsed laser deposition. The reason for the higher ratio found in our films probably has to do with the high

oxygen pressure during deposition, and a connection between oxygen stoichiometry and La /Al ratio.

[1] M.P. Warusawithana, A.A. Pawlicki, T. Heeg, D.G. Schlom, C.Richter, S. Paetel, J. Mannhart, M.

Zheng, B. Mulcahy, J. N. Eckstein, W. Zander, and J. Schubert, Bulletin of the APS 55,nr. 2 (2010), abstract

ID BAPS.2010.MAR.B37.1.

_______________________________________________________________________

*mubeen@physics.leidenuniv.nl

31

Time-resolved optical response of all-oxide 2 3 7 0.7 0.3 3YBa Cu O La Sr MnO

proximitized bilayers

L. Parlato,a R. Arpaia,

a C. De Lisio,

a F. Miletto Granozio,

a G. Pepe,

a P. Perna,

b V. Pagliarulo,

a

M. Radovic,c Y. Wang,

d Roman Sobolewski,

e and U. Scotti di Uccio

a

aCNR-SPIN and Dipartimento di Scienze Fisiche, Complesso Universitario di Monte Sant’Angelo, Via Cinthia, I-

80125 Napoli, Italy

b IMDEA-Nanociencia, Campus Universidad Autonóma de Madrid, 28049 Madrid, Spain,

cSwiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland, Laboratory for Synchrotron and

Neutron Spectroscopy, EPFL, Lausanne, Switzerland

dMaterials Science Graduate Program, and Laboratory for Laser Energetics, University of Rochester, New York 14627-

0231, USA

eDepartments of Electrical and Computer Engineering and Physics and Astronomy, University of Rochester, New York

14627-0231, USA

We present femtosecond pump–probe spectroscopy studies of time-resolved optical reflectivity of all-oxide,

YBa2Cu3O7/La0.7Sr0.3MnO3 (YBCO/LSMO) superconductor/ferromagnet (S/F) bilayers with different

thickness. Samples with a thin ferromagnet overlayer show a photoresponse that is similar to, but faster than,

pure YBCO; the photoresponse of S/F structures with thick (35 nm) LSMO layers resembles instead that of

the pure LSMO. In all cases, the dynamics is described in terms of two characteristic times. Both such times

are peaked close to the superconducting transition. In a wide range of temperature, the application of a thin

ferromagnetic overlayer (10 nm) over YBCO determines a behavior that cannot be interpreted as an

incoherent sum of contributions from the two layers.

32

Properties of oxide ultrathin films on metal supports

L. Giordano a

, J. Goniakowski b, G. Pacchioni

a

a Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Milano, Italy

b CNRS-INSP and Université Pierre et Marie Curie-Paris 6, Paris, France

Oxide ultrathin films of thickness of a few nanometers and below, grown on metallic substrates, may

exhibit peculiar properties that make these systems extremely appealing for new applications in catalysis,

microelectronics and gas sensors. The oxide can exist in unusual atomic structures, stabilized at the

nanoscale and by the interface, with consequences on the electronic and magnetic properties of the film [1].

Electron transfer through the insulating film leads to the spontaneous charging of adsorbates on the

oxide film [2,3], affecting their reactivity and, in some cases, their magnetic properties. The stabilization of

these charged species is enhanced by the polarization of the metal substrate and by the structural response

(polaronic-like distortion) of the oxide film. These charge transfer phenomena are also related to the relative

position of adsorbate and substrate states. For this reason the work function change induced by the oxide is a

key factor in determining the occurrence and the direction of the electron flow through the oxide film [4].

[1] L. Giordano, G. Pacchioni,, Acc. Chem. Res. 44, 1244 (2011).

[2] G. Pacchioni, L. Giordano, M. Baistrocchi, Phys. Rev. Lett. 94, 226104 (2005).

[3] A. Gonchar, T. Risse, H.-J. Freund, L. Giordano, C. Di Valentin, G. Pacchioni, Angew. Chem. Int.

Ed. 50, 2635 (2011).

[4] L. Giordano, F. Cinquini, G. Pacchioni, Phys. Rev. B 73, 045414 (2006).

33

Growth of stoichiometric TiO2 thin films on Au(100) substrates by molecular

beam epitaxy

A Calloni, A Ferrari, A Brambilla, F Ciccacci and L Duò

CNISM and Dipartimento di Fisica, Politecnico di Milano Piazza Leonardo Da Vinci 32, 20133 Milano, Italy

Titanium dioxide (TiO2) has been the subject of intense studies in recent years, due to its photoelectric and

photochemical properties and its high refractive index and dielectric constant. Thanks to such properties,

many applications have highlighted the crucial role played by the TiO2 surface (see Ref. [1] for a

comprehensive review).

Most of the interface works on TiO2 have studied the interactions of different materials with the surface of a

TiO2 substrate. However, these studies can substantially benefit from the use of thin oxide films. The

possibility of synthesize new titanium oxide phases at low coverages has been demonstrated [2,3];

additionally, the oxide surface can be electrically and magnetically coupled with the substrate, thus allowing

for a more reliable and new characterization by electron spectroscopies and microscopic techniques.

We report on the growth of thin TiO2 layers on Au (100) by Ti deposition in a reactive O2 atmosphere. The

oxidation of the substrate is strongly prevented by the use of a noble metal: this allowed us to study titanium

oxide growth at different oxygen pressures and substrate temperatures. Our experimental results [4] show the

presence, together with TiO2, of reduced Ti-oxide species, which gradually disappear with increasing

coverage if growth is performed at high substrate temperature (300 °C). In these conditions, high quality

films are produced, with the correct 1:2 Ti to O stoichiometry and showing a crystalline rutile (100) TiO2

surface. New results are also presented, dealing with TiO2 growth on a magnetic substrate (iron in our case).

The grown oxide is characterized and differences with the gold case are discussed in the light of the higher

reactivity of the substrate towards oxygen.

Our analysis techniques comprises low energy electron diffraction (LEED) and X-ray photoemission

spectroscopy (XPS). Additionally, we employed ultraviolet photoemission spectroscopy (UPS) and inverse

photoemission spectroscopy (IPES) to study the whole valence and conduction electron states of the grown

titanium oxide films.

[1] U. Diebold, Surf. Sci. Rep. 48 (2003) 53.

[2] Finetti et al., Surf. Sci. 602 (2008) 1101.

[3] Papageorgiou et al., J.Phys. Chem. C 111 (2007) 7704.

[4] Calloni et al., Thin solid films 520 (2012) 3922.

34

Magnetic properties of (Ga,Mn)As films in MgO based heterostructures

P. Torellia, B. Rache Salles

a, J. Fujii

a, R. Ciancio

a, E. Carlino

a, M. Sperl

b, C. H. Back

b, C. Rinaldi

c,

M. Cantonic, R. Bertacco

c and G. Panaccione

a

a Istituto Officina dei Materiali IOM-CNR, Laboratorio TASC, in Area Science Park, S.S.14, Km 163.5, I-34149

Trieste, Italy b Institut für Experimentelle Physik, Universität Regensburg, D-93040 Regensburg, Germany

c CNISM and L-NESS - Politecnico di Milano, Via Anzani 42, 22100 Como, Italy

(Ga,Mn)As is one of the most promising diluted magnetic semiconductors (DMS) for spintronics

applications due to the relatively high Curie temperature (190 K) and to the compatibility with the GaAs

MBE technology [1-3]. Despite the promising features (Ga,Mn)As has a Curie temperature well below room

temperature limiting its possible applications. One possible direction to tailor novel properties of DMS thus

making integration in real devices feasible is to exploit interface effects in highly controlled heterostructures

(HS). Following this route FM behaviour of Mn at room temperature in both epitaxial and non-epitaxial

Fe/(Ga,Mn)As interfaces has been demonstrated [4]. This result is of a great interest and gives rise to the

question on the possible magnetic coupling between the ferromagnetic layer and the (Ga,Mn)As layer

through the insulating layer in a Magneto Tunnel Junction like structure: e.g. Fe/MgO/(Ga,Mn)As.

We have grown Fe/MgO/(Ga,Mn)As by MBE in UHV, the Fe layer

had a thickness of 2 nm to ensure RT ferromagnetism while a

spacing layer of MgO had a variable thickness ranging from 0 to 3

nm. The structure and morphology of the different layers were

investigated by TEM. Cross sectional TEM analyses have clearly

shown that the MgO grows in a 2D way on the (Ga,Mn)As substrate

and Fe is in epitaxy on the MgO. Photoemission spectra have been

acquired after each step of the preparation to determine the chemical

state of the interfaces of our system while X-ray Magnetic Circular

Dichroism experiment has been then performed in order to follow

the evolution of the magnetic coupling of the Fe and the Mn

moments as a function of the MgO thickness. A magnetic signal in the Mn was detectable at room

temperature up to about 2 nm of MgO thickness, suggesting that Fe proximity can polarize the (Ga,Mn)As

even across a thin insulating barrier. On gold capped samples we have patterned squared pillars (ranging

from 16 to 225 m2) by optical lithography. On these junctions we have measured the resistance across the

MgO barrier as a function of the applied voltage: the I-V curves of junctions are cubic, i.e. the conductance

versus bias is parabolic thus testifying the tunneling behavior of the transport across the junction. This

measurement, which is preliminary to TMR measurements, rules out the possibility that the observed

coupling is due to the presence of pinholes in the MgO barrier.

[1] Ohno et al. Appl. Phys. Lett. 69 (1996) 363.

[2] A. H. Macdonald et al. Nature Mat. 453 (2008) 899.

[3] Chen et al., APL 95, 182505 (2009)

[4] F. Maccherozzi et al. Phys. Rev. Lett. 101 (2008) 267201

Au

Electrode

GaAs

MTJ

Figure 1: Squared junction

×m2) obtained by optical

lithography

35

Direct fabrication of arbitrary-shaped ferroelectric nanostructures on plastic,

glass and silicon substrates

Suenne Kim and Elisa Riedo

School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA

Ferroelectric (FE) materials represent one of the most versatile smart materials for use at the nanometer scale

owing to their unique properties of switchable spontaneous polarization, pyroelectricity and piezoelectricity.

Specifically, the perovskite-based FEs show a very high piezoelectric response, up to an order of magnitude

higher than non-ferroelectric compositions such as AlN and ZnO. Such high electromechanical coupling

makes ferroelectrics great candidates for sensor, actuator and energy harvesting devices, especially at small

scales in micro- and nano-electromechanical systems (MEMS/NEMS). While the processing of FE thin films

(both epitaxial and polycrystalline) has been extensively examined and developed over the last few decades,

robust techniques that do not require epitaxial substrates for the patterning of 2D and 3D FE nanostructures

remain elusive. Here, we present a nanomanufacturing method based on thermochemical nanolithography

(TCNL) [1, 2] for the creation of arbitrary-shaped planar FE nanostructures without epitaxial-growth

requirements [3]. Ferroelectric lines with widths ≥ 30 nm, spheres with diameter 10 nm and densities up to

213Gb in-2

are directly fabricated on either plain or platinized substrates, ranging from plastic (Kapton) to

silicon and soda-lime glass. This process consists of three steps: i) deposition of a sol-gel precursor film of

Pb(Zr0.52Ti0.48)O3 (PZT) and PbTiO3 (PTO) on any substrate capable of withstanding processing temperature

of ~250 C for one minute (as in step ii); ii) bulk-heating of the substrate at 250-300 C for one minute to

remove most of the organics; and iii) local-crystallization of the precursor film by TCNL, where a

resistively-heated atomic force microscope (AFM) tip is brought in contact (Tcontact ≥ 0 C) with the

precursor to form a crystalline FE nanostructure.

[1] Z. Wei, D. Wang, S. Kim, S.-Y. Kim, Y. Hu, M. K. Yakes, A. R. Laracuente, Z. Dai, S. R. Marder,

C. Berger, W. P. King, W. A. de Heer, P. E. Sheehan, E. Riedo, Science (2010), 328, 1373.

[2] R. Szoszkiewicz, T. Okada, S. C. Jones, T.-D. Li, W. P. King, S. R. Marder, E. Riedo, Nano Letters

(2007), 7, 1064.

[3] Suenne Kim, Yaser Bastani, Haidong Lu, William P. King, Seth Marder, Kenneth H. Sandhage,

Alexei Gruverman, Elisa Riedo, and Nazanin Bassiri-Gharb “Direct fabrication of arbitrary-shaped

ferroelectric nanostructures on plastic, glass and silicon substrates”, Adv. Mat., 23, 3786–3790,

(2011).

36

Resonant Elastic X-ray Scattering investigation of Magneto-Electric materials

C. Mazzolia, R.D. Johnson

b, S. Partzsch

c, J.E. Hamann-Borrero

c, V. Scagnoli

d, M.Allieta

e, M.

Scavinie

a Physics Department, Politecnico di Milano, Milano, Italy

b Oxford University, Oxford, UK

c IFW Dresden, Dresden, Germany

d SLS, Villigen, Swiss

e

Università di Milano, Milano, Italy

Strongly Correlated Electron Systems are the solid state realm of electronic orderings and interactions.

Depending on symmetries, intrinsic electronic degrees of freedom may compete; a high susceptibility of the

electronic state is expected, resulting in rich phase diagrams and exotic phases. Among the latters, the ones

presenting electric and magnetic sublattice polarizations are appealing, in particular Magneto-Electrics

(MEs), which are the subset characterized by electric and magnetic polarization interplay.

Over the last ten years the scientific community has produced a considerable effort to understand in details

the mechanisms at play and the electronic states involved in MEs.

In this respect, Resonant Elastic X-ray Scattering (REXS) is a perfectly suited technique to access long range

electronic ordering (charge, spin and orbital) in this class of materials.

I will present some recent REXS investigations of ME phases, ranging from manganites to ferroborates.

Some interesting possibilities offered by alternative compounds will be discussed, based on recent structural

and magnetic refinements.

37

Growth of strained piezoelectric NaNbO3 thin films by pulsed laser deposition

J. Sellmann, J. Schwarzkopf, A. Duk, A. Kwasniewski, M. Schmidbauer, R. Fornari

Leibniz-Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany

Due to its promising piezoelectric properties and high Curie temperature NaNbO3 has recently attracted

much attention. In contrast to bulk crystals, thin epitaxial films may experience a specific lattice strain which

depends on the used substrate/film combination. In the case of perovskites this deformation of the crystal

lattice is known to have a strong impact on the ferro-/piezoelectric properties.

Fully strained NaNbO3 films with varying lattice strain were epitaxially grown by means of pulsed laser

deposition (PLD). For structural investigations like strain state, film symmetry and orientation we have

deposited the films on bare perovskite substrates. However, in order to exploit the functional properties of

these films it is necessary to embed them in a capacitor structure. To this extent we used pseudomorphically

grown SrRuO3 films as bottom electrodes.

In the present work we report on strained NaNbO3 films which were deposited on several perovskite

substrates as well as on SrRuO3/NaNbO3 heterostructures on SrTiO3 and DyScO3 substrates. In such a way

we achieved films with in-plane compressive and tensile strain. By adjusting the substrate temperature, the

oxygen partial pressure and the laser frequency we have successfully deposited smooth, strained, single

phase NaNbO3 thin films. Reciprocal space mappings in the vicinity of an asymmetric Bragg reflection (fig.

1) display the same in- plane component of the scattering vectors for the substrate and both films, showing

epitaxial growth of fully strained SrRuO3 and NaNbO3 films on the DyScO3 (110) substrates. Step flow

growth mode of the films was achieved, indicated by AFM measurements (fig. 2).

Systematic investigations of the films by atomic force microscopy and high resolution X-ray diffraction

reveal the dependence of the surface morphology and the incorporated lattice strain on the deposition

parameters and the lattice mismatch. Correlations of chemical composition and deposition conditions were

studied by energy dispersive x-ray spectroscopy. All films exhibit piezoelectric properties, as proven by

piezoresponse force microscopy.

Figure 1: HR-XRD reciprocal space mapping in

the vicinity of the (444) Bragg reflection of the

DyScO3 (110) substrate. The Bragg peaks of the

NaNbO3 and SrRuO3 films exhibit the same in-

plane component q as the DyScO3 substrate

Figure 2: Surface morphology of a 15 nm thick

NaNbO3 film on 40 nm SrRuO3 on a DyScO3

substrate

38

Magnetoelectric effects in the spin­driven multiferroic Ba2CoGe2O7

P. Baronea*, K. Yamauchi

b, S. Picozzi

b

aCNR­SPIN­I67100 L’Aquila, Italy

bISIR-SANKEN, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

The magnetoelectric (ME) coupling between magnetic and ferroelectric order parameters hasrecently been

the object of intense research in the context of ME multiferroic oxides, wheremagnetism and ferroelectricity

appear concomitantly.At odds with conventional MEmaterials where the induced polarization is linear in the

applied magnetic field (as, e.g.,prototypical Cr2O3), magnetically induced ferroelectrics may display giant

and peculiar MEeffect. Besides the usually invoked microscopic mechanisms that lead to spin-

drivenferroelectricity relying on symmetric or antisymmetric exchange striction, a third mechanimhas been

devised recently, where the spin-orbit coupling locally “asymmetrizes” the p-dhybridization between the

transition metal (TM) and the surrounding ligand oxygens. Thisspin-dependent hybridization mechanism is

in general difficult to identify, since it can occurconcomitantly with the others in noncollinear spin

configurations. However, it has beenrecently reported that the spin-dependent hybridization can be

responsible for the polarizationobserved in Ba2CoGe2O7 [1]. We report the results of a joint theoretical

approach, combiningmacroscopic symmetry analysis with microscopic methods (density functional theory

and amodel cluster Hamiltonian), employed to shed light on magnetoelectricity in Ba2CoGe2O7 [2].We show

that the magnetic ordering is responsible for the symmetry lowering that allowsferroelectricityto emerge in

the otherwise nonpolar (albeit noncentrosymmetric) crystalstructure. On the microscopic side, two main

ingredients are needed for the cross-couplingbetween magnetic and dipolar degrees of freedom: on-site spin-

orbit coupling and the spin-dependent hybridization between O-pand transition metal dstates. In fact, a local

dipoledevelops on each TM because of the anisotropic p-dhybridization with the surroundingligands,

modulated by the on-site spin-orbit coupling. Being essentially a single-ion effect,the onset of

polarizationthrough this mechanism does not necessarily require a specific spinstructure. However, structural

constraints related to the noncentrosymmetric symmetry andthe particular configuration of CoO4 tetrahedra

provide additional features for a peculiarmagnetoelectricity to develop.

[1] H. Murakawa, Y. Onose, S. Miyahara, N. Furukawa, and Y. Tokura, Phys. Rev. Lett. 105,137202

(2010). [2] K. Yamauchi, P. Barone and S. Picozzi, Phys. Rev. B 84, 165137 (2011).

39

Epitaxial Fe/MgO/Fe tunnelling junctions on BaTiO3 (001)

G. Radaelli a

, S. Brivio b, R. Bertacco

a

a LNESS and CNISM, Physics Department, Politecnico di Milano, Como, Italy

b Laboratorio MDM – IMM – CNR, Agrate Brianza (MB), Italy

Novel schemes for magnetic recording and reading are based on spintronic devices where a determinant role

is played by materials or interfaces displaying magnetoelectric coupling. The first step in this direction is to

develop a method for implementing the control of magnetic properties of a ferromagnetic layer through an

electric field. In this context the study of the Fe/BaTiO3 (BTO) interface is particularly interesting in view of

a possible magnetoelectric coupling at the interface as suggested by Duan et al [1] and Sahoo et al [2].

After an initial study of the magnetoelectric coupling at the interface between thin Fe films and BTO single

crystal substrates [3] epitaxial Fe/BTO//Nb:SrTiO3(001) and Fe/BTO/La0.7Sr0.3MnO3//SrTiO3(001)

interfaces have been grown by combined use of molecular beam epitaxy and pulsed laser deposition. In situ

low energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS) investigation show

that, despite a minor oxidation at the interface, a good epitaxy of Fe on BTO has been achieved.

Finally, fully epitaxial Co/Fe/MgO/Fe/BTO heterostructures have been deposited and magnetic tunnelling

junctions (MTJs) fabricated via optical lithography. I(V) curves clearly indicate that tunnelling is the

dominant mechanism in our MgO junctions. Preliminary experiments testing the electric control of the TMR

have been performed at different temperatures. A modulation of the TMR on the order of 10%, induced by

application of an electric field across the BTO template, has been detected at 150 K. This result attests the

great potential of this system for the electric control of magnetization of ferromagnetic electrodes in

spintronic devices.

[1] C. Duan et al. Phys. Rev. Lett. 97, 047201 (2006)

[2] S. Sahoo et al. Phys. Rev. B 76, 092108 (2007)

[3] S. Brivio et al. Appl. Phys. Lett. 98, 092505 (2011)

40

Noise spectroscopy analysis: a tool for the investigation of transport

mechanisms in novel materials

C. Barone and S. Pagano

Dipartimento di Fisica “E. . Caianiello” and CNR- SPIN UOS Salerno, Università degli Studi di Salerno, Fisciano

(SA), Italy Fluctuations are a manifestation of the thermal motion of matter and discreteness of its structure. The

investigation of fluctuation phenomena, which may be called "fluctuation spectroscopy", is a very

informative method for the study of kinetic processes in matter. Often, it is also a much more sensitive

method than the measurement of mean quantities. Electric noise measurements can give interesting

information on the conduction mechanisms and the dynamic behaviors of the charge carriers in the

investigated systems. In noise studies, the measurement is concentrated on

the ac component of the output voltage signal. In this

case, the standard four-probe technique alone does not eliminate completely the external noise

contributions, due to contact resistance fluctuations,

and a specific experimental procedure has to be

developed [1]. By means of fluctuation spectroscopy, different types of materials have been investigated ranging from Mn oxides, to novel superconductors, to carbon nanotube composites. In La1-xSrxMnO3 ultrathin films, a connection between the electrical transport processes and a two-level tunneling systems model, used for the interpretation of the noise analysis, is found taking into account the presence of miscut induced terraces on the STO substrate [2]. In Pr0.7Ca0.3MnO3 high-quality epitaxial thin films, the broadband 1/f noise and the dependence of resistance on electric field are consistent with a collective electrical transport, as in the classical model of sliding charge density waves [3]. In nonsuperconducting Nd1.83Ce0.17CuO4+δ thin films, a non-standard linear dependence of the 1/f noise on the applied bias current is found at low temperatures, which is interpreted as a signature of the occurrence of weak localization [4]. In FeTe0.5Se0.5 epitaxial thin films, the noise for low bias electric fields and temperatures is compatible with standard resistance fluctuations. Conversely, a more than quadratic dependence of the 1/f noise amplitude is observed at large applied electric fields and temperatures, indicating a possible connection between the hole contribution to the electrical transport and nonlinear noise phenomena [5].

[1] C. Barone, A. Galdi, S. Pagano, O. Quaranta, L. Méchin, J.-M. Routoure, and P. Perna, Rev. Sci.

Instrum. 78, 093905 (2007).

[2] C. Barone, C. Adamo, A. Galdi, P. Orgiani, A. Yu. Petrov, O. Quaranta, L. Maritato, and S. Pagano,

Phys. Rev. B 75, 174431 (2007).

[3] C. Barone, A. Galdi, N. Lampis, L. Maritato, F. Miletto Granozio, S. Pagano, P. Perna, M. Radovic,

and U. Scotti di Uccio, Phys. Rev. B 80, 115128 (2009).

[4] C. Barone, A. Guarino, A. Nigro, A. Romano, and S. Pagano, Phys. Rev. B 80, 224405 (2009).

[5] C. Barone, S. Pagano, I. Pallecchi, E. Bellingeri, M. Putti, and C. Ferdeghini, Phys. Rev. B 83,

134523 (2011).

41

Spin-orbital coupling at triplet superconductor-ferromagnet interfaces

M. Cuocoa, P. Gentile

a, A. Romano

a, C. Noce

a, D. Manske

b, and P. M. R. Brydon

c

a SPIN-CNR, I-84084 Fisciano (Salerno), Italy and Dipartimento di Fisica ``E. R. Caianiello”, Università di Salerno, I-

84084 Fisciano (Salerno), Italy bMax-Planck-Institut für Festkörperforschung, Heisen bergstr. 1, D-70569 Stuttgart, Germany

c Institut für Theoretische

Physik, Technische Unive rsität Dresden, D-01062 Dresden, Germany We determine the proximity effect and the stable magnetic configuration of a triplet superconductor-

ferromagnet (TSC-FM) junction in the clean regime due to the interplay between spin-triplet

superconductivity with px+ipy, px, or py symmetries and itinerant ferromagnetism. We show that the direction

of the magnetization can be controlled by selecting the orbital symmetry of the TSC order parameter and the

strength of the ferromagnetic exchange for a junction with an interface parallel to the y-direction, i.e. [010],

in the xy plane. Using a self-consistent Bogoliubov- de Gennes approach, we demonstrate that the various

effects occurring in a TSC-FM hybrid, i.e. the suppression of the TSC order parameter, the proximity

behaviour, the change of the interface energy spectrum, cooperate to stabilize a magnetization direction in

the ferromagnet with a configuration that is parallel or perpendicular to the d-vector or, equivalently, to the

Cooper pairs spin direction depending on the orbital symmetry. Here the d-vector is assumed to be aligned

along the z-direction. In particular, we show that for chiral spin triplet order parameter the magnetic direction

changes from being perpendicular to parallel to the d-vector at a critical exchange field through a first-order

phase transition. Otherwise for the px (py) spin-triplet order parameter the Gibbs energy is minimized by a

magnetization that is parallel (perpendicular) to the d-vector for any amplitude of the ferromagnetic

exchange. [2] The results are coherent with previous non-self-consistent studies of the Spin Josephson Effect

[3], therefore establishing the robustness of the effect and the relation with the others that play a role in the

physics of the TSC-FM junction.

[1] M. Cuoco, A. Romano, C. Noce, and P. Gentile, Phys. Rev. B 78, 054503 (2008)

[2] M. Cuoco, P. Gentile, A. Romano, C. Noce, D. Manske, and P.M.R. Brydon, preprint (2012).

[3] P. M. R. Brydon, Phys. Rev. B 80, 224520 (2009). P. M. R. Brydon, Y. Asano, and C. Timm, Phys.

Rev. B 83, 180504 (2011).

42

An optical study of the insulator-to-metal transition in single-layer cuprates

D. Nicoletti1, P. Calvani

1, P. Di Pietro

1, O. Limaj

2, S. Ono

3, Yoichi Ando

4, and S. Lupi

2

1CNR-SPIN and Dipartimento di Fisica, Università La Sapienza, Roma 00185, Italy

2CNR-IOM and Dipartimento di Fisica, Università La Sapienza, Roma 00185, Italy

3Central Research Institute of Electric Power Industry, Tokyo 201-8511, Japan

4Institute of Scientific and Industrial Research, Osaka University, Osaka 567-0047, Japan

Presenting author: paolo.calvani@roma1.infn.it

The ab-plane optical conductivity of eleven single crystals, belonging to the families Sr2−xCuO2Cl2,

Y1−xCaxBa2Cu3O6, Bi2Sr2−xLaxCuO6 has been measured with hole concentrations p between 0 and 0.18, and

for 6 K ≤ T ≤ 00 K to obtain an infrared picture of the p, T phase diagram of the Cu-O plane [1]. At extreme

dilution (p = 0.005), a narrow peak is observed at 1570 cm-1

(195 meV), that we assign to a single-hole

bound state. For increasing doping, that peak broadens into a far-infrared (FIR) band whose low-energy edge

sets the insulating gap. The insulator-to-metal transition (IMT) occurs when the softening of the FIR band

closes the gap thus evolving into a Drude term [2]. In the metallic phase, a multi-band analysis identifies a

mid-infrared band which weakly depends on temperature and softens for increasing p, while the extended-

Drude analysis leads to an optical scattering rate larger than the frequency, as found in other cuprates.

The infrared spectral weight W(T) is consistent with a Fermi liquid renormalized by strong correlations,

provided that the T4

term of the Sommerfeld expansion is included above 300 K [3]. In the superconducting

phase, the optical response of single-layer Bi2Sr2−xLaxCuO6at optimum doping is similar to that of the

corresponding optimally-doped bilayer Bi2Sr2CaCu2O8.

Fig. 1. Optical conductivity of several single-layer crystals with increasing hole doping, showing the

evolution of the insulator-to-metal transition through a filling of the insulating optical gap.

[1] D. Nicoletti et al., New J. Phys. 13 123009 (2011).

[2] S. Lupi et al., Phys. Rev. Lett. 102, 206409 (2009).

[3] D. Nicoletti et al., Phys. Rev. Lett. 105, 077002 (2010).

43

Multistate memory devices based on free-standing VO2/TiO2

microstructures driven by Joule self-heating

N. Manca

1, 2, L. Pellegrino

2, T. Kanki

3, H. Tanaka

3, M. Biasotti

1,2, E. Bellingeri

2, A. S. Siri

1,2, D.

Marré 1,2

1Dipartimento di Fisica, Università di Genova, Genova (Italy)

2CNR-SPIN, Genova (Italy)

3ISIR, Osaka University, Osaka (Japan)

Vanadium dioxide (VO2) is a promising material for high speed electronics and optoelectronics due to its

fast (sub-ps) thermally driven Metal-Insulator Transition (MIT) occurring above room temperature (68°C)

where a decrease of more than 4 orders of magnitudes of electrical resistance associated with variations of

optical constants are observed. This MIT presents thermal hysteresis that widens when moving from single

crystals to thin films. We report the fabrication and electrical characterization of two-terminal multistate memory devices based on

VO2/TiO2 thin film microcantilevers [1]. Fabrication of VO2/TiO2 microcantilevers is a multistep process

combining Pulsed Laser Deposition (PLD) and optical microlithography. VO2 films grown on our TiO2

cantilevers show three orders of magnitude resistance change nearby 340 K and hysteretic behavior (width =

6.5 K) during thermal cycles. Experiments are made within the thermal hysteresis region, where phase coexistence of metallic and

insulating domains exists. We study the behavior of the electrical resistance of these cantilevers when current

pulses are applied observing two types of memory effects:

Non-volatile changes of the electrical resistance are observed when a current pulse is applied to the

microcantilever. Pulses of the same magnitude produce only one single change of the electrical resistance,

while we observe that the resistance decreases with pulses of increasing magnitude. These states persist also

if the current is switched-off and can be erased only by cooling the device below the hysteresis region. Volatile multilevel resistance states are instead possible by biasing the device with a fixed powering current

bias and written with reproducibility by current pulses of different magnitude. These states can be erased by

nullifying the bias with a short pulse. The memory mechanism is based on current-induced creation of metastable metallic clusters by self-heating

of micrometric suspended regions and resistive reading via percolation. The higher thermal insulation of

free-standing structures with respect to patterned thin film devices is a key point of these devices. Hot spots

are created at the cantilever center-end where thermal dissipation is lower and efficient Joule heating is

possible. The use of current pulses instead of voltage ones prevents catastrophic effects due to negative

differential resistance and allow setting the system in selected states with good reproducibility.

We will make experimental comparison between free-standing and non free-standing devices as well as

discuss temperature distribution on VO2 calculated by finite element analysis.

[1] L. Pellegrino, N. Manca, T. Kanki, H. Tanaka, M. Biasotti, E. Bellingeri, A. S. Siri, D. Marré, Adv Mater. 2012 (in press)

44

Multiple electronic-valence elements in A-site perovskite manganites: a route to

an high metallicity and an orbital order co-existence

P.Orgiania, C.Aruta

b, A.Galdi

a, V.Cataudella

b, G.De Filippis

b, C.A.Perroni

b,

V. Marigliano Ramagliab, N.B.Brookes

c, M.Minola

d, M.Moretti Sala

d, G.Ghiringhelli

d, and

L.Maritatoa

a CNR-SPIN and University of Salerno, Fisciano (SA), Italy

b CNR-SPIN and Department of Physical Sciences, University of Napoli, Napoli, Italy

c European Synchrotron Radiation Facility, Grenoble, France c Physics Department, Politecnico di Milano, Milano, Italy

With respect to the double-exchange hopping

mechanism, the role of chemical element sitting at

perovskite A-site (generally hosting an alkali-earth or a

rare-earth atom) has always been considered as “silent,”

being the corresponding conduction band of the A-site

element too far from Fermi’s energy level. In order to

make such an atomic site active within the transport

mechanism, a possible strategy calls for a partial insertion

of multiple-valence ions which also show the requested

electronic properties (i.e. conduction band crossing EF).

In manganites, the ideal candidate is indeed Mn-ions

themselves.

Here we show that a partial substitution of Mn

ions at perovskite A-site (therefore named as A-site

perovskite manganites) is indeed possible in both La-

deficient LaxMnO3 and off-stoichiometric LaxSryMnO3

manganite thin films. By combining polarization-

dependent x-ray absorption spectroscopy and resonant

inelastic x-ray spectroscopy, the relevant Mn2+

content is

demonstrated, and it is unambiguously assigned its crystallographic site (namely, the perovskite A-site).

Similarly to traditional manganites, Mn2+

substitution induces the required Mn3+

/Mn4+

mixed population.

However, differently from the latter, the Mn2+

ions at perovskite A-site are electronically involved in the

transport mechanisms, having their electronic bands crossing the Fermi energy. Such an energetic

configuration favours the hopping of electrical charge through that site (usually silent), in addition to the

traditional Mn3+

/Mn4+

hopping path (named Multiple double-exchange mechanism), thus contributing to the

ferromagnetic and metallic state. Furthermore, to an highly metallic and ferromagnetic state, it surprisingly

corresponds also a strong Mn orbital order. Indeed, the tendency of the orbitals to order locally usually

strongly compete with the kinetic energy of the free charge carriers, which however tends to destroy long

range orbital order. Multiple double-exchange mechanism is here demonstrated that destroys such a

dichotomy by sustaining the co-existence of highly metallic states with orbital ordered phase. This will open

unexplored perspectives in both theoretical and experimental possibilities based on such a coexistence of

spin/orbital order (generally in competition with each other), and more in general in fundamental studies on

transport mechanism in strongly correlated electrons systems.

[1] P.Orgiani et al., Phys. Rev. B 82, 205122 (2010).

[2] A.Galdi et al., Phys. Rev. B 83, 064418 (2011).

[3] C.Aruta et al., submitted (2012).

45

Nanoscale modulation of the local density of states at the interface between

LaAlO3 and SrTiO3 band insulators

M. Salluzzoa, Z. Ristic

b, I. Maggio Aprile

c, R. Di Capua

a, G. M. De Luca

a, F. Chiarella

a, M.

Radovicc

a CNR-SPIN, Complesso MonteSantangelo via Cinthia, I-80126 Napoli, Italy

b Département de Physique de la Matière Condensée, University of Geneva, 24 Quai Ernest-

Ansermet, CH-1211 Geneva 4, Switzerland c LSNS - EPFL, PH A2 354 (Batiment PH) Station 3 CH-1015 Lausanne, Switzerland

The discovery of a high-mobility two-dimensional electron liquid (2DEL) at the interfaces between polar

LaAlO3 (LAO) and non-polar SrTiO3 (STO) insulators [1] has motivated a wide research activity due to their

intriguing physical properties [2-4]. Moreover, this breakthrough generated new paradigms and challenges in

condensed matter physics, requiring non-conventional approaches for their understanding. In their seminal

work [1], Ohtomo and Hwang suggested that the appearance of conductivity at the LAO/STO is due to an

electronic stabilization to avoid the divergence of the electrostatic potential by a transfer of electrons to the

STO conduction band at the interface. However, this picture is debated in particular in view of the possible

role of oxygen and cation defects in the phenomenon.

From a theoretical point of view, a pure electronic reconstruction of the LAO/STO interface is

sufficient to stabilize the system [5]. In particular, to eliminate the divergence of the electrostatic potential, a

fraction of titanium ions have to change their valence from Ti4+

to Ti3+

. In view of the similarities with the

properties of doped STO, finding direct experimental proofs of an electronic reconstruction in the LAO/STO

system is quite challenging,

Here we present a scanning tunneling microscopy/spectroscopy (STM/STS) study of the local

density of states of the buried LAO/STO interface with nanometer resolution. We performed experiments on

samples characterized by insulating or metallic character, i.e. we studied bilayers composed by 2uc and 4uc

thick LAO films deposited on TiO2 terminated STO single crystals. STM/STS shows that a distinctive

characteristic of metallic LAO/STO bilayers is the presence a short-range quasi-periodic pattern (6x8 nm2),

not commensurate with lattice [6], which is observed in both topographic as-well-as purely electronic maps

[Fig.1]. Spectroscopy maps, with nm resolution, suggests that the superstructure is due to the combination of

an electronic and structural reconstruction effects. By comparing normalized (dI/dV)/(I/V) data with LDA+U

calculations [5], we conclude that the superstructure is associated to a quasi-ordered arrangement of the 3d

orbitals. These results are in agreement with X-ray absorption spectroscopy (XAS) studies [7] that showed

the LAO/STO interfaces are characterized by an orbital reconstruction removing the orbital degeneracy of

titanium 3d states and pushing down the 3dxy orbitals.

[1] A. Othomo, H. Y. Hwang, Nature 427, 423426 (2004).

[2] A. D. Caviglia, S. Gariglio, C. Cancellieri, B. Sacepe, A. Fete, N. Reyren, M. Gabay, A. F. Morpurgo,

and J.-M. Triscone, Phys. Rev. Lett. 105, 378380 (2010).

[3] S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, and J. Mannhart, Science 313, 1942 (2006).

[4] A. Brinkman, M. Huijben, M. Van Zalk, J. Huijben, U. Zeitler, J. C. Maan, W. G. Van Der Wiel, G.

Rijnders, D. H. A. Blank, and H. Hilgenkamp, Nature Mater. 6, 493 (2007).

[5] R. Pentcheva, W. E. Pickett, Phys. Rev B 74, 035112 (2006).

[6] Z. Ristic, R. Di Capua, G. M. De Luca, F. Chiarella, G. Ghiringhelli, J. C. Cezar, N. B. Brookes, C.

Richter, J. Mannhart and M. Salluzzo, EPL 93, 17004 (2011).

[7] M. Salluzzo, J. C. Cezar, N. B. Brookes, V. Bisogni, G. M. De Luca, C. Richter, S. Thiel, J. Mannhart,

M. Huijben, A. Brinkman, G. Rijnders, and G. Ghiringhelli, Phys. Rev. Lett. 102, 166804 (2009).

46

Spin injection at the LaAlO3/SrTiO3 interface

E. Lesne, N. Reyren, M. Bibes, J.-M. George, C. Deranlot, S. Collin, A. Barthélémy and H. Jaffrès

Unité Mixte de Physique CNRS/Thales, 1 av. Augustin Fresnel 91767 Palaiseau, France and Université Paris-Sud,

91405 Orsay, France

Future spintronics devices will be built from elemental blocks allowing the electrical injection,

propagation, manipulation and detection of spin-based information. Owing to their remarkable

multifunctional and strongly correlated character, oxide materials already provide such building blocks for

charge-based devices such as ferroelectric field-effect transistors, as well as for spin-based two-terminal

devices like magnetic tunnel junctions, with giant responses in both cases. Until now, the lack of suitable

channel materials and the uncertainty of spin injection conditions in these compounds has however prevented

the exploration of similar giant responses in oxide-based lateral spin transport structures. Ohtomo &

Hwang’s 2004 striking discovery [1] of a metallic interface between two band insulators LaAlO3 and SrTiO3

now provides such a conducting oxide-based channel, shown later to be of a quasi-two dimensional

character. To realize such interfaces, LaAlO3/SrTiO3 heterostructures are prepared by pulsed laser deposition

whereby an epitaxial LaAlO3 thin film is grown on a TiO2-terminated (001)SrTiO3 substrate.

The first step towards spintronics devices is efficient spin injection, i.e. the injection of a spin-polarized

current into a non-magnetic material. Here, we present the demonstration of electrical spin injection into the

LaAlO3/SrTiO3 high-mobility quasi-two-dimensional electron system (2-DES). Magnetoresistance

experiments allow us to probe the occurrence of spin injection from a Co/LaAlO3 magnetic tunnel contact.

We analyze, in a local three-terminal measurement scheme, the voltage variation associated with the

precession of the injected spin accumulation driven by perpendicular or longitudinal magnetic fields (Hanle

and inverted Hanle effect [2]). The influence of bias and back-gate voltages reveals that the spin

accumulation signal is amplified by resonant tunneling through localized states in the LaAlO3 barrier

strongly coupled to the 2-DES by tunnelling transfer [3]. This recent achievement indicates that such oxide-

based systems are suitable for future experiments involving spin propagation, manipulation, and detection.

[1] A. Ohtomo, and H. Y. Hwang, Nature 427, 423 (2004)

[2] S.P. Dash et al., Phys. Rev. B 84, 054410 (2011)

[3] N. Reyren et al., to be published in Phys. Rev. Lett.

47

Electronic reconstruction in (SrMnO3)n(LaMnO3)2n digital superlattices probed

by O K edge x-ray linear dichroism

A. Galdi a,b

, C. Arutac, P. Orgiani

a, , C. Adamo

d, V. Bisogni

e, N.B. Brookes

e, G.

Ghiringhellif, D.G. Schlom

d, P. Thakur

e, L. Maritato

a,b

a CNR SPIN u.o.s. Salerno, Università degli Studi di Salerno, Fisciano (SA), Italy

bDipartimento di Ingegneria Industriale, Università degli Studi di Salerno, Fisciano (SA), Italy

cCNR-SPIN and Department of Physical Sciences, I-80126 Napoli, Italy

dDepartment of Materials Science and Engineering, Cornell University, Ithaca, NY-14853, USA

eEuropean Synchrotron Radiation Facility, F-38043 Grenoble, France

fCNR-SPIN and Dipartimento di Fisica, Politecnico di Milano, I-20133 Milano, Italy

The (SrMnO3)n(LaMnO3)2n (SMO/LMO, n=8,5,1) superlattices can be seen as a way to introduce Sr doping

in LaMnO3 (LMO) (La:Sr=2:1, i.e. the optimal doping for the high Curie temperature CMR compound La1-

xSrxMnO3) without introducing substitutional disorder. The metallicity and magnetism of the samples is

tuned by the thickness of the constituent blocks, that determine the density and the coupling between the

interfaces.

X-ray natural and magnetic linear dichroism (XNLD, XMLD) and magnetic circular dichroism (XMCD)

at the Mn L23 edge have been successfully employed to characterize the orbital occupation and the magnetic

moment orientation, both in antiferromagnetic (AF) and ferromagnetic (F) phases.

L edge spectra are dominated by the Mn3+

contribution, so little information is obtained about the strain

and charge reconstruction effect on the energy levels of the SMO (where Mn mainly assumes 4+ valence),

and about new states created at the interfaces[1].

In this respect, because of the hybridization between the oxygen 2p and the empty metal states, the O K

edge XAS can give indication on both components of the SL, and on the interfacial charge transfer. It was

demonstrated that in oxides, the oxygen 1s2p transition absorption spectrum (O K edge) maps the

unoccupied density of states (DOS) of predominant metal character, as in the O2-

ion the 2p orbital is

nominally full [2]. We can then take advantage of O K edge absorption spectra collected with linear

polarized light to study the competition between bulk and interface effects in different period SLs.

Linear dichroism analysis show that the presence of the interfaces not only acts on the constituent blocks

of the superlattices by doping, but also modifies the band structure in a non-trivial way. The interface

contribution strongly emerges in the n=1 sample, also providing an explanation for the observed XNLD and

XMLD signals at the L edge. Furthermore O K edge analysis can provide direct evidence of the validity of

several band structure calculations reported in literature[3].

[1] C. Aruta, C. Adamo, A. Galdi et al., Phys. Rev. B 80, 140405 (2009);

[2] M. Grioni, M. T. Czyzyk, F. M. F. de Groot, J. C. Fuggle, B. E. Watts, Phys. Rev. 39 4886 (1989);

[3] A. Galdi, C. Aruta, P. Orgiani et al. accepted on PRB

48

High magnetic field electric and thermal transport properties of 2DEG in ZnO

based heterostructures

E.Bellingeria, A. Leveratto

a, A. Gadaleta

a, I.Pallecchi

a, L. Pellegrino

a, D.Marrè

a

A. Jostb, U. Zeitler

b

a CNR-SPIN corso Perrone 24, 16152 Genova, Italy and Dipartimento di Fisica, Via Dodecaneso 33, 16146 Genova,

Italy b High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud

University Nijmegen, 6525 ED Nijmegen, The Netherlands

In recent years, two-dimensional electron gas (2DEG) in wide bandgap semiconductors has attracted

much attention for electrical device applications such as high electron mobilitytransistors (HEMTs) and

quantum spin transport devices. Most of previous studies focused on III–nitride materials, but only a few

attempts were made on II–oxide materials [1,2]. Both materials have wurtzite structure and the

heterostructures are usually grown along the polar caxis direction. At the heterointerfaces, for example

AlxGa1-xN/GaN and MgxZn1_xO/ZnO, a strong built-in potential arises from macroscopic polarization

mismatch and bands align because of different bandgaps in the two layers, causing the formation of 2DEGs

at the interface.

Here we present a characterization of electrical and thermoelectrical transport properties of 2DEG in

MgxZn1_xO/ZnO heterostructures. ZnO and MgxZn1_xO (x=0.05; 0.15) layer were deposited by pulsed laser

deposition on ZnO single crystal on Zn polar side. A

difference of the band gap of about 0.6 eV for the doped and

undoped layer was measured, indicating the possibility to

form a quantum well at the interface.

The realized heterostructures have a carrier concentration of

1012 e-/cm2 and a large mobility (up to 4000 V/cm2s).

Measurements up to 32.5 T of electrical and thermoelectrical

transport properties performed at HMFL (Nijmegen).

Quantum Hall Effect and Shubnikov–de Haas oscillations

confirmed the existence of the 2DEG at the interface and have

been observed at temperatures up to 20 K. Quantum

oscillation were observed, for the first time in a oxide based

2DEG, also in the Seebeck and Nernst effects.

[1] A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno, and

M. Kawasaki, Science 315,1388 2007

[2] A. Tsukazaki, A. Ohtomo, M. Kawasaki, S. Akasaka, H.

Yuji, K. Tamura, K. Nakahara,T. Tanabe, A. Kamisawa, T. Gokmen, J. Shavani, M. Shayegan Physical

Review B 78,233308 (2008).

Fig 1 Quantum Hall Effect and

Shubnikov–de Haas oscillations in

Zn/Zn,mMg based 2DEG

49

The 2DEG at the interface of oxide insulators described by First-Principles: structural, electronic, and transport properties.

A. Filippetti

CNR-IOM UOS Cagliari, c/o Physics Department, University of Cagliari, Monserrato (Ca) I-09042, Italy

Advanced First-Principles (FP) calculations [1] are used to describe the fundamental properties of the n-

doped STO/LAO interface. The key features which are at the fundament of 2DEG formation phenomenon

(film polarity, band disalignment, structural and electronic screening, chemistry of 3d orbitals) are

individually evaluated and discussed.

Mapping the electronic properties of the system as function of the sheet charge density confined at the interface is also instrumental to correctly describe transport and thermoelectric properties. Using a combined FP plus BBT approach including a suited energy-dependence modelling of relaxation time, we have determined electrical conductivity and Seebeck coefficient for the STO/LAO interface, in comparison with n-doped STO bulk. In Fig.1 we report an example of BBT calculation at T=300 K for the STO/LAO interface with n=1/2 electrons per unit interface area, that is the charge required, according to the polar catastrophe model, to fully erase the built-in electric field in LAO. The very satisfying agreement with the available experimental data suggest that our BBT method is reliable and efficient, and can be well applied to generic insulating oxide interfaces.

Our theoretical analysis consent to relate the calculated transport properties to the specific features of the band structure at the interface, in particular on-site and inter-site band splitting, which govern the confinement process. It is found in particular that that the specific characteristics of the band alignment in STO/LAO play against the increase of Seebeck with respect to the 3-dimensional case exemplified by the n-doped STO bulk, in agreement with recently reported experimental results [2]. Receipts and design rules for interfaces with enhanced thermopower will be provided. [1] P. Delugas, et al., Phys. Rev. Lett. 106, 166807 (2011).

[2] I. Pallecchi et al., Phys. Rev. B 81, 085414 (2010).

50

Reentrance of magnetism, induced by the Fe:Ru isovalent substitution in the

SmFeAsO0.85F0.15 optimal superconductor at constant doping

De Renzia. S. Sanna

b, P. Carretta

b, P. Bonfà

a, G. Prando

bcg, G. Allodi

a,,T. Shiroka

d, Lamura

e, A.

Martinellif, and M. Putti

e

a Dipartimento di Fisica, Università di Pavia and CNISM, Pavia, Italy

b Dipartimento di Fisica, Università di Parma and

CNISM, Parma, Italy cDipartimento di Fisica ‘‘E. Amaldi’’, Universita` di oma3-CNISM, I-00146 Roma, Italy

dLaboratorium fuer

Festkoerperphysik, ETH-Hoenggerberg, CH- 8093 Zuerich, Switzerland eCNR-SPIN and Universita` di Genova, via

Dodecaneso 33, I-16146 Genova, Italy fCNR-SPIN Corso Perrone 24, I-16146 Genova, Italy

gIFW Dresden, Institute for Solid State Research, P.O. Box 270116, D-01171 Dresden, Germany

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the

superconducting state in optimally F-doped SmFe1 x uxAsO0.8 F0.1 for 0.1 ≤ x ≤ 0. .

Ru was shown to be diamagnetic and isoelectronic with Fe by DFT calculations. Magnetic order is identified

by means of Zero Field Muon Spin Spectroscopy (ZF-µSR).

Superconducting transitions are measured by SQUID magnetometry. Muons show that the two reduced order

parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a

common critical threshold, xc=0.6. Superconductivity

and magnetism are closely related to two distinct local

sites of the 75

As nucleus in the FeAs layers, that are

identified by means of Nuclear Quadrupole Resonance

(NQR) spectra. These local structures display the same

nuclear relaxations, hence most likely they are in close

contact and share the same electronic environment,

suggesting an analogy with the charge-spin stripes of

cuprates.

The superconducting and the magnetic transition

temperatures are controlled by the Ru substitution, and

scale with the volume fraction of the corresponding

environments. This fact suggests that superconductivity

is assisted by magnetic fluctuations, which become

(partially) frozen whenever a short-range static order

appears, and totally vanish above the magnetic dilution

threshold xc.

[1] S. Sanna et al. Phys. Rev. Lett. 107, 227003 (2011)

51

Evidence of 3D – 2D transition in the magneto flux dynamic of

NdFeAsO1-0.14F0.14.

A. Puri a,b

, A. Marcelli a, A. Iadecola

b, N. Saini

b, A. Bianconi

b, and D. Di Gioacchino

a

a INFN - LNF, Via E. Fermi, 40 - 00044 Frascati, Roma, Italy

b Università Sapienza, P.le A. Moro, 4 – 00100, Roma, Italy

In high Tc superconductors one of the main issues for reliable technological applications is the reduction of

the dissipative phenomena due to the vortex motion inside the sample. Therefore the flux dynamic studies

are important. Actually, AC multi-harmonic susceptibility, especially the third harmonic component, is one

of the best tool for this kind of analysis [1].

In this contribution we present a third harmonic susceptibility dynamic analysis and comparison of

representative Fe-based superconductors: the 1111 iron-pinctide group NdFeAsO1-0.14F0.14 and the 11 iron-

chalcogenides FeSe0.88, FeSe0.5Te0.5, FeSe0.25Te0.75, FeTe0.8S0.2.

From the Irreversibility Line (IL) study, using the vortex-glass phase transition approach, we have estimated

the dimensionality of the vortex dynamic in these systems. We have found a 3D to 2D crossover in dynamic

behavior of the NdFeAsO1-0.14F0.14 at Hdc = 1 T, while a 3D behavior for all the systems in a low DC field

was found and good bulk pinning characteristics. In particular the NdOFeAs in the range Hdc ≤ 1 T is always

characterized by a stronger pinning force respect to the 11 iron-chalcogenide group, hence it offers the

possibility to sustain higher critical currents.

We associate its stronger pinning force to the rare earth-oxide spacer layer, which is not present in the

structure of the 11 iron chalcogenides. This plane could give different pinning contributions connected to the

stress on FeAs superconducting layers, or due to the effect Re ions on the flux cores, over imposed to the

weak collective contribution due to the dopant F atoms.

[1] D. Di Gioacchino, A. Marcelli, A. Puri and A. Bianconi, J. Phys. Chem. Solids 71, 1046 (2010).

52

Pressure effect on the magnetic and superconducting properties

of REFeAsO1-xFx (RE=Sm, Ce, La)

S. Sanna,a R. Khasanov,

b G. Prando,

a P. Carretta,

a A. Palenzona,

c M. Tropeano,

c M. Putti,

c N. D.

Zhigadlo,d S. Katrych,

d J. Karpinski,

d Z. Shermadini,

b M. Bendele,

c A. Amato,

c and R. De Renzi

e

a Dipartimento di Fisica A. Volta, Università di Pavia, Via Bassi, 6, I-27100 Pavia, Italy.

b Laboratory for muon spectroscop, PSI, CH-5232 Villigen PSI, Switzerland

c University of Genova and SPIN-CNR Corso Perrone 24,I- 16152 Genova – Italy

d Laboratory for Solid State Physics, ETH Zurich, CH-8093 Zurich, Switzerland

e Dipartimento di Fisica, Università di Parma, Viale G.Usberti, 7A, I-43100 Parma, Italy

Apart from chemical doping, the electronic properties of the Fe-based compounds can also be tuned by

applying an external pressure (P). In particular, it has been shown that LaFeAsO1-xFx (La1111) system, for

0<x<0.15, displays a significant increase of Tc with P already at P<2 GPa [1]. Indeed, the Tc variation upon

pressure for the RE1111 family is found to be RE dependent, showing a minor effect for the case of RE=Sm

[2].

We report on the effect of external pressure on the magnetic properties of REFeAsO1-xFx with RE=La,

Ce, Sm. The microscopic magnetic behaviour of the undoped (x=0) and slightly doped (x~0.04-0.07)

samples has been investigated up to P20 kbar by using the SR technique.

The external pressure modifies the microscopic magnetic properties and the effect is generally rare earth

dependent and the following features are observed:

- The staggered magnetization Ms is diminished for all the REFeAsO samples. The -dMs/dP slope

increases with the RE ionic radius, from La to Sm.

- The reduction of Tm vs. P is the highest for RE=La, sizeable for RE=Ce and marginal for RE=Sm,

whatever the F content is.

The RE dependent behaviour sounds surprisingly since no specific difference of both staggered

magnetization and magnetic transition have been observed at ambient pressure in the pure undoped RE=La,

Sm compounds [3].

The interplay between magnetism and superconductivity in LaFeAsO0.945F0.055 was studied as a function

of hydrostatic pressure up to P = 2.4 GPa by means of muon-spin rotation (μSR) and magnetization

measurements [4]. The application of pressure leads to a substantial decrease of the magnetic ordering

temperature, reduction of the magnetic phase volume and, at the same time, to a strong increase of the

superconducting transition temperature and the diamagnetic susceptibility. From the volume-sensitive μSR

measurements it can be concluded that the superconducting and the magnetic areas, coexisting in the same

sample, are inclined toward spatial separation and compete for phase volume as a function of pressure.

[1] H.Okada et al., J.Phys.Soc.Jap. 77, 113712 (2008)

[2] Wei Yi et al., EPL 83, 57002 (2008)

[3] H. Maeter et al., Phys. Rev. B 80, 094524 (2009)

[4] R. Khasanov et al., Phys. Rev. B 84, 100501(R) (2011)

53

Field effect experiments with electrochemical gating in metallic and

superconducting films

A. Sola, K. Sharda, D. Daghero, G.A. Ummarino, R. S. Gonnelli

Department of Applied Science and Technology, Politecnico di Torino, 10129 Torino, Italy

With respect to the standard FET-like technique with solid dielectric, the polymeric gating technique [1,2]

has allowed a substantial increase (by one order of magnitude) in the maximum surface density of injected

charge achieved in field-effect experiments. This improvement is due to the formation of an electric double

layer (EDL) at the interface between the polymeric electrolyte and the surface of the material under study.

The EDL acts as a parallel-plate capacitor with an extremely small distance between the plates (of the order

of the polymer molecule size) and thus a very large capacitance. The polymeric gating technique has allowed

inducing superconductivity in SrTiO3 [3], ZrNCl [4] and KTaO3 [5]. Here we show that a suitable polymeric

electrolyte solution (PES) allows extending the previous record of surface charge density (4.5 x 1014

cm-2

) up

to about 3.5 x 1015

cm-2

. In Au thin films, such a huge density of carriers results in a 10% change of the film

resistance at low temperature [6]. Moreover, the same technique applied to thin Pb films gives rise to a

reproducible shift in the transition temperature Tc, whose magnitude is in good agreement with that expected

within the Eliashberg theory by using the electron-phonon coupling constant of lead and assuming for

simplicity a free-electron density of states.

[1] M. J. Panzer, C. R. Newman, and C. D. Frisbie, Appl. Phys. Lett. 86, 103503 (2005).

[2] A. S. Dhoot et al., Proc. Natl. Acad. Sci. U.S.A. 103, 11 834 (2006)

[3] K. Ueno et al., Nature Mater. 7, 855 (2008).

[4] J. T. Ye et al., Nature Mater. 9, 125 (2010).

[5] K. Ueno et al., Nature Nanotech. 6, 408 (2011).

[6] D. Daghero et al., Phys. Rev. Lett. 108, 066807 (2012).

54

Poster Contributions

55

Laser heating of HTSC substrates: Challenges, problems, and solutions

W.Steina, T. Heeg

a

aSURFACE systems+technology GmbH & Co. KG, Hueckelhoven, Germany

The thermal preparation of substrates during advanced oxide deposition processes requires a flexible and

precisely controlled heater. In such processes employing high oxygen partial pressures the usual heater

materials are very limited and introduce restrictions of the vacuum range or of the temperature. Platinum and

silicon carbide are the major heater materials if a conventional radiation or contact heater for temperatures up

to 1000-1100 °C is used.

Over the past 20 years applications of laser heating were published in the field of thin film deposition, using

CO2- [1-3], YAG- [4,5], as well as diode [6] lasers. Laser light of high intensity, well focused only to the

substrate, is an extremely vacuum friendly tool, especially for the ultra-high vacuum range of applications.

The heated zone in the UHV chamber is limited to the substrate only and thus has the smallest possible size,

minimizing outgassing and enabling the lowest background pressures and very clean processes.

Using light to transfer energy to the sample requires consideration of the optical properties of the used

substrate, because absorption of the laser light is essential for an effective heating process. YAG and diode

lasers work in the near infrared region. Many substrate materials commonly used for oxide thin film

deposition are partially or fully transparent at near-infrared wavelengths, so direct illumination of the

substrate with these lasers is not efficient. Several more or less acceptable workarounds of this basic problem

have been developed, but partially foil the advantages of laser heating. In addition, problems with these

workarounds are increasing with higher temperatures, so the step to another wavelength is recommended. A

CO2-laser, with 10.6 µm wavelength, offers a good solution for the absorption problem – but the handling of

this light source is not as convenient as that of the frequently used diode laser. This presentation compares

diode- and CO2-laser heater versions, shows solutions for the existing difficulties, and finally demonstrates

results of the use of such heaters in oxide film deposition.

[1] P.E. Dyer, A. Issa, P.H. Key, P. Monk, Supercond. Sci. Technol. 3, 472 (1990)

[2] K.H. Wu, C.L. Lee, J.Y. Juang, T.M. Uen, Y.S. Gou, Appl. Phys. Lett. 58, 1089 (1991)

[3] S.J. Barrington, R.W. Eason, Rev. Sci. Instrum. 71, 4223 (2000)

[4] S. Ohashi, M. Lippmaa, N. Nakagawa, H. Nagasawa, H. Koinuma, M. Kawasaki, Rev. Sci.

Instrum. 70, 178 (1999)

[5] H. Lippmaa, T. Furumochi, S. Ohashi, M. Kawasaki, H. Koinuma, T. Satoh, T. Ishida, H.

Nagasawa, Rev. Sci. Instrum. 72, 1755 (2001)

[6] W. Stein, MRS spring meeting 2006, GG13.6 (Laser Heating, a challenging new Technology for

small Substrates in Oxide Deposition Processes)

56

MgxZn1-xO/ZnO heterostructures: realization and high magnetic field

characterization

Leverattoa,b

, E. Bellingeria, E. Gottardo

a,b, A. Gadaleta

a,b, I. Pallecchi

b, L. Pellegrino

b, Marré

a,b, A.

Jostc, U. Zeitler

c

aCNR-SPIN corso Perrone 24, 16152 Genova, Italy

bDipartimento di Fisica, Via Dodecaneso 33, 16146 Genova, Italy

cHigh Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University Nijmegen, 6525 ED

Nijmegen, The Netherlands

In recent years, two-dimensional electron gases (2DEG) in oxide heterostructures have attracted a lot of

attention. Among the heterostructures investigated only ZnO based multilayers unambiguously showed the

presence of a 2DEG at the interface [1,2]. Such heterostructures resemble to GaAs or GaN based systems; at

the heterointerfaces MgxZn1-xO/ZnO a strong built-in potential arises from macroscopic polarization

mismatch and bands align because of different bandgaps in the two layers (∼0.6 eV), causing the formation

of a potential well at the interface where a 2DEGs is confined. With respect to GaAs or GaN

heterostructures, being ZnO a multifunctional materials with interesting opto-magneto-electronic properties,

the realization of ZnO based quantum devices could lead to significant technological developments. In this poster we present the realization of ZnO and MgxZn1-xO (x=0.5; 0.15) heterostructures deposited by

pulsed laser deposition on ZnO single crystal on Zn polar side. In detail we will discuss the optimization of

substrate treatments and the optimization of deposition parameters necessary to obtain high quality films

growth. The films were in-situ monitored by RHEED and routinely checked ex-situ by AFM, and XRD

measures. The introduction of a low temperature buffer layer before the heterostructure effective layers

resulted a mandatory step in order to produce samples with a smooth interface allowing the formation of a

macroscopically connected high mobility 2DEG. A patterning process was optimized and 50x10 m Hall

bars were obtained by a phosphoric acid (H3PO4) ‘wet etching’ process. An experiment at HFML (High Field Magnet Laboratory - Nijmegen) gave us the possibility of

measurements in field up to 32T and temperature down to 350mK of electric and thermal transport

properties. Quantum phenomena (IQHE and SdHO) were surprisingly observed at temperatures up to 20K.

Moreover thermoelectric power experiments showed for the first time in an oxide based 2DEG quantum

oscillations in the Seebeck and Nerst thermoelectric coefficients. A comparison between both samples, patterned e non-patterned, leads to a study of the different quantum

effects observed in transport measures. [1] A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno, and M. Kawasaki, Science 315, 1388 (2007) [2] A. Tsukazaki, A. Ohtomo, M. Kawasaki, S. Akasaka, H. Yuji, K. Tamura, K. Nakahara, T. Tanabe,

A. Kamisawa, T. Gokmen, J. Shavani, M. Shayegan Physical Review B 78, 233308 (2008).

57

Investigation of resistive switching behavior and nanoscale electronic transport

of Au/Nb-doped SrTiO3 junctions

A. Gerbi a, R. Buzio

a, A. Gadaleta

a,b, L. Anghinolfi

a, F. Bisio

a, E. Bellingeri

a, A.S. Siri

a,b, D.

Marré a,b

a CNR-SPIN Institute for Superconductivity, Innovative Materials and Devices, C.so Perrone 24, 16152 Genova, Italy

b Physics Department, University of Genova, Via Dodecaneso 33, 16146 Genova, Italy

Whenever a Schottky barrier is formed at the junction between large work function metals and electron-

doped SrTiO3 (STO) samples, the macroscopic rectifying transport is accompanied by a resistance switching

(RS) behavior. This is likely due to local, field-induced accumulation or depletion of oxygen vacancies at the

vicinity of the metal/STO interface, that in turn might lead to redox processes responsible for the appearance

of resistance switching [1]. Metal/Nb-doped STO junctions represent a model system for the elucidation of

the physical mechanisms driving RS. Several investigations recently addressed the RS dependence on few

intrinsic and extrinsic physical parameters. There is no doubt that a deeper insight was gained when the

macroscale studies were complemented by the use of local probes, addressing junctions response at the

nanoscale [2].

Here we describe the fabrication and electrical characterization of Au/Nb:STO single-crystal junctions with

nanometer thick metal electrodes. We observe an unexpected phenomenon, namely the coexistence within

the same device of highly rectifying properties - under laboratory air - and bipolar RS - under reducing

vacuum conditions. We explore this phenomenology by systematically studying the junction response under

different oxygen and inert gas atmospheres [3].

Since nanometer-scale alterations of the Schottky barrier represent one of the microscopic mechanisms

proposed to explain RS, we report on novel Scanning Tunnelling Microscopy - Ballistic Electron Emission

Microscopy (STM-BEEM) experiments aimed to directly visualize and quantify the local inhomogeneities of

the effective Schottky barrier height. This approach represents an original strategy to attempt a correlation

with the macroscopic response of the studied system [4].

[1] R. Waser et al. Adv. Mater. 21, 2632 (2009).

[2] K. Szot et al. Nat. Materials 5, 312 (2006).

[3] R. Buzio, A. Gerbi, A. Gadaleta et al. “Oxygen dependence of carrier transport and resistive-switching in

Au/Nb:SrTiO3 Schottky junctions” in preparation

[4] A. Gerbi, R. Buzio, A. Gadaleta et al. “Hot electron transport in Au/Nb:SrTiO3 structure studied by

ballistic electron emission spectroscopy” in preparation

58

Mobility enhancement by UV irradiation in polar/non-polar oxide

heterointerfaces

A. Gadaletaa, C. Aruta

b, E. Di Gennaro

b, F. Miletto Granozio

b, I. Pallecchi

a, M. Riaz

b, U. Scotti di

Ucciob, A. S. Siri

a, D. Marré

a

aDipartimento di Fisica, Università degli Studi di Genova, Via Dodecaneso 33, 16146 Genova, Italy and CNR-SPIN,

Corso Perrone 24, 16152 Genova Italy bDipartimento di Scienze Fisiche, Università di Napoli “Federico II” and CN -SPIN,

Complesso Universitario di Monte Sant’Angelo, Via Cinthia, 80126 Napoli, Italy

In order to clarify the complex physics of SrTiO3/LaAlO3 interfaces, and to unveil the origin of the 2-

dimensional electron layer, a deeper understanding of conduction mechanisms at the interface is needed. The

recent discovery1,2

of novel polar/non-polar oxide heterointerfaces with similar properties, i.e.

SrTiO3/LaGaO3 and SrTiO3/NdGaO3, can represent a significant step forward in this direction. Recent

experimental3 and theoretical

4 insights suggest the possibility of multi-band conduction at the interface,

caused by strong 2-dimensional localization of charge carriers at each TiO2 atomic layer.

In this work we performed Hall effect and resistance measurements, from 2.5 K to 320 K and up to 9 T, on

STO/LAO, STO/LGO and STO/NGO oxide interfaces. The polar oxide films were 12 u.c. thick and were

deposited by PLD in 10-3

mbar O2, with or without a post-annealing treatment. We show that the transport

properties of all three heterointerfaces can be tuned by UV irradiation, displaying a significant

photoconductivity at low temperature, which is larger in more resistive samples. We find that this effect is

not caused by a strong excitation of charge carriers. Instead, an enhancement of Hall mobility at low

temperature is found, similarly to the case of back-gate electric field effect, together with a counterintuitive

decrease of the single-channel effective carrier density. We explain these findings by multi-band conduction

in the 2D electron liquid at the interface, showing that a simple model, including multiple bands with

different electron mobilites, can reproduce the experimental results.

[1] P. Perna et al., Appl. Phys. Lett. 97, 152111 (2010)

[2] U. Scotti di Uccio et al., submitted to Appl. Phys. Lett.

[3] T. Fix et al., Phys. Rev. Lett. 103, 166802 (2009)

[4] P. Delugas et al., Phys. Rev. Lett. 106, 166807 (2011)

59

Atomic resolution by scanning tunnelling microscopy on ultrathin

La0.7Sr0.3MnO3 films

A. Gambardella a

, P. Graziosi a, I. Bergenti

a, Mirko Prezioso

a, F. Biscarini

a and V. Dediu

a

a CNR - ISMN, Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati, v. Gobetti 101,

40129, Bologna, Italy

Nanoscale investigations of hole-doped manganites are nowadays strongly desirable, since the exotic

properties of these highly correlated systems arise from the coexistence of microscopic effects [1]. Insulating

ultrathin La0.7Sr0.3MnO3 (LSMO) films are studied at room temperature by Scanning Tunnelling Microscopy-

Spectroscopy (STM-S) techniques, for the first time with atomic resolution. We mapped the sample surface

with tunnel conductance curves and topographies, by evidencing large atomically-flat surface regions with

quite homogeneous spectroscopic behaviour. Nevertheless, we observed that atomic contrast is obtained at

some surface locations only. These findings were previousely proposed as markers for trapped polarons [2]

on bulk samples whose surfaces were optimized in cleanliness and flatness. We justify our results by the

presence of local intrinsic inhomogeneities, that would act on a homogeneously conducting surface as charge

density reliever, by allowing or participating to atomic contrast in a tunnelling measurement. Furthermore,

our findings suggest that the effect of strain is to induce the manganite to a transition from weak- to strong-

electron-phonon coupling regimes, making fully-strained films as interesting model-systems to investigate

the nature of the electron correlation directly at nanoscale lengths, opening interesting tehoretical and

technological issues, the latter with a view in the optimization of surfaces and interfaces to be employed in

the fabrication of hybrid innovative devices, as sensors and spin-valves.

Figure 1 a) Unit cell observed at 1.2V and 120 pA at RT on a 5 cells –thick LSMO deposited on SrTiO3 substrates;

atomic resolution, together with strong lattice distortions is observed on larger regions as in the image b). c) Resistance

vs Temperature plot showing the lack of the Metal-Insulator transition under about 6nm of thickness. d) Tunnelling

Spectroscopic I(V) and dI/dV curves showing homogeneous conductivity.

[1] E. Dagotto, T. Hotta and A. Moreo, Phys. Rep. 344, Issue 1-3, p. 1-153 (2001).

[2] H. M. RØnnow, et al., Nature (London) 440, 1025 (2006)

60

Magnetic tunneling junctions for biomolecular detection

E. Albisettia, D. Petti

a, R.Bertacco

a

(a)L-NESS Center, Department of Physics of Politecnico di Milano and CNISM – via Anzani, 42 22100 Como, Italy

Early stage diagnostics is becoming one of the major goals of medicine. This relies on the development of

highly sensitive, low cost and portable diagnostic tools allowing for a wide screening of patients, involving

point of care diagnosis closer to citizen. Magnetic labels in combination with spintronic biosensors offer the

opportunity to combine sensitivities in the zeptomolar range with fast performances, integrability and

portability.

Magnetic biosensors were made possible by the fast development of devices based on physical effects that

relate an electrical resistance to the external magnetic field, namely the giant magnetoresistance (GMR) and

the tunnelling magnetoresistance (TMR) [1]. MTJs are based on two ferromagnetic electrodes separated by

an insulating barrier. Due to the spin-dependent tunneling process, the electrical resistance of the junctions

depends on the relative orientation of the electrodes' magnetization direction. Extremely high TMR values at

room temperature are achieved exploiting the coherent tunneling through crystalline fcc (100) MgO barriers

when transition metal ferromagnetic electrodes are used (CoFe, CoFeB, Fe).

For this reason, MgO based MTJs are a viable biosensing tool, provided that one takes into account that their

performances, i.e. TMR value and linearity of the sensor response curve, are strongly influenced by the

growth conditions which need to be optimized and controlled properly for each material.

In this work, magnetic tunneling junctions (MTJs) have been

deposited in a high vacuum magnetron sputtering system. The stack

structure is (thickness in Angstrom) the following:

Si/SiO2(1000)/Ta(50)/Ru(180)/Ta(30)/IrMn(200)/CoFe(20)/Ru(9)/C

oFeB(30)/MgO(20)/CoFeB(30)/Ru(50)/Ta(50). The layers

roughness has been optimized by checking the surface topology by

Atomic Force Measurements; the correct MgO layer texturation,

which is crucial for enhancing the TMR value, was investigated by

X-Ray diffraction measurements.

Devices with rectangular shape and dimensions of 2.5 x 120 mm2

have been fabricated by optolithography and Cr/Au contacts were e-

beam evaporated [2]. For achieving the desired linear sensor

response (Figure 1), the bottom CoFeB layer has its magnetization

pinned in one direction by the sinthetic antiferromagnet (SAF)

structure. The top CoFeB layer has a linear magnetization response

with no hysteresis; this feature is achieved combining the control on

the magnetocrystalline anisotropy direction in CoFeB during growth,

the shape anisotropy, and the thickness of the layer.

[1] Butler W. H., Zhang X.-G. et al. Phys. Rev. B, 63, 054416 (2001).

[2] Donolato M. et al. Appl. Phys. Lett., 98, 073702 (2011).

Figure 1: Top panel: linear and

hysteresis-free sensor response with

respect to external magnetic field.

Bottom panel: schematic of the chip

with 8 MTJ sensors.

61

Infrared phonon activity and Fano interference in multilayer graphenes

E. Cappellutia,b

, L. Benfattob, A.B. Kuzmenko

c

a Instituto de Ciencias de Materiales de Madrid, CSIC, Cantoblanco, Madrid, Spain

b Istituto dei Sistemi Complessi, CNR, Roma, Italy

b DPMC, Université de Genève, 1211, Genève, Switzerland

The detection and analysis of the spectral properties of optical phonon in single-layer and multilayer

graphene provides a powerful tool not only for a careful characterization of the systems but also for

investigating the role of the underlying electron-phonon interaction. Recent experiments in gated bilayer

graphene revealed a clear phonon resonance at 1590 cm-1

with several interesting features, as for instance a

giant enhancement of the phonon intensity as a function of the gate voltage as well as a pronounced Fano

lineshape asymmetry [1,2].

In this contribution we will discuss how these features can be analyzed and predicted on a microscopic

quantitative level using a charge-phonon theory applied to the specific case of graphene systems. We show in

particular how the phonon intensity and the Fano asymmetry are strictly related, stemming out from the

quantum interference between the electronic and phononic degrees of freedom [3]. Within this context we

are also able to elucidate the relative role of the Eu and Eg phonon modes in regards to the infrared activity

and the Fano asymmetry of the observed phonon peaks. We present thus a complete phase diagram for the

strength of the phonon modes and their Fano properties as functions of the chemical potential and of the

gated-induced electronic gap, showing that a switching mechanism between the dominance of the Eu or Eg

mode can be controlled by the external gate voltage [3].

We discuss how the present analysis can be generalized as well to multilayer systems with different stacking

order [4]. Such controlled quantitative theory can provide thus an useful roadmap for the characterization of

graphenic systems by optical infrared means.

[1] A.B. Kuzmenko, L. Benfatto, E. Cappelluti, I. Crassee, D. van der Marel, P. Blake, K.S. Novoselov,

and A.K. Geim, Phys. Rev. Lett. 103, 116804 (2009).

[2] T.-Ta Tang, Y. Zhang, Ch.-H. Park, B. Geng, C. Girit, Z. Hao, M.C. Martin, A. Zettl, M.F. Crommie,

S.G. Louie, Y.R. Shen and F. Wang, Nat. Nanotech. 5, 32 (2010).

[3] E. Cappelluti, L. Benfatto, and A.B. Kuzmenko, Phys. Rev. B 82, 041402 (2010).

[4] Z.Q. Li, C.H. Lui, E. Cappelluti, L. Benfatto, K.F. Mak, G.L. Carr, J. Shan, and T.F. Heinz, Phys. Rev.

Lett. 2012 (in press),

62

Defects associated with the ultrathin La2/3Sr1/3MnO3/SrTiO3

A Positron Annihilation Spectroscopy Study

F. Moia, R. Ferragut, G. Radaelli, R. Bertacco

LNESS, Dipartimento di Fisica, Politecnico di Milano, via Anzani 42, I-22100, Como, Italy

The ferromagnetic metallic oxide La2/3Sr1/3MnO3 (LSMO) is a system with very complex and rich

physics, whose electronic and magnetic properties may be altered by applying various perturbations such as

electric and magnetic fields, strain, light, etc. [1]. The high degree of spin polarization and colossal

magnetoresistance make manganese perovskites, in particular, the optimally doped ferromagnetic manganite

LSMO, appealing ingredients for spintronic devices [2]. Such devices are based on heterostructures that

involve contact of thin manganite LSMO layers with other material as SrTiO3 (STO) and, in general,

conductors, superconductors, organic materials, etc.

The positron annihilation depth profiling technique is an established method for investigating ultrathin

layers. In the present work, we used this technique to investigate vacancy-like and interface defects in

LSMO/STO heterostructures. When a positron is implanted into condensed matter, it annihilates with an

electron and emits two 511 keV -rays. The energy spectrum of the annihilation -rays is broadened due to

the Doppler effect associated with the momentum component of the annihilating electron–positron pair.

Coincidence Doppler broadening was used to obtain information of the average chemical composition in the

annihilation region. Other parameter sensitive to trapping is the positron lifetime of positrons, which is

longer when positrons are trapped by vacancy-like defects because of the reduced electron density in such

defects. Positron annihilation spectroscopy measurements can thus help to identify the existing defects and to

obtain quantitative information on their density.

The results presented here suggest that vacancy-like defects mainly associated with oxygen are the

dominant positron traps in perovskite oxide heterostructures. LSMO single-crystals films were grown by

pulsed laser deposition on a STO commercial substrate with different growth conditions. It was possible to

identify the optimum growth condition to minimize the vacancy concentration into the film. We have also

observed large vacancy clusters with an average dimension of about 6 Å into the perovskite oxide [3].

[1] C. H. Ahn et al. Rev. Mod. Phys. 78, 1185 (2006).

[2] W. Prellier et al. J. Phys.: Condens. Matter 13, R915 (2001).

[3] R. Ferragut, A. Dupasquier, S. Brivio, R. Bertacco, W. Egger. J. Appl. Phys. 110, 053511 (2011).

rafael.ferragut@polimi.it

63

Magnetic Proximity effect as a pathway to spintronic applications of topological

insulators

I. Vobornik a, U. Manju

b, J. Fujii

a, F. Borgatti

c, P. Torelli

a, D. Krizmancic

a,

Y.S. Hord, R.J. Cava

d and G. Panaccione

a

aIOM-CNR, TASC Laboratory, Trieste, Italy

bInternational Center for Theoretical Physics (ICTP), Trieste, Italy

cISMN-CNR, Bologna, Italy

dDepartment of Chemistry, Princeton University, Princeton, NJ, USA

Spin-based electronics in topological insulators (TIs) is highly favored by the long spin coherence

originating from the topologically protected surface spin environment [1,2] and consequently fault-tolerant

information storage. The control of the magnetic properties of TIs was realized by the inclusion of magnetic

metal impurities (i.e. Mn in Bi2-xMnxTe3): the sdoped material is ferromagnetic up to 13 K [3,4], well

below any practical operating condition. Here we report on the magnetic proximity effect driving the

impurity-doped TIs interface ferromagnetic up to room temperature [5]. Our X-ray magnetic circular

dichroism experiments clearly evidence that the long range ferromagnetism on the Mn atoms in Bi2-

xMnxTe3 is induced by the magnetic Fe overlayer, opening a new path to interface controlled

ferromagnetism in novel TI-based spintronic devices.

[1] M.Z. Hasan, C.L. Kane, Rev. Mod. Phys. 82 (2010) 3045 [2] J.E. Moore, Nature 464 (2010) 194 [3] Y.S. Hor et al., Phys. Rev. B 81 (2010) 195203 [4] D. Hsieh et al, Phys. Rev. Lett. 103 (2009) 146401 [5] I. Vobornik et al., Nano Lett., 2011, 11 (10), 4079

64

Superconducting heterostructure with hybrid magnetic interlayer

A.E. Sheyermana, Yu.V. Kislinskii

a, A.V. Shadrin

a,b, G.A. Ovsyannikov

a,b,

K.Y. Constantiniana, and A. Kalabukhov

b

a Kotel’nikov Institute of adio Engineering and Electronics, ussian

Academy of Sciences, Moscow, Russia b Department of Microtechnology and Nanoscience – MC2, Chalmers

University of Technology, Gothenburg, Sweden We present experimental data on multilayer superconducting heterostructure with hybrid interlayer

exhibiting Josephson effect at mm wave frequencies. The base electrode of heterostructure was oxide d-wave

superconductor YBa2Cu3O7-δ with thickness 190 nm grown epitaxially on NdGaO3 substrate by means of

laser ablation. Then 8-9 nm SrRuO3 and 6 nm thick La0.7Sr0.3MnO3 layers were in situ deposited over the

base electrode. The top electrode was Nb/Au bilayer (Au - 20 nm). The hybrid interlayer consists of La0.7Sr0.3

MnO3 and SrRuO3 ferromagnetic films that have different directions of magnetization. The I-V curve

measured at T=4.2 K showed no hysteresis for a heterostructure with in-plane size 10x10 m2; critical

current IC=88 A and normal resistance RN=0.16 Ω results in critical frequency fC=(2e/h)ICRN=6.8GHz.

Oscillating critical current and Shapiro steps with power of applied microwaves were registered. At fe= 41

GHz (ratio fe/fC=6 very well corresponds to high frequency limit) maximum of the first Shapiro step was as

large as I1=94 A. Half-integer Shapiro step was observed as well, and the maximal amplitude was I1/2 =15

A. Taking into account the negligible small impact of heterostructure capacitance on high frequency

dynamics this apparently hints at non-sinusoidal current-phase relation. Obtained results are discussed in

terms of possible generation of long-range spin triplet superconducting current component at the interfaces

of singlet superconductors in contacts with ferromagnetic bi-layer consisting of ferromagnetic materials with

different spatial directions of magnetization.

The work was supported by Russian Academy of Sciences, President Grant MK-5266.2011.2, Scientific

School Grant 2456.2012.2, RFBR project 11-02-01234а, and VISBI Program of Russian-Swedish

collaboration.

65

Superconductivity in electron-doped infinite-layer cuprate Sr1-xLaxCuO2 thin

films deposited by shuttered molecular beam epitaxy

L. Maritatoa,b,c

, P. Orgianib, A. Galdi

a,b, J.M. Harter

d, D.G. Schlom

c

a Università di Salerno, Via Ponte don Melillo, 84084 Fisciano (SA), Italy

b CNR-SPIN, Unità Operativa di Salerno, Via Ponte don Melillo, 84084 Fisciano (SA), Italy

c Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA

d Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853,

USA

We report the growth of IL epitaxial Sr1-xLaxCuO2 thin films on GdScO3 (100) substrates with

superconducting critical temperatures as high as 35 K by using shuttered MBE technique. The ACuO2

“Infinite Layer” (IL) compounds, where the CuO2 sheets, with four-fold Cu atoms coordinated by oxygen,

are separated by Ca and/or Sr cations (A), have the simplest structure at the base of all the presently known

high-temperature superconducting cuprates. To obtain good superconducting IL films, the use of suitable

substrates is critical. In particular, while compressive in-plane epitaxial strain (as in the case of SrTiO3

substrates) generally gives IL films with inferior superconducting performances, in-plane tensile strain IL

superconducting films with critical temperatures close to the bulk value and with low and metallic

resistivities. The produced films have been structurally characterized by X Ray diffraction analysis and their

transport properties have been measured down to low temperatures (10 K) and in the presence of external

magnetic fields. In situ performed ARPES analysis have shown interesting behaviors with a strong influence

of the antiferromagnetic fluctuations on the superconducting order parameter even for optimally doped

samples. The availability of this technique to fabricate n-doped superconducting IL films, opens the way to

future studies on the interface effects in heterostructures based on this class of materials.

66

Metal-to-superconductor transition in two-dimensional electron systems:

mesoscopic disorder and intrinsic charge instability from Rashba coupling

S. Caprara, M. Grilli, D. Bucheli, F. Peronaci

Physics Department, University of ome “Sapienza”, ome, Italy

Motivated by experiments in thin films and oxide interfaces, we first propose a random-resistor network to

describe the occurrence of a metal-to-superconductor transition in a two-dimensional electron system with

disorder on the mesoscopic scale [1]. Disorder induces a distribution of local superconducting critical

temperatures. With lowering the temperature, global superconductivity establishes as soon as percolation

occurs within the superconducting cluster. We explore the interplay of the statistical distribution of local

critical temperatures and of the occurrence of a lower-dimensional (e.g., fractal) structure of the

superconducting cluster embedded in the two-dimensional system [2].

Then, to explain the systematic occurrence of mesoscopically disordered regions, we model the electron gas

at the interface of superconducting oxide heterostructures considering a twodimensional electron gas in the

presence of a Rashba spin-orbit coupling. Under simple general assumptions we show that a phase separation

instability occurs for realistic values of the spin-orbit coupling and of the band parameters [3]. This could

provide an intrinsic mechanism for the recently observed inhomogenous phases at the LaAlO3/SrTiO3 or

LaTiO3/SrTiO3 interfaces.

[1] S. Caprara, M. Grilli, L. Benfatto, and C. Castellani, Phys. Rev. B 84, 014514 (2011).

[2] D. Bucheli, S. Caprara, C. Castellani, and M. Grilli, submitted to Phys. Rev. B

[3] S. Caprara, F. Peronaci, and M. Grilli, submitted to Phys. Rev. Lett.

67

Optical conductivity of Bismuth-based topological insulators

Di Pietro 1, D. Nicoletti

2, F. M. Vitucci

1, L. Baldassarre

3, P. Calvani

1, R. Cava

4, S. Hor

4 and S.

Lupi 1

1Department of Physics, University of Rome La Sapienza, Piazzale A. Moro 2, I-00185 Roma, Italy

2Max Planck Research Department for Structural Dynamics Center for Free Electron Laser Science &

University of Hamburg, Notkestrasse 85 - 22607 Hamburg, Germany 3Sincrotrone Trieste, Area Science Park, I-34012 Trieste, Italy

4Department of Chemistry, Princeton University, New Jersey 08544, U.S.A.

Topological Insulators (TI) are new quantum materials with a bulk insulating-gap of spin-orbit origin and 2-

dimensional metallic states at the surface. These states are characterized by a Dirac dispersion and strikingly

stable against disorder, since they are topologically-protected via time reversal symmetry from

backscattering by a chiral spin texture [1-3]. Such properties make TI good materials for several applications

in quantum computing, photonics, and spintronic devices [4-6].

TI belonging to the V2VI3 (V= Bi, Sb, S; VI = Se, Te, S) family recently emerged as the first candidates for

the study of topological surface states thanks to their large bulk insulating gap (~300 meV).

However the as-grown crystals usually show an extrinsic degenerate semiconducting behavior, due the

unstoichiometry related to the growing process, emerging in the presence of some impurities. These

impurities affect the low energy transport properties, rendering difficult the separation between the intrinsic

2D metallic behavior of the topological surface states and the 3D metallic conduction induced by extrinsic

charges.

In this talk I will discuss the optical conductivity σ1(ω) and the optical spectral weight SW of four

topological insulators with an increasing chemical compensation (Bi2Se3, Bi2-xCaxSe3 with x=0.0002,

Bi2Se2Te, Bi2Te2Se), in order to identify the most compensated sample to detect the maximum contribution

of the surface states vs that one of extrinsic carriers.

The effect of compensation is clearly visible in the infrared spectra, through the suppression of the extrinsic

Drude term and the appearance of strong peaks in the terahertz range, assigned to electronic transitions

among impurity states. The SW of the most compensated sample (Bi2Te2Se) is still nearly two orders of

magnitude higher than that expected from surface states [7-8].

[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).

[2] C. L. Kane, and E. J. Mele, Phys. Rev. Lett. 95 226801 (2005).

[3] J. E. Moore, Nature 464, 194 (2010).

[4] L. Fu, and G. P. Collins, Sci. Am. 294, 57 (2006).

[5] A. Kitaev and J. Preskill Phys. Rev. Lett. 96, 110404 (2006).

[6] Y. L. Chen et al., Science 325, 178 (2009).

[7] R. Valdes Aguilar et al., arXiv:1105.0237v3 (2011).

[8] N. Bansal et al., arxiv:1104.5709 (2011).

68

Electrical spin injection in all-oxides crystalline heterostructures.

F.Telesioa,b

, E. Espositoc, I. Pallecchi

b, L. Pellegrino

b, D. Marré

a,b

a Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy

b CNR-SPIN, Corso Perrone 24, 16152, Genova, Italy

c Istituto di Cibernetica E. Caianiello del CNR, Via Campi Flegrei 34, I-80078 Pozzuoli (NA), Italy

The search for a suitable semiconducting non magnetic material for spin injection devices is so far still

unaddressed. The crucial role of interfaces in spin depolarization has been partially understood only recently,

in the case of electrical spin injection from ferromagnetic electrodes. Indeed, such issues as conductivity

mismatch, electron wavefunction symmetry, band alignment, presence of disorder and traps may severely

suppress the spin polarization of injected charge carriers. In this context, the realization of fully crystalline

devices with epitaxial ferromagnetic electrodes and active channels may yield high quality interfaces and

thus enhanced spintronic performances. In this work we present pioneering experiments of spin injection in

fully crystalline oxide heterostrutures, either in vertical or planar spin valve geometries. The ferromagnetic

electrodes are made of perovskite manganites, while for non-equilibrium spin transport we test

semiconducting Cu2O and SrTiO3. These prototype devices have the two-fold aim of measuring the spin

diffusion length in Cu2O and SrTiO3, as well as of testing the actual spintronic device feasibility.

69

Mechanical detection of phase transitions in transition metal oxides thin films

N. Manca1,2

, L. Pellegrino2, R. Buzio

2, A. S. Siri

1,2, D. Marre

1,2, T. Kanki

3, H. Tanaka

3

1Dipartimento di Fisica, Università di Genova, Genova (Italy)

2CNR-SPIN, Genova (Italy)

3ISIR, Osaka University, Osaka,(Japan)

Correlated electron systems like Transition Metal Oxides (TMO) exhibit a wide range of physical

phenomena thanks to the interplay between spin, charge and lattice degrees of freedom. TMO have broad

lattice compatibility and can be grown in complex heterostructures having high structural quality. TMO

heterostructures show also enhanced or even new properties that arise from the mechanical interactions

between the single layers or by electrical or magnetic couplings.

Phase transitions in these systems often determine multiple changes of their physical properties including

mechanical ones. In this work, we present a study on the detection of mechanical resonances of

microcantilevers fully made with TMO heterostructures, whose fabrication combines Pulsed Laser

Deposition (PLD) and micrometric optical lithography. The mechanical resonance frequency of these

microcantilevers changes during phase transitions. As an example, across its well known Metal Insulator

Transition at 68 C the Young’s modulus of VO2 exhibits a sharp change of its value, affecting the

resonance frequency of the microstructure. Furthermore, the magnetic transition occurring in manganites

nearby room temperature can be mechanically detected by monitoring shifts of the resonance frequency due

to the different coupling with an external magnetic field.

We will report the fabrication process of TMO microcantilevers and details of the experimental setup based

on optical lever detection. Our system is able to measure shifts of the resonance frequency of about 1 Hz in

different atmospheres (vacuum, gas) and from room temperature to 100°C. This frequency shift method is

based on the same principles of Frequency Modulation Atomic Force Microscopy and allows very precise

detection of external forces and bulk properties. Preliminary measurements on (La,Sr)MnO3 and VO2/TiO2

free-standing cantilevers across their phase transition will be shown and discussed.

70

Skewness in the current-phase relation of double-barrier Josephson junctions

R. De Luca

Dipartimento di Fisica “E. . Caianiello”, Università degli Studi di Salerno, Fisciano, Italy

Skewness in the current-phase relation of a SISIS superconducting structure (where S stands for

superconducting layer and I for insulating interface) has been predicted by De Luca and Romeo [1] by means

of a semi-classical approach adopted for the first time by Ohta in 1976 [2]. Very recently, a similar

behaviour has been experimentally detected in Superconductor-graphene-Superconductor (SgS) Josephson

junction (JJ) by a direct interferometric method [3]. In the case of graphene-based JJs, however, appropriate

analyses are needed [4], due to the peculiarities of the interstitial electrode. Nevertheless, the tentative to

formally define skewness in ref. [3] is useful also for identifying the behaviour of SISIS double-barrier

Josephson junctions (DBJJ). In fact, deviations from the sinusoidal single-junction behaviour in the CPR of

DBJJs depends essentially on two parameters [5]. The first parameter 0

21

2I

II represents the semi-

difference between the maximum Josephson currents, 1

I and 2

I , normalized to their average value 0

I , of

the two single junctions of the DBJJ; the second

parameter, , represents the normalized

superconducting coupling strength between the

outer electrodes of the SISIS structure.

In the present work, adopting the definition

given in ref. [3] of the skewness S of the curves

representing the CPR of composite or

unconventional junctions, we find an analytical

expression for S in DBJJ in terms of and ,

by considering very small. A detailed

dependence of the maximum Josephson current

of the device on and is also found.

[1] R. De Luca and F. Romeo, Phys. Rev. B 79

(2009) 094516.

[2] H. Ohta, IC- SQUID 76 (1976) 35.

[3] C. Chialvo, J. C. Moraru, D. J. Van Harlinger, N. Mason, Current-phase relation of graphene Josephson

junctions, arXiv:1005.2630v2 [cond-mat.supr-con].

[4] M. Titov, C. W. J. Beenakker, Phys. Rev. B 74 (2006) 041401.

[5] R. De Luca, Physica C 470 (2010) 487.

Figure 1: Current-phase relation of a DBJJ for 01.0 .

From the top full-line curve to the bottom dotted-line

curve, the parameter is as follows: 1.0 , 2.0 ,

3.0 , 4.0 .

71

Optical spin orientation in Ge probed by the spin filtering in Fe/MgO/Ge

photodiodes

C. Rinaldi1*

, M. Cantoni1, M. Marangoni

2, G. Cerullo

2, R. Bertacco

1

1 CNISM and L-NESS - Politecnico di Milano, Via Anzani 42, 22100 Como, Italy

2 Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Como, Italy

Spin-polarized carriers can be photo-excited in a semiconductor by monochromatic circularly polarized

light. To date, several techniques have been introduced to study the optical degree of spin polarization (DSP)

of holes and electrons. In this work, the spin filtering effect at the Fe/MgO interface has been employed both

to evaluate the DSP in Ge and to study the efficiency of spin-photodiodes versus wavelength.

We have fabricated photodiodes by means of optical lithography, starting from fully epitaxial

Fe/MgO/Ge(001) heterostructures [1,2]. Spin-detection experiments are performed by illuminating

photodiodes with circularly polarized light in a magnetic field applied parallel to the light helicity. Photo-

generated carriers in Ge are spin-filtered by the MgO barrier depending on the direction of their spin with

respect to the magnetization of Fe, giving rise to a photocurrent related to the DSP.

The efficiency of photodiodes in distinguishing between right and left circularly polarized light results

from the competition between two wavelength-dependent parameters: (i) the initial DSP created by circularly

polarized light; (ii) the attenuation length Latt of light

in the semiconductor compared to the spin-flip length.

Rioux and Sipe have calculated the DSP as a function

of the wavelength [3]: it is maximum when the photon

energy is resonant with the direct energy band-gap of

Ge and decreases when the photon energy increases.

On the contrary, the reduction of Latt with the photon

energy increases the efficiency of the photodiodes. The

competition between DSP and Latt defines the optimal

wavelength range for using such photodiodes as

detectors of light helicity.

A theoretical model based on Fert-Jaffrès theory

[4] has been developed to calculate the behavior of the

efficiency. As shown in figure, the agreement between

model prediction (solid line) and experiment (dots) for both electrons and holes confirms the calculation of

Rioux and Sipe and also validate our model.

Although the scientific community has always expected the maximum spin filtering at the band-gap, we

demonstrated that the competition between DSP and absorption length could even favor higher photon

energies. To conclude, we found that Fe/MgO/Ge spin-photodiodes are suitable for integrated detection of

light helicity at room temperature in novel spin-optoelectronics systems working in a wide spectral range,

from about 540 nm to 1500 nm.

[1] M. Cantoni et al., Appl. Phys. Lett. 98, 032104 (2011).

[2] D. Petti et al., J. Appl. Phys. 109, 084909 (2011).

[3] J. Rioux and J. E. Sipe, Phys. Rev. B 81, 155215 (2010).

[4] A. Fert and H. Jaffrès, Phys. Rev. B 64, 184420 (2001).

[*] christian.rinaldi@mail.polimi.it

600 900 1200 1500

0

5

10

Calculation for electrons

Experiment for electrons

Effic

iency (

%)

Wavelength (nm)

Calculation for holes

Experiment for holes

72

MgO-based spin photodiodes on Ge: growth and characterization

M. Cantoni*, C. Rinaldi, D. Petti, R. Bertacco

CNISM and L-NESS - Politecnico di Milano, Via Anzani 42, 22100 Como, Italy

Spin-optoelectronics exploits the coupling between the angular momentum of photons and the spin

angular momentum to obtain a connection between optical and spintronic devices, adding a new degree of

freedom (the photon helicity) to integrated optical communication systems. A

very promising structure is the fully epitaxial Fe/MgO/Ge(001)

heterostructure. Ge has recently attracted great attention, thanks to the large

carrier mobility and the opportunity of both spin optical pumping and electrical

spin manipulation The insertion of a thin epitaxial MgO tunnel barrier between

Fe and Ge overcomes the problem of conductivity mismatch and exploits the

efficient spin filtering effect at the MgO/Fe interface: spin polarized photo-

carriers, coming from Ge illuminated by circularly polarized light, can be

selectively filtered, depending on their spin orientation, in order to produce a

spin-dependent (and thus helicity-dependent via the optical selection rules in

Ge) electrical signal.

In this contribution we report on the realization of epitaxial Fe/MgO/Ge(001) heterostructures by

Molecular Beam Epitaxy [1]. The lowest oxidation and highest sharpness of the MgO/Ge interface is

obtained for room temperature growth of MgO followed by annealing in a vacuum at 500°C. The MgO layer

grows epitaxially on Ge(001) with the [110] direction parallel to the [100] direction of Ge, at variance with

the cube-on-cube growth on Si(001) and GaAs(001), as revealed by transmission electron microscopy and

diffraction techniques (XPD, RHEED) [2]. For the Fe overlayer, room temperature growth followed by

annealing up to 200 °C gives rise to a sharp interface and the well-known 45° rotation of the Fe lattice with

respect to the MgO lattice. In Figure is reported a TEM image of a Fe/MgO/Ge heterostructure, showing the

very good epitaxial quality of the stack.

Spin-photodiodes have been realized starting from the Fe/MgO/Ge heterostructure by means of optical

lithography, ion milling and e-beam evaporation. Transport and photocurrent measurements, with both CW

and pulsed laser light, have been performed in order to characterize the device and its temporal response.

Spin detection experiments have finally been performed by illuminating spin-photodiodes with left or right

circularly polarized light in a magnetic field applied parallel to the light helicity. We found that efficient spin

transport of carriers photo-generated in Ge occurs at room temperature, with an helicity-dependent electrical

signal mainly due to spin filtering of carriers across the MgO layer [3] and only partially to Fe dichroism;

moreover, we found a strong dependence of the spin filtering efficiency on the MgO layer thickness, that we

explained by means of a theoretical model based on Fert-Jaffrès theory [4].

[1] M. Cantoni et al., Appl. Phys. Lett. 98, 032104 (2011).

[2] D. Petti et al., J. Appl. Phys. 109, 084909 (2011).

[3] Rinaldi et al., submitted to Adv. Mat.

[4] A. Fert and H. Jaffrès, Phys. Rev. B 64, 184420 (2001).

[*] matteo.cantoni@polimi.it

Fe

MgO

Ge

1 nmFe

MgO

Ge

1 nm

73

Development of chemical vapor- and atomic layer-deposition methods for

spintronic applications

R. Mantovan a

, S. Vangelista a

, B. Kutrzeba-Kotowska a

, A. Lamperti a

, S. Cocco a

, and

M. Fanciullia,b

.

a Laboratorio MDM, CNR-IMM, Via C. Olivetti 2 - 20864 Agrate Brianza, Italy

b Scienza dei Materiali, Università Milano-Bicocca, Milano, Italy

Despite the great potentiality offered by chemical methods in providing efficient, cost effective and

large-area depositions of a wide range of thin films, the employment of chemical routes for the synthesis of

thin films and multilayer stacks for spintronic applications have been scarcely attempted so far. In particular,

very few reports are available on the use of chemical vapor- and atomic layer- deposition (ALD/CVD) of

ferromagnetic (FM) thin films and Magnetic Tunnel Junction (MTJs) stacks [1,2].

We developed a versatile deposition system which manages the synthesis of “standard” 3d-FMs (Fe, Co)

and “half metals” (Fe3O4) by CVD, and MgO thin films by means of pure ALD process [3]. By combining

CVD and ALD process performed in situ without any vacuum-break, a wide range of FM/oxide and

FM1/oxide/FM2 multilayered stacks can be synthesized [3,5,6]. The structural, chemical, morphological and

magnetotransport characterization of single layers and multistacks produced in the CVD/ALD reactor will be

reported. In particular, we will present the possibility of performing the CVD of pure Fe3O4 showing

reasonable magnetoresistance (MR) performances (-2/-5 %), comparable to those observed in films

synthesized by more conventional techniques like magnetron sputtering [4,6]. The functionality of as

deposited MTJs with MgO barrier and Fe, Co, and Fe3O4 FM electrodes has been demonstrated by showing

non-zero tunnel magnetoresistance (TMR) values. Even if displaying limited TMR (fraction of percent), our

results show the feasibility of the proposed CVD/ALD process for the synthesis of functional simple MTJs.

On the other hand, in junctions with FeOx-based FM electrodes and Ta/TaOx contacts with large area

(hundreds μm2) extremely large TMR (up to 200 %) has been observed. This enhanced TMR can be

explained as a non-uniform perpendicular-to-plane current distribution occurring in high-resistance junction

electrodes, and potential employment of the phenomenon for applications has been recently proposed [7].

FM/oxide stacks (i.e. Co/MgO) showing sharp interfaces with negligible chemical mixing can be synthesized

in the developed CVD/ALD reactor, and our work towards the synthesis of simple devices for exploiting

spin injection into Si will be presented.

The developed CVD/ALD system opens the possibility for a cost-effective synthesis on large areas of a

wide range of FM, FM/oxide, and FM/oxide/FM thin films and structures, potentially targeting a range of

different spintronic applications.

[1] R. Bubber et al., IEEE Trans. Magn. 38(5), 2724 (2002).

[2] C. A. M. Knechten, Ph. D Thesis, Eindhoven University of Technology, 2005.

[3] R. Mantovan et al., Thin Solid Films (2011), doi:10.1016/j.tsf.2011.08.037.

[4] R. Mantovan et al., J. Phys. D: Appl. Phys. 42, 065002 (2010).

[5] S. Vangelista et al., Thin Solid Films (2011), doi: 10.1016/j.tsf. 2011.10.128.

[6] R. Mantovan et al., J. Appl. Phys, 111, 07B107 (2012).

[7] M. Opel et al., Phys. Status Solidi A 208, 232 (2011).

74

Synthesis of Au/NiO/Au nanowire arrays for ReRAM applications

D. Peregoa, S. Franz

a*, M. Bestetti

a, S. Brivio

b, G. Tallarida

b, S. Spiga

b

a Dipartimento di Chimica, Materiali ed Ing. Chimica “G. Natta”, Politecnico di Milano, Via Mancinelli 7 – 20131

Milano (Italy) b Laboratorio MDM, IMM-CNR, Via C. Olivetti 2 – 20864 Agrate Brianza (Italy)

The Resistive switching Random Access non-volatile Memories (ReRAM) are considered among the most

promising candidates for future high density data storage. ReRAM memories are based on Metal-Insulator-

Metal (MIM) heterojunctions which show electrical switching between high and low conductive states under

applied electrical pulses. So far, one of the most studied insulating layer is Nickel Oxide (NiO), giving

Metal-Oxide-Metal (MOM) heterojunctions.

Whereas the resistive switching behaviour of MOM thin films has been widely investigated, a full

comprehension of the phenomenon has not been achieved yet and to this aim a downscaling of the devices is

needed. Furthermore nanoscaled Metal/Oxide/Metal (MOM) heterostructures can be considered possible

building blocks for such memory devices.

In this work, an innovative approach is described to synthesize MOM nanowire (NWs) arrays with

measurable electrical properties likely down to the single nanowire.

MOM NW arrays were fabricated by electrodeposition of either Ni or Au/Ni/Au multilayers into Anodic

Aluminum Oxide (AAO) templates (pore diameter 50 nm).

The electrodeposition of Ni and Au was carried out from sulphate-based and cyanide-based electrolytes,

respectively. The thickness of the Ni layer was controlled down to 20 nm. Mechanical and chemical

polishing of the AAO templates was performed after the electrochemical synthesis of metallic NWs in order

to expose them from the AAO template surface. Thermal oxidation was carried out without removing the

AAO template, at 400°C in oxygen atmosphere for times ranging from 3 to 30 minutes, as well as at 300°C

in air for 6 hours.

Ni/NiO nanowire arrays embedded in the AAO templates, with NiO layers in the 20-200 nm thickness range,

were obtained by tailoring the annealing times. On the other hand Au/ NiO/Au NW arrays with NiO ranging

from 20 to 200 nm were obtained by controlling the Ni deposition times (Figure (a)). The AFM image shows

Au/NiO/Au NWs stick out of the AAO surface for about 20 nm (Figure (b)), and can be directly

characterized by conductive AFM (C-AFM).

Structure and morphology of the heterojunction NWs characterized by Scanning Electron Microscopy

(SEM), High Resolution Transmission Electron Microscopy (HRTEM), X-Ray Diffraction (XRD) and

Atomic Force Microscopy (AFM). Electrical properties were studied by C-AFM.

This work was partially supported by Fondazione Cariplo (MORE Project n°2009-2711).

(a) (b) (c)

Figure (a) SEM cross section of Au/100nm NiO/Au NW arrays embedded in AAO template; (b) AFM scan and

(c) surface profile of Au/NiO/Au NW array embedded into the AAO template.

Corresponding author: silvia.franz@polimi.it

75

Spin mixing, spin triplet proximity effects and spin polarisation: Can CrO2

continue to surprise?

K.A. Yates a

, L.F. Cohen a

, M.S. Anwarb, J. Aarts

b

a Physics Department, Blackett Laboratory, Imperial College London, London, UK, SW7 2AZ

b Physics Department, University of Leiden

The behavior of superconductor-ferromagnet (S-F) interfaces has been intensively studied in recent years. Of

particular interest is the detection of a long range spin triplet proximity effect (LRSTPE) whereby even fully

spin polarized materials appear to support a proximity induced supercurrent when in contact with a

superconducting layer [1,2]. Point contact Andreev reflection spectroscopy (PCAR) measures the electrical

conductance of a contact as a function of bias across single S-F interfaces. We have recently studied the spin

spiral Ho and have seen unusual characteristics in the PCAR spectra as a function of tip pressure [3]. The

challenge is to clarify what the signature of a proximity induced triplet behavior might be. A new

theoretical model, applicable to highly spin polarized systems such as CrO2, has been proposed that suggests

trends in the current-voltage (IV) characteristic and temperature dependence of the conductance curves that

should be observed in the presence of the strong spin mixing necessary for LRSTPE generation [4]. Here we

review data on CrO2 films grown on sapphire [5] and on new films grown onto TiO2 to examine whether the

experimental data is sufficiently exacting (thermal and non-thermal smearing allowing) to provide a

definitive answer as to whether these signatures of spin mixing have been observed.

[1] M.S. Anwar, F. Czeschka, M. Hesselberth et al., Phys Rev B, 82, 100501 (2010)

[2] J.W.A Robinson, J.D.S. Witt, M. G. Blamire, Science, 329, 59 (2010)

[3] ITM Usman, KA Yates, JD Moore et al., Phys Rev B, 83, 144518 (2011), Phys Rev B, 84,

139904(E) (2011)

[4] T Lofwander, R. Grein, M. Eschrig, Phys Rev Lett., 105, 207001 (2010)

[5] K.A Yates, WR Branford, F Magnus et al., Appl Phys Lett., 91, 172504 (2007)

76

The Magnetocaloric Effect: Prospect for Manganites

J.Turcaud,a K.Morrison,

a A.Berenov,

b K.G. Sandeman,

a & L.F. Cohen*

a

aThe Blackett Laboratory, Imperial College, London SW7 2BZ, UK

b Department of Materials, Imperial College, London, SW7 2BZ, UK

The magnetocaloric effect is the change in temperature of a material caused by an applied magnetic field,

and is largest when a phase transition is induced, particularly a first order phase transition. The

magnetocaloric effect has been highlighted as a potential system for efficient and environmentally friendly

solid state cooling. Manganites are a potential magnetocaloric material family although they only exhibit a

modest temperature change in 0-1 Tesla [1], and it is an open question whether compositions such as

La0.67Ca0.33MnO3 show evidence of first order transition behaviour [2]. Here we will review the desirable

characteristic a magnetocaloric material requires to make it attractive for application and examine whether

there are prospects for the use of manganites in this regard. We will report on the routes we have explored to

control thermal management in La0.67Ca0.33MnO3 by incorporation of silver and MgO and work to date to

explore the order of the phase transition.

Acknowledgements: This work is supported by the UK EPSRC grant no EP/G060940/1

[1] Manh-Huong Phan and Seong-Cho Yu, Journal of Magnetism and Magnetic Materials 308 (2007) 325–

340

[2] K. Morrison et al., 2010 MRS Fall Meeting Symposium Proceedings

77

Defining the first order phase transition

K. Morrison,a A. Berenov,

b J. Turcaud,

a A.D. Caplin,

a & L.F. Cohen*

a

aThe Blackett Laboratory, Imperial College, London SW7 2BZ, UK

b Department of Materials, Imperial College, London, SW7 2BZ, UK

For application as a magnetic refrigerant the ideal material should possess a tunable Tc, and large

values of ∆S and ∆T. This is most easily achieved in systems that exhibit a first order phase

transition, which is why they are often the most explored. But how can we reliably define a first order phase

transition? The manganites are a promising material system with regards to durability, low field

performance, and cost, but it remains to be seen how they can be optimized. If we can understand how to

drive a phase transition towards first order behavior then we are one step closer to developing a

better understanding of how to tailor Tc and maintain the entropy change.

It is common practice to use the Banerjee criterion to assess a new material, where if a negative

gradient is seen in Arrott plots the phase transition is considered first order. This method is unreliable should

there be disorder broadening of the phase transition, and we cannot discount the impact of spin

fluctuations.[1],[2]

We discuss the impact of both these effects on the manifestation of the phase transition

with respect to magnetization and heat capacity data and demonstrate a new calorimetric technique to

identify first order behaviour should the latent heat be distributed in field.[3]

Finally an alternative criterion

will be suggested.

Figure 1 – Arrott plots for two manganite systems, La2/3Ca1/3MnO3 and RMn1.05O3 , where R is a

combination of rare earth elements, ‘mischmetal’. At some temperatures there are negative slopes in both

systems, but no hysteresis was observed for La2/3Ca1/3MnO3.[4]

The curve closest to Tc is highlighted in red.

[1] J.B. Goodenough, J.-S. Zhou, Chem. Mater., 10, 2980-2993 (1998)

[2] M. Shimizu, Proc. Phys. Soc. 84, 397 (1964)

[3] K. Morrison, M. Bratko, J. Turcaud, A. Berenov, D.A. Caplin, and L.F. Cohen, Rev. Sci. Instrum.,

83, 033901 (2012)

[4] K. Morrison, A. Berenov, and L.F. Cohen, Mater. Res. Soc. Symp. Proc.,1310, mrsf10-1310-ff02-05

(2010)

RxMn1.05O3 La2/3Ca1/3MnO3

78

Low frequency noise in La0.7Sr0.3MnO3 thin films on various substrates at 300 K

S. Wua , B. Guillet

a , S. Liu

a, C. Adamo

b, D.G. Schlom

b,c, L. Méchin

a , and

J.M. Routourea

a GREYC (UMR 6072) – CNRS - ENSICAEN - Université de Caen Basse-Normandie, 6 Bd Maréchal Juin, 14050

Caen Cedex, France b Department of Materials Science and Engineering, Cornell University

Ithaca, NY 14853-1501, USA c Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA

Manganite oxides are promising for next generation devices and sensors [1-3]. In the case of

La0.7Sr0.3MnO3 (LSMO), it has been demonstrated that high resolution bolometers can be realized [4]. A

systematic study of the impact of length, width, thickness, substrate and geometries on the low frequency

(LF) noise level is necessary for future sensor development.

We report measurements of LF noise at 300 K performed in LSMO thin films of various thicknesses

(5 nm up to 200 nm), patterned in the form of lines of various lengths (50 μm to 300 μm) and widths (20 μm

to 400 μm), and deposited on various substrates: SrTiO3 (001), SrTiO3 (110), vicinal SrTiO3 (001), LSAT

(001), buffered silicon (001). For each film, the spatial homogeneity was checked by electrical resistivity

measurements.

Magnetization measurements versus temperature have revealed a Curie temperature close to 350 K

and the temperature of maximal resistance sensitivity was found to be about 390 K. These values are very

close to the bulk ones proving the good crystallinity of all the tested thin films.

The noise measurements have been performed using a 4-probe setup. For each geometry, the film LF

noise was extracted for different values of the DC voltage across the device. We found that these spectral

densities vary as the square of the DC voltage across the device, thus following the Hooge relation [5]:

f

K

V

S2

V

, where the K parameter, also called the normalized Hooge parameter /n can be used as a

figure of merit to quantify the quality of the films. Normalized Hooge parameters as low as to 10-30

m3

(among the lowest reported values in the literature) have been measured.

Some details about the measurements of the noise level will be presented. It will also be shown that

the LF contact resistance noise is not negligible and some discussions will be given. The systematic study of

the impact of the geometry and of the substrate will then be presented and discussed. It will also be shown

that the intrinsic noise level depends on the LSMO film thickness.

[1] M. Bibes and A. Barthélémy, IEEE Trans. Electron Dev. 54, 1003 (2007)

[2] F. Yang et al. , J. Appl. Phys. 99(10) (2006)

[3] L. Méchin et al., Appl. Phys. Lett. 87 204103 (2005)

[4] S. Liu et al. this conference

[5] F. N. Hooge, Phys. Lett. 29A, 139 (1969)

79

Rigorous results on the Hubbard model with spin orbit coupling: a lattice gauge

theory approach

G. Guarnaccia C. Noce

Dipartimento di Fisica ``E.R. Caianiello'', Università di Salerno, I-84084 Fisciano (Salerno), Italy

We study the symmetry properties of the Hubbard model with spin-orbit interactions of Rashba and

Dresselhaus type. Since these interactions break the rotational symmetry in spin space, the magnetic order

cannot be excluded by using the Bogoliubov inequality method. Nevertheless, we rigorously show that the

existence of the magnetic long-range orders may be ruled out when the Rashba and Dresselhaus coupling

constant are the same, whereas the -pairing can be always ruled out, regardless of the microscopic

parameters of the model. These results are obtained by imposing locally the SU(2) gauge symmetry on

lattice, and rewriting the spin-orbit interactions in such a way that they are included in the path ordered of the

gauge field on lattice. When the coupling constants of Rashba and Dresselhaus interactions are the same, the

SU(2) global invariance is restored allowing for the use of the Bogoliubov inequality and for the derivation

of upper bounds for the order parameters.

80

Effective electronic Hamiltonian for a dimer with on- and inter-site electron-

phonon interactions: a non-perturbative evaluation of renormalized electronic

interactions.

M. Acquarone a,b

a Physics Department, Università di Parma, Parma, Italy

b IMEM-CNR, Parma, Italy

In the SC cuprates both electronic correlation and phonons play a role. However, assessing their respective

relevance, and the reciprocal renormalization effect, is still a subject of hot debate. In this context, a

simplified model which can be exactly solved analytically may be of help in orienting more realistic

investigations.

We therefore study a toy model for the basic unit of SC cuprates, consisting in a dimer where one or two

fermions occupy a non-degenerate band. Besides interacting through all admissible fermionic one- and two-

body interactions, the fermions interact also with both on-site (“à la Holstein” ) and inter-site (“à la Su-

Schrieffer-Heeger”) harmonic phonons, mimicking , respectively, the apical out-of-plane, and inter-plaquette

in-plane, Cu-O vibrations. Assuming arbitrary but comparable values for the two electron-phonon

interactions (EPI) implies the need of treating them on equal footing. This we do by means of unitary

transformations, for both Fermi and Bose operators, exactly summed to all orders in the EPI’s amplitudes.

The resulting transformed operators are highly non-linear functions of the phononic quantities. Next, the

transformed Hamiltonian is averaged over squeezed phonons yielding the effective electronic Hamiltonian

with fully renormalized electronic interactions. A variational minimization of the ground state energy with

respect to the free parameters characterizing the unitary transformations yields the optimal values for the

renormalized electronic interaction parameters, and the physical value of the ground state energy.

81

Electric-field induced structural transition in Bi1-xLaxFeO3 ceramics

D.V. Karpinsky, A.L. Kholkin

CICECO/Department of Ceramics and Glass Engineering, University of Aveiro, 3810-193 Aveiro, Portugal

Remarkable physical properties of the BiFeO3-based multiferroic solid solutions near the

morphotropic phase boundaries (MPB) attract significant attention in the last years [1, 2]. Increased electric and magnetic susceptibilities estimated for the compositions in the vicinity of the MPB significantly improve magnetoelectric (ME) coupling within the compounds [3]. At the same time the ME coupling within one structural state based on intrinsic mechanisms result in weak ME coefficients and remain unpromising for practical applications. We used a new approach to improve multiferroic properties and magnetoelectric coupling based on recently revealed electric-field induced structural transitions in BiFeO3-based films.

Ceramics of Bi1-x LaxFeO3 systems (0<x<0.5) were synthesized from stoichiometric mixtures of high purity oxides. The crystal structure investigations carried out using X-ray diffraction measurements (XRD) at room temperature testify rhombohedrally distorted structure (polar R3c space group) for x≤0.15. The structure of the compounds in compositional range 0.16<x<18 has been refined assuming a coexistence of polar (R3c) and antipolar (Pnam) phases. Above a concentration x=0.18 an incommensurately modulated antipolar orthorhombic phase (Pnam) was detected from XRD data.

Piezoresponse force microscopy (PFM) was used to estimate evolution of electromechanical properties across the rhombohedral-orthorhombic phase boundary. PFM measurements were performed in different parts of various granules in order to diminish an effect of grain orientation. Well saturated local piezoresponse hysteresis loops were obtained for parent BiFeO3 and for the compounds with x≤0.15. It should be noted that the piezoelectric coefficient d33

eff estimated for the x= 0.15 compound

testified three-fold increase in comparison with those for initial BiFeO3. The PFM loop obtained for the compounds in two-phase structural region are characterized by step-like behavior of the d 33

eff dependence. We suggest that the anomalous local

hysteresis loops can be caused by electric-field driven transition from orthorhombic antipolar to rhombohedral polar structural state. Assuming effective volume of the sample subjected to the electric field during PFM loop measurements the local electric field can reach 500kV/cm that is in accordance with theoretically calculated energy difference between these structural states [4].

Magnetic field dependencies obtained at room temperature testify significant increased remanent magnetization for the compounds in orthorhombic phase. An increased magnetization can be explained considering changes in the location of oxygen and iron ions. An estimated decrease in Fe – O bond length promotes larger spin canting and subsequent remanent magnetization increase. The magnetic anomaly concomitant with rhombohedral to orthorhombic transition in Bi1-xLaxFeO3 solid solutions testifies a close correlation between magnetic and crystal structures and opens up new possibilities to increase magnetoelectric properties of the BiFeO3-based materials.

[1] G. Catalan and J. F. Scott, Adv. Mater. 21, 2463 (2009). [2] S. Fujino et al. Appl. Phys. Lett. 92, 202904 (2008).

[3] M. Fiebig, J. Phys. D: Appl. Phys. 38, R123 (2005).

[4] D. Kan et al. Adv. Funct. Mater. 20, 1108 (2010).

82

Influence of oxygen vacancies on the magnetoelectric coupling in the Fe/BaTiO3

interfaces

E. Plekhanov and S. Picozzi

CNR-SPIN, L'Aquila, Italy

Fe/BaTiO3 interfaces are a promising candidate for the spintronics and, in particular, for design of future

non-volatile random-access memory (NVRAM) devices. The possibility to control the magnetization

electrically through the magnetoelectric coupling (MEC) on ferromagnetic-ferroelectric interface is currently

under intense investigation [1-2]. The success in realization of such futuristic devices depends to a great

extent on the control over the impurities. Indeed, in pure BaTiO3, the impurities are know to spoil the

ferroelectricity by causing, among others, the polarization fatigue – the decrease of switchable polarization

during normal operation conditions. In addition, many proposed mechanisms for MEC in heterostructures

rely heavily on subtle energy balance and would surely suffer from the presence of impurities. On the other

hand, the oxygen vacancy redistribution is thought to be one of the main mechanisms responsible for the

polarization fatigue [3]. For the practical realization of MEC interfaces it is important to know how robust

the effect is with respect to the presence of the impurities and, in the first place, of the oxygen vacancies. In

the present work we analyze the influence of various types of oxygen vacancies on the MEC in a

prototypical Fe/BaTiO3 heterostructure by means of the Density Functional Theory (DFT) ab-initio

calculations.

[1] Chun-Gang Duan, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 97, 047201 (2006)

[2] K. Yamauchi, B. Sanyal, and S. Picozzi, App. Phys. Lett. 91, 062506 (2007)

[3] M. Dawber and J. F. Scott, Appl. Phys. Lett. 76, 1060 (2000)

83

Mechanism of ferroelectricity in d3 perovskites: a model study

P. Baronea*

, S. Kanungob, S. Picozzi

a and T. Saha-Dasgupta

b

aCN SPIN, I67100 L’A uila, Italy

bS. N. Bose National Centre for Basic Sciences, Sector III, Block JD, Salt Lake, Kolkata 700 098, India

Since the revival of interest in the magnetoelectric effect due to the possibility ofmultiferrocity with

the simultaneous presence of ferroelectric and magnetic order, manyattempts have been made to

create multiferroic materials focusing on the wide variety oftransition-metal (TM) ABO3

perovskites. The classical ferroelectrics as well as manymagnetic materials belongto this class of

compounds. However, overlap betweenferroelectric and magnetic perovskites is rare, leading to an

empirical “exclusion” rule,according to which ferroelectricity occurs for d 0 B-site TM cations,

while magnetismrequires d n B-site cations with n ≠ 0. This mutual exclusion of ferroelectricity and

magnetismis rationalized by the cases of conventional ferroelectric perovskites, for which the

drivingforce behind the ferroelectric distortion comes from the tendency of TM empty d states

toestablish a strong covalency with the surrounding oxygens. The covalency-driven mechanismis

maximally efficient when the antibonding states, with a predominant TM d character, arecompletely

empty, hence for d 0 B-site perovskites, but does not vanishes completely forpartially occupied d

shells. Since the ferroelectric instability arises due to the delicate balancebetween the covalency

energy gain and the repulsive electrostatic energy, the generalapplicability of the “exclusion

principle” for a d n perovskite with n < 5 is difficult to predicta priori.

By means of a model Hamiltonian approach combined with DFT calculations we study therole of

volume expansion, Hund’s coupling, and electron correlation in the covalency-drivenmechanism for

ferroelectricity in cubic CaMn O3, a prototypical magnetic non- d 0perovskite[1]. In general,

alkaline-earth manganites are possible candidates, since Mn is in a d 3electronic configuration with

electrons occupying, within a cubic crystal field, thethreefold degenerate t2g states, leaving the eg

states unoccupied. Our results establish that theferroelectric instability arises from a subtle balance

between different energy contributions,explaining the origin of its enhancement under negative

pressure that had been previouslytheoretically predicted[2]. An expansion of the volume is found to

cause a strong reduction inthe elastic energy, while leaving almost unchanged the tendency of Mn

states to formcovalent bonds with the surrounding oxygens. On the other hand, Hund’s coupling

with localspins of magnetic cations can reduce a d even suppress the instability toward the

ferroelectricstate.

[1]P. Barone, S. Kanungo, S. Picozzi and T. Saha-Dasgupta, Phys. Rev. B 84, 134101(2011).

[2]S. Bhattacharjee, E. Bousquet, and Ph. Ghosez, Phys. Rev. Lett. 102, 117602 (2009).

84

Effect of Pr and Mn (co)substitution on crystal structure and magnetic

properties of BiFeO3 multiferroic

V. A. Khomchenko a

, I. O. Troyanchuk b

, M. Kopcewicz c, J. A. Paixão

a

a CEMDRX/Department of Physics, University of Coimbra, Coimbra, Portugal

b SSPA “Scientific-Practical Materials esearch Centre of NAS of Belarus”, Minsk, Belarus

c Institute of Electronic Materials Technology, Warsaw, Poland

BiFeO3 remans the only material with the proven multiferroic behavior far above the room temperature. The

compound crystallizes in the noncentrosymmetric rhombohedral structure (S.G. R3c) compatible with the

polarization directed along the main hexagonal axis. Magnetic moments of Fe3+

ions form the cycloidal-

modulated G-type antiferromagnetic structure. Possibility to tune and control the multiferroic behavior of

BiFeO3 via a chemical substitution motivated numerous investigations of the related solid solutions. In this

communication, we are reporting on crystal structure and magnetic properties of the Bi1-xPrxFe1-yMnyO3 (x,

y≤0.3) compounds at room temperature. In the parent Bi1-xPrxFeO3 series, a rhombohedral (R3c) and two

orthorhombic (Pnam and Pnma) structural modifications were found. The R3c phase is stable at 0≤x<0.12.

The homogeneous Pnam structure is realized at 0.16≤x≤0.2 . Single-phase compounds with the Pnma

structure can be obtained at x≥0.28. In the intermediate compositional ranges, a two-phase structural state is

formed. The rhombohedral compounds possess the main antiferromagnetic phase and exhibit a metamagnetic

behavior associated with the field-induced removal of the spatially-modulated antiferromagnetic structure;

decrease of the threshold field of the magnetic phase transition and appearance of the small remnant

magnetization are observed with increasing Pr content. The orthorhombic compounds with the Pnam and

Pnma structures are weak ferromagnetic and demonstrate the spontaneous magnetization close to that

releasing upon the field-induced suppression of the spin cycloid in the rhombohedral phase. Mn doping

effectively modifies the initial R3c and Pnam structures to stabilize new phase demonstrating

incommensurable structural modulation. Magnetic behavior of the Mn-containing samples changes in

correlation with the evolution of their structural state. Within compositional range of the rhombohedrally

distorted compounds, the manganese substitution gives rise to suppression of the dominant antiferromagnetic

interaction. In the weak ferromagnetic Mn doping-induced phase, a gradual decrease of room temperature

spontaneous magnetization takes place with increasing Mn content.

85

Multiferroicity and Magnetoelectricity in a Metal-Organic Framework

A. Stroppaa, P.Jain

b, P. Barone

a, M. Marsman

c,

J.M. Perez-Matod, A. K. Cheetham

e, H. W. Kroto

b, S. Picozzi

a

aCNR-SPIN. L'Aquila, Italy

b Department of Chemistry and Biochemistry, Florida State University

Tallahassee, FL 32306 (USA)

cUniversity of Vienna, Faculty of Physics and Center for

Computational Materials Science (Austria)

dDepartamento de Fisica de la Materia Condensada

Facultad de Ciencia y Tecnologia, UPV/EHU, Bilbao (Spain)

eDepartment of Materials Science and Metallurgy

University of Cambridge (UK)

Metal-organic frameworks (MOFs) show increasing promise as materials for applications in catalysis, gas

storage and molecular recognition, and are also interesting from fundamental condensed-matter physics

point of view. Of particular interest are MOFs with the perovskite architecture, which show hydrogen

bonding-related multiferroic phenomena, exhibiting both magnetic and ferroelectric ordering. Here, we focus

on [C(NH2)3]Cu(HCOO)3 , a compound that crystallizes in polar space group. We have performed ab-initio

calculations[1] and show theoretically, for the first time in a MOF, that the ferroelectricity related to this

polar structure induces a weak ferromagnetic coupling. In inorganic perovskite-like compounds with the

ABX3 composition, octahedral tilting and Jahn-Teller effects are usually non-polar distortions. However, in

this MOF cooperative interactions between the formate framework and the central guanidinium cation via

hydrogen bonding breaks the inversion symmetry and induces a ferroelectric polarization. Moreover, we

show that the switching of polarization direction implies the reversal of the weak ferromagnetic component,

therefore allowing long sought-after electrical control of the magnetization. These results therefore offer an

important starting point for tailoring multiferroic properties in this emerging class of materials for various

technological applications.

References

[1]A. Stroppa et al, Angew. Chemie Int. Ed. 50, 5847 (2010).

86

Correlation between growth dynamics and dielectric properties of epitaxial

BaTiO3 films

G. Radaelli a

, S. Brivio a, I. Fina

c, D. Chrastina

a, R. Bertacco

a

a LNESS and CNISM, Physics Department, Politecnico di Milano, Como, Italy

b Laboratorio MDM – IMM – CNR, Agrate Brianza (MB), Italy

c Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Bellaterra, Spain

Room temperature high-quality ferroelectric (FE) epitaxial films have enourmous potential for applications.

Their switchable electric polarization is ideal for use in nanoscale devices for storage applications such as

FERAMs [1]. More recently it has been suggested that ferroelectrics can be combined with magnetic films to

create outstanding spintronic memory devices based on the electric control of the magnetization of

ferromagnetic electrodes [2]. In particular BaTiO3 (BTO) has been proposed essentially because it is a

prototypical room temperature FE perovskite oxide, with good lattice matching with some of the widely used

ferromagnets such as Fe and La0.67Sr0.33MnO3 (LSMO). In this contest the comprehension of the impact of

the growth parameters on the growth dynamics and on the ferroelectric properties of BTO films a relevant

issue.

A detailed study of the effect of the substrate temperature on the growth dynamics and ferroelectric

properties of epitaxial BTO films grown on Nb:SrTiO3(100) substrates by Pulsed Laser Deposition has been

performed. Via time-dependent reflection high energy electron diffraction (RHEED) intensity analysis, X-

Ray Diffraction, Impedance Spectroscopy (IS), C-V and I-V measurements we show that the growth

dynamics plays a fundamental role in determining the dielectric and FE quality of the films. The increase of

the surface diffusion length of adatoms, obtained by raising the substrate temperature up to 680°C, causes a

considerable increase of the dielectric constant and saturation polarization. Above this critical temperature

the dielectric and FE properties strongly deteriorate, due to the activation of Ba re-evaporation, as shown by

X-Ray Photoemission Spectroscopy (XPS) analysis [3].

This work shed light on thermally activated physical mechanisms which determine the ferroelectric

properties of BTO films, and pave the way to the development of devices based on epitaxial FE BTO films.

[1] C. H. Ahn, K. M. Rabe, and J. M. Triscone, Science 303, 488 (2004).

[2] J. Ma, J. Hu, Z. Li, and C. W. Nan, Adv. Mat. 23, 1062 (2011).

[3] G. Radaelli, S. Brivio, I. Fina, and R. Bertacco, Appl. Phys. Lett. 100, 102904 (2012)

87

Microstructural analysis of the tetragonal to orthorhombic transition and

nematic orbital ordering in 1111 Fe superconductors

A. Martinelli a

, A. Palenzona a,b

, M.Putti a,c

, C.Ferdeghini a

a SPIN-CNR, corso Perrone 24, 16152 Genova - Italy

b Dept. of Chemistry and Industrial Chemistry, University of Genoa, via Dodecaneso 31, 16146 Genova – Italy

c Dept. of Physics, University of Genoa, via Dodecaneso 33, 16146 Genova - Italy

The microstructural evolution throughout the first order tetragonal to orthorhombic structural transition is

analyzed by powder diffraction analysis for two different systems belonging to the class of compounds

referred to as 1111 oxy-pnictides: (La1-yYy)FeAsO [1] and electron doped SmFeAs(O1-xFx). A careful

microstructural analysis reveals that the first order tetragonal to orthorhombic structural transition is not

suppressed in SmFeAs(O1-xFx), even at optimal doping [2]. An abrupt broadening of the full width at half

maximum of the 110 diffraction peak marks symmetry breaking. Orthorhombic distortion is decreased but

not suppressed by the increase of F content.

Both systems are characterized by a similar behaviour: on cooling microstructural strain along the tetragonal

hh0 direction takes place and increases as the temperature is decreased (Figure 1). Just above the structural

transition strain reaches its maximum value and then is abruptly suppressed by symmetry breaking. No

volume discontinuity throughout the transition is observed and a group-subgroup relationship holds between

the tetragonal and the orthorhombic structures, thus suggesting that orbital ordering drives symmetry

breaking. Microstrain is probably related to a progressive tendency of 3d Fe orbitals to order (nematic orbital

order), reflects a distribution of lattice parameters in the tetragonal phase, explains the occurrence of

anisotropic properties commonly attributed to nematic correlations and suggests the occurrence of a nematic

orbital ordering [3,4].

Figure 1: Evolution with temperature of the structural strain affecting the 110 peak in SmFeAsO (from

synchrotron powder diffraction data, on the left) and 220 peak in (La0.9Y0.1)FeAsO (from neutron powder

diffraction data, on the right); lines are guide to eyes.

[1] A. Martinelli, A. Palenzona, M. Tropeano, C. Ferdeghini, M. R. Cimberle, C. Ritter, Phys. Rev. B 80,

214106 (2009)

[2] A. Martinelli, A. Palenzona, M. Tropeano, M. Putti, C. Ferdeghini, G. Profeta, E. Emerich, Phys.

Rev. Lett. 106, 227001 (2011)

[3] Weicheng Lv, P. Phillips Phys. Rev. B 84, 174512 (2011)

[4] M. S. Laad, L. Craco, Phys. Rev. B 84, 054530 (2011)

88

Superconducting FeSe0.5Te0.5 thin films: a morphological and structural

investigation with scanning tunnelling microscopy and X-ray diffraction

Andrea Gerbi a, Renato Buzio

a, Emilio Bellingeri

a, Shrikant Kawale

a, DanieleMarrè

a,b, Antonio

Sergio Siri a,b

, Andrea Palenzona c, and Carlo Ferdeghini

a

a CNR-SPIN, Corso Perrone 24, 16152 Genova, Italy

b Dipartimento di Fisica, Università degli Studi di Genova, Via Dodecaneso 33 Italy

c Dipartimento di Chimica, Università degli Studi di Genova, Via Dodecaneso 31 Italy

It is so far established that high quality, epitaxial FeSexTe1-x thin films can be grown by pulsed laser

deposition (PLD) and that tensile and compressive stresses build up during thickening of the film and affect

the critical temperature (Tc) in a remarkable way [1-3]. Accordingly, a complex phenomenology has been

reported and Tc values up to 21 K have been observed. It is therefore interesting to address the mechanisms

of stress formation and release during the deposition of such materials. Similar studies have been challenged

for metal and semiconductor thin films, where a detailed investigation of the growth and surface structure

allowed an exploration of the relationship between morphology, stress and transport properties [4]. In this

context it has been shown that the deposition rate effectively controls the amount of strain accumulated

during the growth process [5]. Here we report a low-temperature (4.8K) scanning tunnelling microscopy

(STM) and x-ray diffraction (XRD) investigation of 150 nm thick FeSe0.5Te0.5 superconducting films

epitaxially grown by PLD. We describe surface morphology and discuss its relationship with film structure

and Tc for two deposition rates. Samples with critical temperature Tc above the bulk value (>16 K) show

large atomic terraces, and a square lattice of periodicity 3.8 A associated with the Se/Te surface termination.

Differences in the height coordinate of the chalcogenide atoms are clearly visible at the atomic level. On the

contrary, samples with lower Tc (11 K) show hillocks generated by a spiral surface growth driven by

threading dislocations of screw character. A comparative x-ray diffraction analysis reveals differences of

compressive strain for the two classes of specimens. Variations in the deposition rate are found to affect film

growth and inner strain, which ultimately tune Tc [6].

[1] Bellingeri E. et al., Appl. Phys. Lett. 96 102512 (2010)

[2] Wang M. J. et al Phys. Rev. Lett. 103 117002 (2009)

[3] Huang S. X. et al Phys. Rev. Lett. 104 217002 (2010)

[4] Springholtz G. and Wiesauer K. 2002 Phys. Rev. Lett. 88 015507 (2002)

[5] Tello J. S. et al Phys. Rev. Lett. 98 216104 (2007)

[6] Gerbi et al. Supercond. Sci. Technol. 25 012001 (2012)

89

Microwave response of coaxial cavities made of bulk MgB2

A. Agliolo Gallitto a

, A. Figini Albisetti b, G. Giunchi

b, M. Li Vigni

a

a Dipartimento di Fisica, Università di Palermo, Palermo, Italy

b EDISON SpA R & D, Milano, Italy

We report on the microwave (mw) properties of coaxial cavities built by using bulk MgB2 superconductor.

The bulk MgB2 specimens have been produced by Reactive Liquid Mg Infiltration process [1]. Three

different coaxial cavities have been prepared by using bulk MgB2. In particular, two of them are composed

by an outer copper cylinder and an inner MgB2 rod; two inner rods of lengths 45 mm and 94 mm have been

used. The third cavity is constituted by the MgB2 rod of 45 mm and an outer MgB2 cylinder about 15 mm

longer than the inner rod. Figure 1 shows a picture of the outer MgB2 cylinder.

The resonant cavities have been characterized measuring their

frequency response in the range 1 – 13 GHz by an hp-8719D

Network Analyzer, in the temperature range 4.2 – 50 K. The

cavities built using the 45 mm rod exhibit four resonant modes, in

the range 2.5 - 11 GHz; the spectrum of the cavity with the 94 mm

MgB2 rod shows eight resonant modes in the range 1.3 -11 GHz.

Preliminary results have shown that, at T = 4.2 K, the highest

unloaded quality factor of the cavity entirely made of MgB2 is Q

80000 at the resonant frequency f = 2.55 GHz. It remains of the

order of 104 up to about 30 K and reduces by a factor of 60 when

the superconductor goes into the normal state.

The results obtained in the coaxial cavity entirely made of MgB2

will be compared with those already obtained in a MgB2

cylindrical cavity [2] and discussed with the aim to exploit the material in mw applications.

Recently, we have built a tunable coaxial cavity using a rod of BSCCO and have shown that it can be

conveniently used to investigate the mw response of the inner superconducting rod, in both linear and

nonlinear regimes [3]. We will use the longer cavity with this aim. In particular, it will be possible to

determine the frequency dependence of the mw surface resistance of the MgB2 in the frequency range 1 - 11

GHz; to our knowledge, this issue is not widely discussed in the literature.

[1] G. Giunchi, G. Ripamonti, T. Cavallin, E. Bassani, Cryogenics 46 (2006) 237.

[2] G. Giunchi, A. Agliolo Gallitto, G. Bonsignore, M. Bonura, M. Li Vigni, Supercond. Sci. Technol.

20 (2007) L16.

[3] A. Agliolo Gallitto, G. Bonsignore, M. Li Vigni, A. Maccarone, , Supercond. Sci. Technol. 24

(2011) 095008 (8pp).

Figure 1: Bulk MgB2 cylinder used for

assembling the coaxial cavity

90

Metal-to-superconductor transition in two-dimensional electron systems:

mesoscopic disorder

D. Bucheli, S. Caprara, C. Castellani, M. Grilli, G. Lemarié

Physics Department, University of Rome “Sapienza”, ome, Italy

Motivated by experiments in thin films and oxide interfaces, we propose a random-resistor network to

describe the occurrence of a metal-to-superconductor transition in a two-dimensional electron system with

disorder on the mesoscopic scale [1]. We assume that disorder induces a distribution of local

superconducting critical temperatures. With lowering the temperature, global superconductivity establishes

as soon as percolation occurs within the superconducting cluster.

Disregarding the physical origin (classical or quantum mechanical) of the inhomogeneous distribution of the

superconducting regions, we phenomenologically focus on low-dimensional (fractal) structures of the

superconducting cluster. By varying the geometry of the clusters, as well as the probability distribution of the

critical temperatures, the influence of these two aspects on the temperature dependence of the resistivity is

studied [2].

From a more microscopic point of view, we explore the possibility that inhomogeneous

superconductivity arises in a system of preformed pairs in a homogeneously disordered square lattice [4]. In

particular we address the issue of the order parameter distribution, which has been recently studied by

Feigel’man, Ioffe and Mezard [3] using the cavity method on an effective spin model on a Bethe lattice. We

investigate the possible occurrence of a glassy superconducting phase taking place on a low-dimensional

subset of the square lattice.

[1] S. Caprara, M. Grilli, L. Benfatto, and C. Castellani, Phys. Rev. B 84, 014514 (2011).

[2] D. Bucheli, S. Caprara, C. Castellani, and M. Grilli, submitted to Phys. Rev. B

[3] M. V. Feigel’man, L. B. Ioffe, and M. Mézard, Phys. Rev.B 82, 184534 (2010)

[4] G. Lemarie, et al. , in preparation

91

µSR in RECoAsO and RECoPO under hydrostatic pressure

Prando,a,b,c

S. Sanna,a G. Profeta,

d P. Carretta,

a

R. De Renzi,e P. Bonfà,

e R. Khasanov,

f A. Pal,

g V. P. S. Awana

g

a Department of Physics “A. Volta,” University of Pavia-CNISM, I-27100 Pavia, Italy

b Department of Physics “E. Amaldi,” University of oma Tre-CNISM, I-00146 Roma, Italy

c Leibniz-Institut für Festkörper- und Werkstoffforschung (IFW) Dresden, D-01171 Dresden, Germany

d SPIN-CNR-Department of Physics, University of L’A uila, I-67100 L’A uila, Italy

e Department of Physics, University of Parma-CNISM, I-43124 Parma, Italy

f Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

g National Physical Laboratory (CSIR) Dr. K.S. Krishnan Marg, New Delhi 110012, India

The discovery of high-Tc superconductivity (SC) in iron-containing pnictide materials has triggered an

intense worldwide research activity about the physics of these compounds. In spite of the sizeable

improvements in the understanding of the most relevant macroscopic properties of these materials, anyway,

several questions are still open on the microscopic point of view, the main one concerning the mechanism

leading to the SC pairing. SC is typically reported to emerge close to the disruption of a antiferromagnetic

spin-density-wave (SDW). Similarly to the scenario possibly holding for cuprate materials, then, a

magnetically-mediated SC pairing can be envisaged. It is then clear how the local investigation of the

microscopic magnetic features across the whole phase diagram of these materials can be of extreme

importance and interest for a better microscopic understanding of SC itself.

In this respect, oxy-pnictide materials REFeAsO1−xFx belonging to the so-called 1111 family offer a rich

phenomenology possibly associated to the interplay of both itinerant and localized magnetic degrees of

freedom. It must be remarked that a wealth of different magnetic ground states can be achieved in non-SC

1111 materials like CeFeAs1−xPxO.1 This can be interpreted as a result of the different extent of the f − d

hybridization whose size is mainly governed by chemical pressure effects associated with the As/P

substitution. It is then clear how the interest in 1111 oxy-pnictides goes well beyond SC alone and that the

systematic study of the interplay between localized and itinerant degrees of freedom should be deepened also

in the case of non-SC compounds. At this aim, a comparison among different classes of 1111 materials could turn out to offer a more complete

overview of the actual physics in the considered materials. REFeAsO pnic-tides are known to experience

antiferromagnetic correlations on the FeAs bands resulting in a SDW phase in the undoped parental limit.

Surprisingly, in Co-based RECoAsO compounds ferromagnetic correlations are at work2 and, at variance

with the former materials, SC is not displayed in their phase diagram. In RECoAsO compounds, interesting

magnetic properties due to the simultaneous presence of both Co and RE magnetic sublattices have been

reported.3 A somewhat similar phenomenology has been reported in the isostructural compounds RE-CoPO

4

even if, in this case, detailed systematic local investigations are still lacking. Here we present the results of a local µSR investigation on RECoAsO and RECoPO sam-ples under applied

external pressure P . The application of P , in fact, seems to be of great importance in order to mimick the

effects of the chemical pressure increased by the As/P sub-stitution that, as recalled above, is able to sizeably

enhance f − d interaction in turn. At this stage of the work, LaCoPO appears to be the most interesting

compound due to its high sensi-tivity to P , even if DFT calculations do not evidence any sizeable effect of P

on the structure of the electronic bands.

1Y. Luo et al., Phys. Rev. B 81, 134422 (2010)

2H. Ohta, K. Yoshimura, Phys. Rev. B 79, 184407 (2009)

3J. Sugiyama et al., Phys. Rev. B 84, 184421

(2011) 4A. Pal et al., Journ. Appl. Phys. 110, 103913 (2011)

92

Current-current fermi-liquid corrections to the superconducting fluctuations on

conductivity and diamagnetism

L.Fanfarillo a

, L. Benfatto b,a

, C.Castellania

a Department of Physics, Sapienza University of Rome, P.le A. Moro 2, Rome, Italy

b Institute for Complex Systems (ISC), CNR, U.O.S. Sapienza , P.le A. Moro 2, Rome, Italy

It is generally believed that, owing to their low superfluid densities and short correlation lengths,

superconducting fluctuations in underdoped cuprates should be significant for transport and thermodynamic

properties. Although there has been ample experimental evidence for such correlations, prompted mainly by

measurements of a large Nerst effect [1], there is still disagreement about the range of temperatures in which

they are relevant, their magnitude and their role in the phenomenology of the pseudogap [1,2]. Recently

Bilbro et al. [3] presented a detailed THz time-domain spectroscopy study of the fluctuation

superconductivity in LSCO. They showed that not only the fluctuation conductivity persists up to

temperatures lower than Nernst signal, but more specifically it is about two orders of magnitude smaller than

the fluctuating diamagnetism in the same system.

We showed that the quantitative disagreement between the superconducting fluctuation contribution to

conductivity and diamagnetism could be understood as a consequence of current-current interactions in a

doped Mott insulator [4].

By explicitly computing the superconducting-fluctuation contribution to diamagnetism and conductivity

above Tc we demonstrate that their different magnitude is a direct outcome of the proximity to the Mott-

insulator in the presence of current-current interactions.

[1] L. Li et al., Phys. Rev. B 81, 17 (2010).

[2] L. S Bilbro et al., Nature Phys. 7, 298-302 (2011).

[3] L. S Bilbro et al., Phys. Rev. B 84, 100511 (2011).

[4] L. Fanfarillo et al., Phys. Rev. B 85, 024507 (2012).

93

Insight into non linearly shaped superconducting micro-crystals via synchrotron

nano-probe

S. Caglieroa, E. Borfecchia

a, L. Mino

a, L. Calore

a, M. Truccato

b, C. Lamberti

a , A. Agostino

a, L.

Opertia

a

Department of Chemistry, University of Torino, NIS-Centre of Excellence, Torino, Italy b Department of Physics, University of Torino, NIS-Centre of Excellence, Torino, Italy

Nano-stacks of superconducting and insulating planes naturally occur along the c-axis in high-temperature

superconducting cuprates. This feature has been recently exploited for solid-state THz applications based on

sequences of Josephson Junctions. We managed to synthesize non-linear YBa2Cu3Ox whiskers, which are

promising candidates for these devices. Here we report a complete characterization of their structural

properties by synchrotron nano-probe diffraction and laboratory single-crystal diffraction. The

superconducting properties are also evaluated in comparison with the straight counterpart.

Figure 1: Results obtained by nano-beam synchrotron XRD investigation of a curved YBCO

whisker. (a) SEM micrograph of the analyzed semicircular shaped whisker, with indicated the spatial

positions along the profile where diffraction patterns were collected (numbers from 1 to 6).

Corresponding XRD 2D-patterns (after correction for CCD distortions) are reported around the SEM

image. Intensity of diffraction spots has been enhanced for graphical reasons. (b) Indexed reflections

on the averaged image of XRD frames 1-6 in (a); on the right a magnification of the (110) reflection

is reported. (c) Schematics of possible cell arrangements resulting in a non-linear whisker shape: (1)

bowed profile originating from a continuous succession of growth twins in the ab-plane, (2)

curvature resulting from lateral translation of crystal cells, always aligned along the initial growth

direction.

94

Ferronematic order in underdoped cuprates

M. Capatia, S. Caprara

a, C. Di Castro

a, M. Grilli

a, J. Lorenzana

b, G. Seibold

c.

a Dipartimento di Fisica, Università di oma “Sapienza”, Piazzale Aldo Moro 2, I-00185 Roma, Italy

b Istituto dei Sistemi Complessi, CNR, Dipartimento di Fisica,

Università di oma “Sapienza”, Piazzale Aldo Moro 2, I-00185 Roma, Italy c Institut für Physik, BTU Cottbus, PBox 101344, 03013 Cottbus, Germany

We study a model for underdoped lanthanum cuprates where doped holes form magnetic vortices and

aggregate into stripe segments which are composed of vortex-antivortex pairs.

These segments are aligned by the underlying

low temperature orthorhombic lattice distortion

and can thus be viewed as Ising dipoles. We

argue that due to the short-range interaction

between segments a state with macroscopic

polarization is stabilized which we call a

ferronematic.

This state can be characterized as a charge

nematic, but due to the net polarization also

exhibits an incommensurate spin modulation.

Our calculation can reproduce the doping

dependent spin structure factor of lanthanum

cuprates in excellent agreement with experiment.

A Monte Carlo analysis provides us information

on the thermodynamic phase transitions.

[1] S. Wakimoto et al., Phys. Rev. B 61, 3699 (2000).

Figure 1: Spin phase distribution generated by

ferroelectrically aligned segments, each one made by 4

vortex-antivortex pairs. The resulting spin structure factor is

in agreement with the experimental data by Wakimoto [1].

95

Fluctuation conductivity of SrFe2As2 single crystal in a magnetic field

P. Marraa, A. Nigro

b,c, Z. Li

d, G. F. Chen

d, N. L. Wang

d, J. L. Luo

d, and C. Noce

b,c

aInstitute for Theoretical Solid State Physics, IFW, D01171 Dresden, Germany

bSPIN-CNR, I-84084 Fisciano (Salerno), Italy

cDipartimento di Fisica “E. . Caianiello" Università di Salerno, I-84084 Fisciano (Salerno), Italy

dBeijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,

Beijing 100190, China

The in-plane excess conductivity of the optimally K-doped SrFe2As2 superconductor has been measured in

high quality single crystals, in the presence of an external magnetic field up to 14 T. We show that the

fluctuation conductivity data in zero field, and for temperatures close to the critical temperature, can be

modeled within a three-dimensional Lawrence-Doniach theory, with a negligible Maki-Thompson

contribution. In the presence of the magnetic field, it is shown that paraconductivity obeys the three-

dimensional Ullah-Dorsey scaling law, above 2 T and for H∥ c. The estimated upper critical field and the

coherence length nicely agree with the available experimental data.

96

Probing the ground state of undoped and optimally doped REFe1-yRuyAsO1-xFx

(RE=La, Sm) by isoelectronic Ru/Fe substitution.

Bonfàa, G. Prando

a,b, P. Carretta

a, S. Sanna

a, R. De Renzi

c, A. Martinelli

d, M. Tropeano

e,

G. Lamurae, A. Palenzona

e, M. Putti

e

a Physics Department “A. Volta”, University of Pavia – CNISM, Pavia, Italy

b Physics Department “E. Amaldi”, University of oma Tre – CNISM, Roma, Italy

c Physics Department, University of Parma, Parma, Italy

d CNR-SPIN, Genova, Italy

e CNR-SPIN and University of Genova, Genova, Italy

The appearance of high-temperature superconductivity in REFe1-yRuyAsO1-xFx close to or coexisting with

an antiferromagnetic order has renewed the interest in investigating the possible interplay between

magnetism and superconductivity.

To shed light on this intriguing topic, here we present a new approach to characterize the microscopic

properties of the new iron-based superconducting compounds. The evolution of the magnetic and

superconducting ground states have been studied by keeping a constant electronic doping as a function of the

magnetic dilution/frustration. This result has been achieved by Ru for Fe substitution in undoped LaFe1-

yRuyAsO [1] and optimally doped SmFe1-yRuyAsO0.85F0.15 [2]. It has been shown indeed, that the

electronic structure is only marginally affected by Ru substitution around the Fermi level [3] even for high

Ru concentrations. This guaranties that Ru for Fe substitution is effectively isoelectronic, thus the electronic

doping is only governed by fluorine doping. Moreover it has also been predicted that the Ru atoms do not

sustain any magnetic moment [3].

muSR and NMR experiments were conducted on the aforementioned compounds on a wide range of Ru

concentrations. The results show that the spin-density-wave ordering typical for the undoped parent

compound is progressively suppressed as Ru is added. We discuss this behaviour considering both the

dilution model and the frustration mechanism [1] since these compounds are characterized by competing

interactions between nearest neighbour and next nearest neighbour atoms in the Fe square lattice [3-5]. The

role of the spinless impurity and its capacity to induce further frustration of the coupling among Fe spins are

also considered.

Thanks to the results of the undoped sample, it is possible to show that a strong degradation of the

superconducting state is produced by a isoelectronic Ru/Fe substitution in optimally

e--doped SmFe1-yRuyAsO0.85F0.15 samples, concomitant with the development of short range re-entrant

static magnetism throughout the whole sample volume, as observed by zero-field muon-spin relaxation

measurements for 0.15<y<0.4. These results are discussed in the framework of the strong competition

between magnetism and superconductivity.

[1] P. Bonfà et al., Phys. Rev. B 85, 054518 (2012) [2] S. Sanna et al., Phys Rev. Lett. 107, 227003 (2011) [3] M. Tropeano et al., Phys. Rev. B 81, 184504 (2010)

[4] T. Yildirim, Phys. Rev. Lett. 101, 057010 (2008) [5] Q. Si and E. Abrahams, Phys. Rev. Lett. 101, 076401 (2008)

97

Tuning the interplay between magnetism and superconductivity in optimally F

doped REFe1-xRuxAsO with RE=Sm, Nd, La

S. Sannaa, P. Carretta

a, P. Bonfà

b, G. Prando

c, G. Allodi

b, R. De Renzi

b, T. Shiroka

d, G. Lamura

e, A.

Martinellif, M. Putti

e, Y.Kobayashi

g

a Dipartimento di Fisica, Università di Pavia and CNISM, Pavia, Italy

b Dipartimento di Fisica, Università di Parma and CNISM, Parma, Italy

c Laboratorium fuer Festkoerperphysik, ETH-Hoenggerberg, Zuerich, Switzerland

e CNR-SPIN and Universita` di Genova, Genova, Italy

f CNR-SPIN, Genova, Italy

g Department of Physics, Nagoya University, Nagoya, Japan

In the REFeAsO family it is found that SC and static magnetism strongly compete and hardly coexist

simultaneously, apart for RE=Sm and Ce within a small doping range where both order parameters are

depressed [1-5]. In case of coexistence both cuprates and pnictides display a short range magnetic order,

related to magnetic clusters nanoscopically mixed with SC regions. A combination of recent experiments of

muon spin relaxation and nuclear quadrupolar resonance on REFe1-xRuxAsO0.85F0.15 with RE=Sm show that

superconductivity and magnetism are tightly related to two distinct well defined local electronic

environments of the FeAs layers, which can be finely tuned by isoelectronic and diamagnetic Ru substitution

[6]. It is also shown that, by going from RE=Sm to Nd to La, the re-entrant static magnetism is progressively

weakened, while the superconducting is less perturbed. In addition the residual resistivity is increased when

magnetism is strongly pinned, displaying that electrons and magnetic excitations are intimately coupled.

Indirect evidence is given that superconductivity is assisted by Fe magnetic fluctuations, which are at least

partially frozen when static order appears and are absent above the Fe/Ru spin dilution threshold [6-7].

[1] S. Sanna, et al., Phys. Rev. B 80 052503 (2009).

[2] G. Prando et al., Phys. Rev. B 81 100508(R) (2010).

[3] S. Sanna et al., Phys. Rev. B 82, 060508(R) (2010).

[4] R. Khasanov et al., Phys. Rev. B 84, 100501(R) (2011).

[5] T. Shiroka et al., Phys. Rev. B 84, 195123 (2011).

[6] S. Sanna et al., Phys. Rev. Letters 107, 227003 (2011).

[7] P. Bonfà et al., Phys. Rev. B 85, 054518 (2012).

98

Correlation between structural and transport properties in epitaxial films of

Nd2-xCexCuO4±.

A. Guarino a,b

, R.Fittipaldi b

, A. Romanoa,b

, A. Vecchionea, b

, A. Nigroa,b

a Dipartimento di Fisica “E. . Caianiello” Università di Salerno, Fisciano, Italy

b CNR-SPIN, presso Università di Salerno, Fisciano, Italy

The majority of the high-temperature superconductors (HTS) discovered so far are antiferromagnetic Mott

insulators with perovskitic structure doped with hole-like charge carriers injected in the CuO2 planes.

However, as first shown in 1989 [1], a small number of other perovskitic cuprates can become

superconducting, though with lower critical temperatures, when doping is made with electrons rather than

with holes. The most relevant of these systems are those with chemical formula RE2-xCexCuO4± (RE-CCO),

where RE denotes a trivalent lanthanide rare earth element (RE=Nd, Pr, Sm, Eu). A partial substitution of

this latter with tetravalent cerium leads, for suitable values of x, and T, to the superconducting transition

thanks to the related injection of electron-like charged carriers. Though in the last two decades the attention

has predominantly been devoted to hole-doped cuprates, the electron-doped ones have received a renewed

interest in recent years [2], since it has been realized that a powerful route towards the understanding of the

physics of HTS may come through a careful analysis of the analogies and the differences between the

superconducting phases induced by the two kinds of doping [3].

In this framework, we have performed a combined analysis of the structural and the transport properties of

the electron-doped Nd2-xCexCuO4± (NCCO) thin films grown on (001)-oriented SrTiO3 (STO) substrates via

dc sputtering technique, focusing on the effects induced by the procedure of oxygen reduction needed for the

establishment of the superconducting phase [4]. Our main finding is that the transition appears to be

triggered by a structural reorganization of the system, rather than by a gradual reduction of the disorder

associated with the removal of oxygen ions in excess [5]. In particular, the oxygen loss occurring at

temperatures lower than ~ 850 °C leads to an improvement of the metallic behavior, but neither a

superconducting transition nor an appreciable lattice modification takes place. On the other hand, when the

annealing temperature is raised at or above ~ 850 °C a further elimination of oxygen ions, probably located

at different lattice positions compared to those of the ions eliminated at lower T, takes place. This gives rise

simultaneously to the establishment of the superconducting phase and to a structural reorganization

producing an increase of the lattice parameter in the c direction. This observation was possible due to the

high epitaxy of our samples. These effects are accompanied by the appearance of a linear contribution in the

temperature dependence of the resistivity, which we conjecture to be associated with the formation of hole-

like carriers along the nodal direction of the Brillouin zone, coexisting with electron-like ones already

present at the antinodal points when the sample is non-superconducting.

[1] Y. Tokura, H. Takagi, and S. Uchida, Nature (London) 337 (1989) 345; H. Takagi, S. Uchida, and Y.

Tokura, Phys. Rev. Lett. 62 (1989) 1197.

[2] N.P. Armitage, P. Fournier, and R.L. Greene, Rev. Mod. Phys. 82 (2010) 2421.

[3] C. Weber, K. Haule, and G. Kotliar, Nature Physics 6 (2010) 574.

[4] A. Guarino, R. Fittipaldi, A. Romano, A. Vecchione, and A. Nigro, submitted to Thin Solid Films.

[5] J.S. Higgins, Y. Dagan, M.C. Barr, B.D. Weaver, and R.L. Greene Phys. Rev. B 73 (2006) 104510.

99

Quasiparticle energy relaxation time in type-II superconductors

A. Leoa, G. Grimaldi

a, R. Citro

a, A. Nigro

a, S. Pace

a

a CNR-SPIN Salerno and Dipartimento di Fisica ’E Caianiello’, Università di Salerno, Via Ponte Don Melillo, I-

84084, Fisciano (SA), Italy We review the results of a study on quasiparticle energy relaxation time τ in different type-II

superconductors. The contributions to τ from the electron-phonon scattering and the recombination processes

are evaluated using the tool of flux-flow instability phenomenon [1, 2]. Experimentally, we measure current-

voltage (IV) characteristics deriving the vortex critical velocity behaviour as a function of the temperature

v*(T) in a fixed magnetic field. To interpret such behaviour, we numerically calculate the quasiparticle

energy relaxation rates and compare the results with the experimental data. Our findings show that different

energy scales are involved in the quasiparticle energy relaxation of superconducting materials with different

electron mean free path.

[1] G. Grimaldi, A. Leo, A. Nigro, S. Pace and R. P. Huebener, Phys. Rev. B 80, 144521 (2009)

[2] A. Leo, G. Grimaldi, R. Citro, A. Nigro and S. Pace, Phys. Rev. B 84, 014536 (2011).

100

Effect of Inhomogeneous Magnetization on the Superconducting Properties of

Nb/Py/Nb Trilayers: Evidence of Spin-Triplet Superconductivity

C. Cirilloa, E. A. Ilyina

a, J. M. Hernandez

b, A. Garcia-Santiago

b, J. Tejada

b, C. Attanasio

a

a CNR-SPIN Salerno and Dipartimento di Fisica “E. . Caianiello”,

Università degli Studi di Salerno, Fisciano (Sa), Italy b Grup de Magnetisme, Departament de Fisica Fonamental,

Facultat de Fisica, and Institut de Nanociencia i Nanotecnologia IN2UB,

Universitat de Barcelona, Barcelona, Spain

We report measurements of the temperature dependence of the parallel upper critical field, Hc2||(T), in

proximity coupled Nb/Py/Nb trilayers in which the thickness of the Py layer, dPy, changes from 20 nm up to

432 nm. When dPy is in the range 150-250 nm a coupling between the two superconducting outer layers is

observed with Hc2||(T) showing a linear behavior from Tc down to temperatures relatively far from the critical

transition temperature. We believe that this is due to a long-range proximity effect generated by the

inhomogeneous magnetization related to the presence of stripe domains of proper size in thick Py layers.

[1] C. Cirillo, E. A. Ilyina, J. M. Hernandez, A. Garcia-Santiago, J. Tejada, C. Attanasio, submitted.

101

Tuning of superconducting properties of Fe(Se,Te) by thin film technology.

E. Bellingeria, S. Kawale

a,b, I. Pallecchi

a, A. Gerbi

a, R. Buzio

a, V. Braccini

a, A. Palenzona

a,c, M.

Puttia,b

,A. Salaa,b

, D. Marréa,b

and C. Ferdeghinia

aCNR-SPIN, Corso Perrone 24, 16152 Genova, Italy

b Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33 16146 Genova

c Dipartimento di Chimica, Università di Genova, Via Dodecaneso 31 16146 Genova

Epitaxial FeSe0.5Te0.5 thin films were grown by Pulsed Laser Ablation deposition on different

substrates. High purity phase and fully epitaxial growth were obtained.

We show how it is possible to tune the properties of the 11 thin films enhancing Tc up to 21K, the upper

critical field Hc2 to more than 55T with low anisotropy and the Jc in-field dependence by introducing strong

correlated pinning centers.

Superconducting transition temperatures up to 21K were observed, significantly larger than the bulk

value 16.2 K. Structural analyses indicated that the a axis changes significantly with the film thickness and is

linearly related to the Tc . The latter result indicates the important role of the compressive strain in enhancing

Tc. Tc is also related to both the Fe-(Se,Te) bond length and angle, suggesting the possibility of further

enhancement.

The critical current density Jc is another key property for applications in superconductors: the values

measured in thin films - which are free from grain boundaries and may have stronger pinning centers - are

promising also in pnictides and its investigation may yield important clues to enhance the performances of

bulk samples, tapes, and wires. We found Jc values up to 4 105 A/cm2 in self field at T=4K and weak field

dependence. The analysis of the activation energy for vortex motion indicates that the single pinning regime

holds up to 9 T, suggesting that correlated pinning centers are more effective than the vortex-vortex

interaction even at the largest applied fields. Scanning tunneling microscope analysis indicates threading

dislocations as possible pinning centers.

102

Nernst effect of Iron-based superconductors

F.Caglieris1, G.Lamura

1, I. Pallecchi

1, M.Tropeano

1, E.Galleani

1,2, A.Jost

3, S. Wiedmass

3, U.

Zeitler3, M.Putti

1,2

1CNR-SPIN, Corso Perrone 24, 16151, Genova Italy

2University of Genova, Physic Department, via Dodecaneso 33, 16146 Genova Italy

3 High Field Magnet Laboratory, Radboud University of Nijmegen NL- 6500 GL Nijmegen

The Nernst effect, defined as the transverse electric field in response to a longitudinal temperature gradient,

in the presence of a perpendicular magnetic field H, is related to the thermal drift of quasiparticles shifted by

the Lorenz force. The Nernst effect is usually small in metals but it is enhanced in compensated metal and

exhibits a huge contribution in the mixed state of superconductors due to the motion of vortices.

We measure the Nernst effect of Fe-based superconductors and related parent compounds of the “11”,

(Fe(Te,Se) compounds), and “1111”, (SmFeAs(O,F)compounds), families.

We observed different behaviours between the two families, both in the magnetic and superconduting

regimes.

As for the superconducting samples, we explore the vortex regime below Tc and in we find an enhancement

of the Nernst effect in the mixed state corresponding to the vortex motion in the 1111 phase that was not

observed in the 11 compounds.

As for the parent compounds we explore the regime below the antiferromagnetic transition. We find an

enhanced signal in the SmFeAsO compound whereas in the FeTe compound the Nernst signal change sign in

correspondence of the transition. We interpret the curves in a framework of almost compensated metals and

we suggest a possible relationship with the presence of Dirac cones in the band structure of the 1111 family,

that are not been evidenced in the 11 phase.

103

The effect of the AC field frequency and of the grains shape on the bulk inter-

grain current of polycrystalline iron-based superconductors.

M. Polichetti a, J. Luo

b, G.F. Chen

b, Z. Li

b, N.L. Wang

b, C. Noce

a’, S. Pace

a

a Dipartimento di Fisica “E. . Caianiello,” Universitá di Salerno and CN -SPIN, Unità di Salerno, via Ponte don

Melillo, I-84084 Fisciano (SA), Italy

b Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,

Beijing 100190, China

The detailed magnetic response of a granular LaFeAs0.92F0.08 sample to the application of ac magnetic fields

with different frequencies has been analysed by using the multi-harmonic AC susceptibility analysis. The

response coming from the bulk sample has been compared with the analogous one coming from the powders

obtained by grinding it.

The study of both the 1st and the 3

rd harmonics of the ac susceptibility has shown that the electromagnetic

coupling of the grains in the bulk granular sample can be strongly influenced by the frequency of the applied

ac field. In particular, at the highest frequencies the harmonic ac response of the bulk sample resembles that

of the unconnected grains of the powders even at low frequencies. The peculiar shape of the 3rd

harmonic

curves as function of the temperature corresponds to what expected from numerical simulations in which the

geometrical characteristics of the grains are considered.

Moreover, the effect of the superconducting grains geometry has been taken into account to interpret the

peak effect (Fig.1) found in the field dependence of the inter-granular critical current. More precisely, the

interpretation has been done in terms of the occurrence of magnetic field focusing in the region between the

grains, which can be ascribed to both superconducting shielding and demagnetisation effect of the grains.

Finally, in the range of fields corresponding to the peak effect in the M(Hdc) curve, the ac response is just

slightly depending on the dc field (Fig. 1), but the light slope of the ’(Hdc) curve increases with the

frequency of the ac field, as shown in Fig. 2.

The results of our analysis can be applied, in principle, to any kind of granular sample composed of flat

grains connected by weak links, and they show that a peak effect in a granular sample can exist without

necessarily invoke the existence of vortex dynamic processes.

1 10 100-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

'(

a.u

.)

Hdc

(kOe)

T=10 K

Hac = 15 Oe

=17 Hz

-25

-20

-15

-10

-5

0

5

1 10 100-0.3

-0.2

-0.1

0.0

Powder

'(

a.u

.)

Hdc(kOe)

Bulk

M(m

em

u)

Figure 1: M(Hdc) loop compared with the

dcH'

1 curve at =17 Hz and Hac= 15 Oe

parallel to Hdc. Inset: dcH'

1 curves on the

bulk sample and on the powders measured under

the same conditions to identify when the field

penetrates into the grains of the bulk..

1 10 100-0.30

-0.25

-0.20

-0.15

-0.10

-0.05 17 Hz

107 Hz

1607 Hz

'(

a.u

.)

Hdc

(kOe)

T=10 K

Hac

= 15 Oe

Figure 2: dcH'

1 curves measured at different

frequencies on the bulk sample. The dotted lines

indicates the slopes of the curves in two particular

regions of the transition.

104

Odd-Frequency Triplet Pairing in Mixed-Parity Superconductors

P. Gentilea, C. Noce

a, A. Romano

a, G. Annunziata

a, J. Linder

b and M. Cuoco

a

aDipartimento di Fisica “E. . Caianiello”, Università degli Studi di Salerno and CNR-SPIN, I-84084 Fisciano

(Salerno), Italy bDepartment of Physics, Norwegian University of Science and Technology, N-7491 Trondheim, Norway

The topic of unconventional proximity effect and quantum states in hybrid ferromagnet/superconductor

systems [1] is at the center of a wide investigation especially for the possibility of generating spin triplet odd-

in-time (STOT) superconducting pairs which propagate within strong diffusive ferromagnets over very long

distances, comparable with the penetration length scale of spin singlet pairs in a normal metal. This long-

range proximity effect may offer the possibility to exploit completely polarized super-currents in the rapidly

developing field of spintronics. The recent experimental efforts addressing the generation and detection of

STOT pair correlations have been based on the theoretical prediction that magnetic inhomogeneity is a key

aspect to generate odd-in-time pairing, at least for singlet superconductors. However, in all devices so far

realized the fine tuning of the magnetic profile is a complicate task, and this in turn probably hampers the

controlled manipulation of spin triplet super-currents. To avoid this problem, we propose an alternative

perspective for the generation of STOT pair correlations, looking at the almost unexplored context of

unconventional superconductors (US), and also considering hybrid ferromagnet superconductor systems. We

show that the generation of STOT pair correlations is intimately connected to the symmetry breaking in US.

We found, in particular, that the main source of STOT correlations is provided by the presence of mixed-

parity states [2]. Such feature is of fundamental importance in the context of symmetry determination of US

and it turns out to be relevant also for hybrids, since it can lead to STOT correlations without invoking

magnetic inhomogeneities or non-collinearity. The great variety of US available at the present time, from

high-temperature cuprates, to f-electrons superconductors, cobaltates, ruthenates, iron based, organics and

many others, allows a realistic experimental implementation of our predictions, that can thus help to make a

significant advance in a reliable production of a new generation of experiments and devices.

[1] M. Cuoco et al., Phys. Rev. B 78, 054503 (2008); A. Romano et al., Phys. Rev. B 81, 064513 (2010).

[2] P. Gentile et al., e-print arXiv:1109.4885v1 (2011).

105

Point-contact Andreev-reflection spectroscopy in Fe-based superconducting thin

films

P. Pecchioa , D. Daghero

a, R. S. Gonnelli

a, K.Iida

b, B. Holzapfel

b, C. Ferdeghini

c, R. Cristiano

d

a Department of Applied Science and Technology, Politecnico di Torino, 10129 Torino, Italy bLeibniz Institute for Solid State and Materials Research (IFW) 01069 Dresden, Germany

c CNR-SPIN, 16152 Genova, Italy

d CNR-Istituto di Cibernetica “E. Caianiello”, 80078 Pozzuoli (NA), Italy

The results of point-contact Andreev reflection spectroscopy (PCARS) measurements in thin films of Fe-

based superconductors have been recently published in literature. In SmFeAs(OF) and LaFeAs(O,F) films of

good quality [1], the PCARS spectra look very similar to those obtained in polycrystals of the same materials

[2], and give consistent values of the gaps if analyzed within the same two-band Blonder-Tinkham- Klapwijk

(BTK) model [3]. In other cases, namely in Fe(SeTe) films [4] and Ba(Fe,Co)2As2 films [5], anomalous

effects were observed such as the emergence of a zero-bias peak disappearing above Tc, that was ascribed to

exotic phenomena such as Andreev reflection from uncondensed pairs in the pseudogap state [5].

Here we report the results of PCARS measurements in thin films of Fe(Se,Te) and Ba(FeCo)2As2 (with and

without a Fe buffer layer). We observed effects similar to those described in literature and, in addition, a

systematic shift of the tails of the conductance curves on increasing temperature. We then compare these

PCARS spectra to those obtained in bulk samples of the same compounds and in more conventional

superconducting films (e.g. NbN). The comparison suggests that these features might not be related to

specific features of the Fe-based compounds, but rather to extrinsic effects such as, for example, the small

thickness of the films (that make the critical current density be easily reached in the vicinity of the contact),

the effect of a magnetic buffer layer, or the formation of a conducting layer at the interface between the film

and the substrate.

[1] Yu. G. Naidyuk et al., Supercond. Sci. Technol. 24 065010 (2011)

[2] R. S. Gonnelli et al., Phys. Rev. B 79 184526 (2009).

[3] R. S. Gonnelli et al., Rep. Prog. Phys. 74 124509 (2011)

[3] W. K. Park et al., arXiv:1005.0190 (2010)

[4] G. Sheet et al., Phys. Rev. Lett. 105, 167003 (2010)

106

Phenomenology of LiFeAs explained in the framework of multiband s+-

Eliashberg theory

Sara Galasso

a , G.A. Ummarino

a, A. Sanna

b

a Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino and CNISM, Corso Duca degli Abruzzi 24,

10129 Torino, Italy. bMax-Planck-Institut fr Mikrostrukturphysik, Weinberg 2, D-06120 Halle (Germany)

The phenomenology of LiFeAs superconductor can be explained in the framework of a four bands s +- wave

Eliashberg theory with only a small number of free parameters. We have examined experimental data from

literature and we have found that it is possible to reproduce the experimental critical temperature, the gap

values and the temperature dependence of the upper critical field in a moderate strong-coupling regime:

λtot=1.60. The total electron-boson coupling can be separated in a purely interband spin-fluctuaction

contribution with λtot SF

=1.39 and a purely intraband phonon contributions with λtot PH

=0.21.

We have also calculated the superconductive density of states and the temperature dependence of the

penetration depth, of the spin-lattice relaxation rate from NMR and of the spin susceptibility, and compared

the results with available experimental data.

107

Synthesis of MgB2 enhanced by monomodal microwave oven

A. Agostinoa,c*

, E. Bonomettia, V.Pau

a, M.Truccato

b,c, A. Pagliero

b, S. Cagliero

a, L.

Opertia,c

a Department of Chemistry, University of Torino - Via P.Giuria 7 b

Department of Physics, University of Torino - Via P.Giuria 1 c NIS- Center of Excellence, Via Quarello,11/A -Torino

In past years MgB2 superconductor was synthesized by various methods, very useful but nevertheless rather

expensive. Microwave technique is instead a low cost, clean and time saving technique[1]. Because the

heating of material originates from interaction of an electromagnetic field with induced and permanent

molecular dipoles energy absorption is distributed simultaneously over all the material [2].

MgB2 bulks were synthesized in several shapes (pellets, slab, cylinders, etc.)[3] starting from elemental

precursors powder. Samples are realized by monomodal oven custom-tailored in 20 minutes, further details

are described in the patent application. Sample was characterized by XRD (X-ray diffraction) (fig.1) that

revealed MgB2 was the only phase; with the Rie tveld refinement, using the Maud program, we get MgB2 as

main phase and an accessory phase of MgO (about 6.5% wt) in the matrix. Electrical measurements showed

a TC of 38.5°K (fig.2) with values of RRR of 3.38.

SEM (scanning electron microscopy) revealed MgB2 grains ranged from 300 to 500

micrometers in diameter.

So by MW we can realized MgB2 in various shapes in an extremely short time and this means

above all cost savin g from industrial point of view.

* Author to whom any correspondence should be addressed: angelo.agostino@unito.it

[3] Agostino A., Panetta M., et all., IEEE Trans. on Appl. Superc., Vol. 17, NO.2, 2774 -2777, 2007 [4] Agostino A, Volpe P., et all., M., Mat. Res. Innovat., 8, 2, 75-77, 2004 [5] Gozzelino Laura; Minetti Bruno; Gerbaldo Roberto; et al.IEEE TRANSACTIONS ON APPLIED

SUPERCONDUCTIVITY , 21,3, 3146-3149, Part 3, 2011

108

List of presenting authors

Author Contribution Page

Aarts Jan Oral 30

Acquarone Marcello Poster 80

Agliolo GallittoAurelio Poster 89

Agostino Angelo Poster 107

Albisetti Edoardo Poster 60

Avella Adolfo Oral 22

Barone Carlo Oral 40

Barone Paolo Oral 38

Barone Paolo Poster 83

Bellingeri Emilio Oral 48

Bellingeri Emilio Poster 101

Bonavolontà Carmela Oral 9

Bonfà Pietro Poster 96

Bossoni Lucia Oral 12

Brivio Stefano Oral 18

Bucheli Daniel Poster 90

Büchner Bernd Plenary talk 3

Caglieris Federico Poster 102

Calloni Alberto Oral 33

Calore Leandro Poster 93

Calvani Paolo Oral 42

Cantoni Matteo Poster 72

Capati Matteo Poster 94

Capone Massimo Oral 11

Cappelluti Emmanuele Poster 61

Chikoidze Ekaterina Oral+Poster 25

Cirillo Carla Poster 100

Cohen L.F. Poster 76, 77

Constantinian Karen Poster 64

Cuoco Mario Oral 41

Davidson Bruce Oral+Poster 24

De Luca Roberto Poster 70

De Renzi Roberto Oral 50

Di Castro Daniele Oral 28

Di Pietro Paola Poster 67

Fanfarillo Laura Oral 23

Fanfarillo Laura Poster 92

Filippetti Alessio Oral 49

Fontcuberta Joseph Plenary talk 5

Forte Filomena Oral 21

Franz Silvia Poster 74

109

Gadaleta Alessandro Poster 58

Galasso Sara Poster 106

Galdi Alice Oral 47

Gambardella Alessandro Poster 59

Gentile Paola Poster 104

Gerbi Andrea Poster 57, 88

Ghiringhelli Giacomo Oral 19

Giordano Livia Oral 32

Graziosi Patrizio Oral+Poster 26

Grilli Marco Poster 66

Guarino Anita Poster 98

Guarnaccia Giuseppe Poster 79

Karpinsky Dzmitry Poster 81

Khomchenko Vladimir Poster 84

Koelle Dieter Oral 20

Leo Antonio Poster 99

Lesne Edouard Oral+Poster 46

Leveratto Alessandro Poster 56

Liu Shuang Oral+Poster 17

Manca Nicola Poster 69

Manca Nicola Oral 43

Mannhart Jochen Plenary talk 6

Mantovan Roberto Poster 73

Maritato Luigi Poster 65

Martinelli Alberto Poster 87

Mazzoli Claudio Oral 36

Modreanu Mircea Oral 29

Moia Fabio Poster 62

Noce Canio Poster 95

Orgiani Pasquale Oral 44

Parlato Loredana Oral 31

Pecchio Paola Poster 105

Pepe Giovanni Piero Oral 14

Petti Daniela Oral 16

Plekhanov Evgeny Poster 82

Polichetti Massimiliano Poster 103

Prando Giacomo Oral+Poster 13

Prando Giacomo Poster 91

Puri Alessandro Oral 51

Radaelli Greta Oral 39

Radaelli Greta Poster 86

Riedo Elisa Oral 35

Rinaldi Christian Poster 71

Russo Roberto Oral 15

Sala Alberto Oral 8

Salluzzo Marco Oral+Poster 45

Sangaletti Luigi Oral 27

110

Sanna Samuele Poster 97

Sanna Samuele Oral+Poster 52

Sellmann Jan Oral 37

Sigrist Manfred Plenary talk 4

Sola Alessandro Oral+Poster 53

Stein Wolfgang Poster 55

Stroppa Alessandro Poster 85

Telesio Francesca Poster 68

Torelli Piero Oral 34

Vobornik Ivana Poster 63

Wu Sheng Poster 78

Yates Karen Poster 75

Yates Karen Oral+Poster 10