Ph. D. in Physics: Cycle XXIX

Ph. D. student: Marco BilardelloSupervisor: Angelo Bassi

Temporary title of the research project: Continuous spontaneous wave function collapse models: phenomenological analysis.

Continuous spontaneous wave function collapse models provide a dynamics which is different from the Schroedinger dynamics. For that reason their physical predictions are different, although slightly, from the physical predictions of quantum mechanics.

Among the several ways to test these models – and in general every model which breaks the linearity of Schroedinger equation – experiments involving quantum superposition of a large number of particles/atoms/molecules play a leading role. The most important experiments of this type are:

• Bose Einstein condensations• Interferometry involving macromolecules • Josephson junctions and SQUIDs• Superconductors• Superfluids • Oscillating microresonators• Entanglement involving macroscopic structures (for example,

diamonds)

The following methodology will be used for every type of experiment:

1. Theoretical analysis of the experiment2. Theoretical prediction of collapse models about the experiment

outcome3. Comparison with experimental data: finding an upper bound of the

parameters of the collapse models

The results of the different experiments will be compared to achieve the better strategy to test continuous spontaneous wave function collapse models.

The Ph. D. student will study in deep several topics such as stochastic processes and stochastic differential equations, foundations of quantum mechanics, open quantum systems, approximative/numerical methods to solve differential equations. Moreover, the Ph. D. student will learn how to compare theoretical and experimental works in the scientific research. All of this – together with the research activity – will provide the Ph. D. student with solid basis for his future scientific career.

Doctorate School in PhysicsResearch proposal

Davide Curcio

The field in which I mean to bring forward the three years of research activ-ity is experimental Condensed Matter Physics, and in particular, Surface Science.

My PhD research proposal, which is partially intended as the follow-up of theresearch activity brought forward during my master thesis in Physics, is centeredon the study and the growth of graphene by using epitaxial growth techniques,and by using different hydrocarbon species as precursors. Ethylene will be used,as will Propylene and more complex polycyclic hydrocarbons. There are twomain goals which we mean to achieve: on one hand, me mean to synthesise,exploring various techniques, single layers of graphene of high structural quality,that is, with the minimum possible defect density (single and double vacancies,domain boundaries, or contaminants). On the other, we mean to manipulatesome chemical and physical properties of this allotropic form of carbon throughthe control of its interaction with the substrate and its functionalization withdifferent atomic species.

Ever since graphene’s discovery in 2004, for which K. Novoselov and A. Geimwere awarded the Physics Nobel Prize in 2010, this material has attracted an evergrowing scientific interest. Its stability at high temperatures, the high thermaland electrical conductivity [1], as the electronic structure’s sensibility to interac-tion with the substrate, make graphene one of the most promising materials fromthe point of view of the possible applications in various sectors of nanotechnol-ogy. However, to this day, a method for the mass production of single layered,high quality graphene for industrial use is lacking. Of the many techniques re-cently developed for its synthesis, epitaxial growth on metallic surfaces seemsto be the most promising. The so obtained graphene, in fact, differs from freestanding graphene both in the electronic and morphologic properties, which canbe directly influenced by the substrate on which it is grown. A careful choice andmanipulation of the substrate allows one to finely tune this material’s properties.

Among the main objectives it is important to mention the possibility ofopening a gap in the density of states at the Fermi level (essential for itsemployment in nanoelectronic devices) and the possibility of using graphene asa template for the growth of nanostructures [2].

During my Graduate Course in Physics, I plan to use different substratesfor the epitaxial growth of graphene. This is useful for probing the effects of

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interaction with the substrate on its properties. As an example, we mean to usesingle crystals of transition elements (like W, Co, Ta), and compare the propertiesof these layers with those of graphene grown on Platinum group metal surfaceslike Ir, Ru, Rh or Pt, on which it is possible to grow extremely high qualitygraphene. It is our interest to use both single crystal transition metals with lowMiller index for the creation of a standard, and stepped surfaces of these samemetals, with the aim of examining the growth mechanisms at an atomic level [3],and of inducing and controlling with precision the corrugation in graphene whichis known to influence the electronic properties [4].

Moreover, we plan to also use bimetallic alloys, which offer the possibility ofgrowing high quality graphene, and afterwards to intercalete oxygen underneaththe graphene in order to grow an isolating oxide layer between the substrate andgraphene. This allows for a decoupling of the substrate from graphene, makingit possible to modify its electronic properties.

Another objective is to use quasicrystals as template for the growth ofgraphene. Quasicrystals have been given a lot of attention and controversy sincetheir dicovery. This class of solids is, to this day, largely unexplored because ofthe significant difficulties both in the theoretical description (numerical meth-ods depend heavily on the presence of periodicity, which quasicrystals lack), andin the experimental field, because of technical difficulties which render samplepreparation a challenge. Taking account of some papers which predict the ex-istence of a gap in the electronic states of bidimensional quasicrystals, due tothe specific geometry of the aperiodic structure [5], we will try to grow grapheneon the surface of different quasicrystal surfaces, with the objective of creating aquasiperiodic order (and hence a gap) in the structure of graphene. As alreadymentioned previously, this could solve one of the greatest obstacles in the appli-cation of graphene in the semiconductor industry, since for the greatest part ofthe applications in devices a gap in the semiconductor is a prerequisite. In thecase of graphene, the gap is present, although it is degenerate and has no width.A large portion of research on graphene has the aim of creating a gap in thissemiconductor, and commonly used approaches include functionalization withother atomic species [6], or the use of graphene nanoribbons [7]. Using aperiodicorder for the same purpose is a completely novel and unexplored approach.

In parallel with the comparative approach with different substrates, differentprecursors will be used for the synthesisation of graphene. In addition to usingsimple hydrocarbons commonly employed for chemical vapor deposition [8]like ethylene or propylene, polycyclic hydrocarbons (like pentacene, coronene,rubrene, ecc.) will be used to control and tune in a very fine manner thenucleation process and the structural properties of graphene.

A further objective is to use carbon nanoclusters in very controlled conditions(being sensible to the number of atoms they are made up of) as precursorsof graphene. In fact, during my PhD courses, I will have the opportunity ofparticipating in the development of a mass-selected nanocluster source, presentlyunder development at the research center Elettra - Sincrotrone Trieste. This will

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Figure 1: Nanocluster source which is presently under development. It will be ableto generate carbon and metal nanuclusters, by ablation of an appropriate target witha Nd:YAG laser and by supersonic expansion in He atmosphere of the resulting metalvapors.

be able to generate, among other things, fullerene molecules of different sizes.The nanocluster source (Fig. 1) represents in itself an opportunity to study nan-oclusters, which are currently the subject of cutting edge research in condensedmatter physics. One of the primary objectives of nanotechnology resides in factin the understanding, the control and manipulation of objects with intermediatesizes, in between single atoms and bulk solids. Among these, nanoclustersmade up of a few tens or hundreds of atoms have a prominent importance inthis field. Recent studies have shown how small (tens or hundreds of atoms)nanocluster properties actually depend on the exact number of atoms, and theircomposition and nucleation process also have a great influence on these clusters’properties [9]. In fact, thank to the large surface to volume ratio, they havechemical and physical properties which greatly depend on their size. Our doubleobjective is to use the nanocluster source both for the controlled depositionof carbon aggregates and for the growth of metal and metal alloy nanoclus-ters in order to deposit them on graphene and manipulate its electronic structure.

Moreover, we will try to control and tune the electronic properties ofgraphene by doping it with other atomic species, in particular single atoms oflight elements like O or N introduced in the crystal structure of graphene duringthe epitaxial growth by using a plasma source, and of transition metals obtainedwith the nanocluster source.

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The research activity I mean to bring forward in the three PhD years isgoing to take advantage of various cutting edge experimental techniques, inparticular photoemission spectroscopy (XPS) (both from a conventional sourcesand from synchrotron radiation), photoelectron diffraction (XPD) and of lowenergy electrons (LEED, SPA-LEED) and programmed desorption spectroscopy(TPD). This activity will be carried out at the Surface Science Laboratoryand at the beamlines of the synchrotron facility Elettra in Trieste, under thesupervision of the prof. Alessandro Baraldi. In fact, I will have the chanceto work both at the Surface Science Laboratory, where the nanocluster sourcewill be commissioned, and at the SuperESCA beamline where synchrotronlight is used to carry out photoemission experiments. Furthermore, thanksto international collaborations, it will be possible to carry out angle resolvedphotoelectron spectroscopy measurements at other synchrotron facilities inorder to aid the comprehension of the electronic structures of the examinedsystems.

From Table 1 one can see how I mean to temporally organise the aforemen-tioned activities. In parallel with the work on the nanocluster source, necessaryfor many of the planned experiments, we mean to proceed with the synthesisof graphene on different metal surfaces. We mean to use at first single crystaltransition metal surfaces, to employ at a later time bimetallic alloys and qua-sicrystals. The project plans to use in a first phase simple hydrocarbons, and ina later phase polycyclic hydrocarbons and carbon nanoclusters for the synthesisof graphene.

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References

[1] AK Geim and KS Novoselov. The rise of graphene. Nature Materials,6(3):183–191, 2007.

[2] AT N’Diaye, S Bleikamp, PJ Feibelman, and T Michely. Two-dimensional Ircluster lattice on a graphene moire on Ir(111). Phys. Rev. Lett., 97:215501,Nov 2006.

[3] H Chen, W Zhu, and Z Zhang. Contrasting behavior of carbon nucleationin the initial stages of graphene epitaxial growth on stepped metal surfaces.Phys. Rev. Lett., 104:186101, 2010.

[4] I Srut, VM Trontl, P Pervan, and M Kralj. Temperature dependence ofgraphene growth on a stepped iridium surface. Carbon, 56(0):193 – 200,2013.

[5] RF Sabiryanov and SK Bose. The decagonal plane-wave model for 2d and3d quasicrystals. Journal of Physics: Condensed Matter, 6(31):6197, 1994.

[6] V Georgakilas, M Otyepka, AB Bourlinos, V Chandra, N Kim, KC Kemp,P Hobza, R Zboril, and KS Kim. Functionalization of graphene: Covalentand non-covalent approaches, derivatives and applications. Chemical Reviews,112(11):6156–6214, 2012.

[7] F Schwierz. Graphene transistors. Nature Nanotechnology, 5(7):487–496,2010.

[8] M Batzill. The surface science of graphene: Metal interfaces, {CVD} syn-thesis, nanoribbons, chemical modifications, and defects. Surface ScienceReports, 67(3–4):83 – 115, 2012.

[9] F Baletto and R Ferrando. Structural properties of nanoclusters: Energetic,thermodynamic, and kinetic effects. Rev. Mod. Phys., 77:371–423, 2005.

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Programmed)activities

1st )year,)1s

t )sem

ester

1st )year,)2n

d )sem

ester

2nd )year,)1s

t )sem

ester

2nd )year,)2n

d )sem

ester

3rd )year,)1s

t )sem

ester

3rd year,)2n

d )sem

ester

Graphene)(different)substrates)Graphene)growth)in)single)crystal)transition)metal)surfaces)with)low)Miller)indexGraphene)growth)in)single)crystal)transition)metal)stepped)surfaces

Graphene)growth)on)bimetallic)alloy)surfaces

Graphene)growth)on)quasicrystal)surfaces

Graphene)(different)precursors)Graphene)growth)using)simple)hydrocarbons)(Ethylene,)Propylene)Graphene)growth)using)polyaromatic)hydrocarbons)(coronene,)pentacene)Nanocluster)SourceAssemply)of)mechanical)and)optical)components)of)the)source,)machine)setupCarbon)deposition)on)low)Miller)index)surfaces)))))))))))))

Deposition)of)carbon)and)metals)on)stepped)surfacesDeposition)of)carbon)and)metals)on)bimetallic)and)quasicrystal)surfaces

Table 1: Gantt chart for the research proposal.

Trieste, January 22, 2014 Davide Curcio

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Scuola Dottorato Fisica

N DOTTORANDO Supervisor

1 CURCIO Davide FIS/03

2 RANDI Francesco/FSE FIS/03

3 TOROŠ Marko FIS/02

4 ROMELLI Erik FIS/05

5 STERZI Andrea FIS/03

6 MANZONI Giulia

7 BILARDELLO Marco FIS/02

8 MONGARDI Chiara FIS/05

9 VINCENZO Fiorenzo FIS/05

Research projectGraduate program in physics

University of Trieste

Student: Francesco Randi

Non-equilibrium superconductivity

Ultrashort light pulses enable us to potentially control properties of materials on sub-

picosecond timescales. By shining ultrashort light pulses on a material (e.g. in pump-probe

experiments), we can excite states on timescales much shorter than their characteristic

relaxation times. In particular, it is possible to photoinduce phase transformations or other

highly off-equilibrium states not attainable through quasistatic adiabatic transformations.

In the standard pump-probe experiment the excitation is done by means of high photon-

energy light pulses (>1 eV), which 'photoinject' high energy electronic excitations in the

material. In this configuration the 'injection' of energy into the low energy degrees of freedom

(vibrational, magnetic, …) which are relevant for thermodynamical phase transitions is left to

their intrinsic coupling to the excited electrons. Therefore, the triggering of phase transitions

by means of high energy optical excitation can be achieved only indirectly. Hence, the phase

transition and the equilibrium recovery are limited, both in timescales and efficiency, by

thermodynamical restrictions.

The key to bypass this limitation is the production of low photon-energy (1 – 100 meV) intense

ultrashort light pulses to be used for the excitation in pump-probe experiments. Such pulses

allow for direct electric-dipole coupling to the vibrational degrees of freedom in the material

and hence for the photo-injection of low energy excitations in the system. This kind of

interaction, being linear and resonant, 'prepares' the excited sub-system (e.g. a given

vibrational mode or other quasi-particle modes) in a quantum mechanically coherent state [1].

In pioneering works following this approach, it has recently been shown that the optical

control of, e.g., the vibrational state of the system could be used to change its electronic

ground state and thereby to control metallicity [2] or, eventually, superconductivity [3].

Moreover, producing phase-stable infrared pump pulses (i.e. whose electric field's phase is

stable with respect to its intensity envelope from pulse to pulse) it is possible to study the

evolution of the system as a function of amplitude and phase the e.m. field that is driving the

excitation.

Transition metal oxides are the ideal playground to achieve such an optical control. It has

been shown that the excitation with infrared light pulses can lead to phase transformations in

these compounds, and different metastable phases have been reported: non-thermal

structural phases in cuprates [4], insulator-to-metal phase transitions in manganites and

vanadium based oxides [5][6][7] and non-thermal suppression of superconductivity [8][9][10]

[11].

The response of the material to the excitation can be studied in two different ways. Most

basically, the variation of its reflectivity at a given wavelength can be measured using a quasi-

monochromatic probe pulse. However, the usage of broadband probe pulses to study the

evolution of the properties of the material over a whole spectral region gives access to the

direct calculation of the time dependent optical conductivity, a quantity that can be easily

compared with the results of theoretical calculations [12].

[1] Ferraro et al., arXiv:quant-ph/0503237

[2] Rini et al., Nature 449, 72 (2007)

[3] Fausti et al., Science 14, 2011: 189-191

[4] Gedik et al., Science 316, 425 (2007)

[5] Ehrke et al., PRL 106, 217401 (2011)

[6] Ichikawa et al., Nature Materials 10, 101 (2011)

[7] Cavalleri et al., Phys. Rev. Lett. 87, 237401 (2001)

[8] Perfetti et al., Phys. Rev. Lett. 97, 067402 (2006)

[9] Kusar et al., Phys. Rev. Lett. 101, 227001 (2008)

[10] Giannetti et al., Phys. Rev. B 79, 224502 (2009)

[11] Coslovich et al., Phys. Rev. B 83, 064519 (2011)

[12] Capone et al., Science 296, 2364 (2002)

Activity

The key point of such an approach is the generation of intense low photon-energy ultrashort

light pulses. Far infrared (1 to 8 meV) ultrashort light pulses can be generated exploiting the

tilted wave front generation scheme [13] in LiNbO3, while mid-infrared pulses (40 to 500 meV)

can be obtained by difference frequency generation of the outputs of two twin optical

parametric amplifiers [14]. Being the two twin optical parametric amplifiers seeded with the

same pulse, the difference frequency-pulse produced will be phase-stable and will allow the

investigations mentioned above.

The activity of the project will be divided in three main work-packages (WP).

WP1 Development of the spectroscopic tools: WP1 has the target of developing

the two twin optical parametric amplifiers and the difference frequency generation for the

production of infrared light pulses to be used in experiments and combining it with broadband

white light probes. I will also develop an improved acquisition system for the measurements of

quasi-monochromatic probe pulses. It will allow pulse per pulse measurements to be

performed and hence statistically relevant data to be acquired. This means higher quality

measurements, better noise handling and in particular, reliable noise analysis for the

extraction of information on the quantum states prepared in the material.

WP2 Experimental activities: The experimental activities will focus on the studying

photoexcitation processes of superconductors with e.m. Fields close to the superconducting

gap. I will study the 'incoherent' evolution of the optical conductivity and in a second time the

coherent effects, mapping the response to the amplitude and phase of the e.m. field.

WP3 Development of the theoretical models: The theoretical models will be

developed using dynamical mean field theory methods and density matrix formalism in

collaboration with theoretical groups.

[13] Hebling et al., J. Opt. Soc. Am. B 25, B6–B19 (2008)

[14] Kaindl, JOSA Vol. 17, Issue 12, Dec. 2000, (2086--2094)

Università degli Studi di Trieste Scuola di Dottorato di Ricerca in Fisica, XXIX Ciclo

Giulia Manzoni

Research Project

“TR-ARPES studies on the nature of polarons in selected materials”

The study of the ground state properties of systems at equilibrium has represented the key issue of the scientific community for a long time. In the last two decades, growing attention has been devoted to the study of the non-equilibrium dynamics of materials impulsively brought out of equilibrium. This condition can be achieved by impulsively injecting a finite amount of energy into the system, by means of ultrashort (~ 100 fs) light pulses that perturb the electronic and phononic population. The pump and probe technique is an experimental technique that allows to collect information about the out-of-equilibrium behaviour of a material. Using a laser beam that is splitted into two parts, the sample is brought out-of-equilibrium by the absorption of the pump beam, which is the most intense. Indeed, after photo-excitation, a material starts recovering its equilibrium state through different relaxation channels with different and characteristic relaxation times: for example electron - electron scattering (few femstosecond, fs), electron - phonon scattering (few picosecond, ps), phonon - phonon anharmonic interaction (several ps), diffusion (nanosecond, ns). Therefore, information on the timescale of these processes is achievable only through non equilibrium spectroscopy, that has been demonstrated to be effective in disentangling the contribution of the various scattering mechanisms, and in providing a deeper understanding of their physical origin. During my PhD activity I am going to focus on both Angle Resolved Photo-Emission Spectroscopy (ARPES) and Time Resolved and Angle Resolved Photo-Emission Spectroscopy (Tr-ARPES). ARPES is the most direct method for studying the electronic band structure of a solid. The technique is based on the photoelectric effects. It consists in illuminating a sample with sufficiently high photon energy radiation in order to photo-emit electrons, after light absorption, as free particles. By measuring their kinetic energy and their angular distribution, information on both the energy and momentum of the electrons inside the material can be recovered.[1] Tr-ARPES is a pump and probe spectroscopy which allows to directly map the transient modification of electronic band structure ( function of momentum and energy) for different time delays after photo-excitation. Moreover, this technique provide access to the non-equilibrium carrier distribution and allows to measure the band occupancy at all energies and times.

Figure 1: Principle of ARPES. Taken from en.wikipedia.org

It is known that the band occupancy close to the Fermi level is described, in a metal, by a Fermi Dirac distribution that is function of the chemical potential and of the temperature of the electrons. The latter quantifies the average energy in the electronic bath. After the absorption of the pump beam the electrons are excited at very high energies. Then, they scatter through electron-electron scattering event on a very short time scale. This leads to the electronic thermalization that gives a new distribution with an elevated temperature. By studying how the temperature of the electronic bath relaxes with time, one can obtain information on the flow of energy from the electronic bath to the other degrees of freedom, for example the lattice phonons. The larger is the coupling between electrons and phonons, the faster is the process. In solids, electron-phonon interaction is well known to affect the electronic band dispersion in proximity of the Fermi level, in an energy range comparable with the phonon energies. In the case of weak coupling constant, the interaction results in a band renormalization and in a remarkable change in the electron velocity approaching the Fermi level (kink [2]). When the electron-phonons coupling is strong, it may lead to large lattice distortion. These lattice distortions are responsible for the formation of exotic ground state, such as the Charge Density Waves (CDW)[3][4] and polarons. The latter are quasi-particles describing electrons self-trapped in the lattice distortion caused by their motion in the solid. The mobility of the electrons is largely reduced, because the electrons is dressed with the phonons describing the distortion, and this results in a large effective mass, in the suppression of the spectral weight of the quasi-particle peak at the Fermi level and in peculiar temperature related features of the electronic transport properties (resistivity, conductivity). Polarons can be classified in small polarons and large polarons as the radius of the polaron changes in comparison to the unit cell size (a radius larger than the unit cell size corresponds to a large polaron and viceversa).[5] The physics of polarons is common to many materials like cuprates and manganites, semiconductors and insulators. Studies about polaron-related dynamics with TR-ARPES (and in general out-of-equilibrium techniques) are still lacking. The timescale of this kind of phenomena well matches with the temporal resolution of the femtosecond time-resolved techniques. The first candidate material for the study of the out-of-equilibrium dynamics of the polaronic bound states will be TiO2. At low temperatures, TiO2 presents an anomaly in its electronic transport properties with a remarkable increase of the resistivity. This has been recently proposed to be the fingerprint that the charge carriers are not bare electrons but polarons. Understanding the properties of such composite particles in TiO2 is important to better engineer the material for targeted applications. In particular, the low electron mobility often represents the overall performance bottleneck. A lot of works have been focused on its photo-catalytic behaviour and several devices, such as transparent conducting electrodes and photovoltaic cells. Nevertheless, in this respect, a detailed study of the TiO2 electronic properties, and the electron-phonon coupling is still missing. In particular, the anatase structural phase of TiO2 has been recently suggested as a candidate for replacing the Indium-based technology for transparent metals, in a wide range of applications from solar cells, to light-emitting devices, to flat panels, to touch-screen controls.[6] Similarly, also in HfTe5 an anomalous increase of the resistivity has been experimentally observed at low temperature, but a fully theoretical description is not available even thogh a possible scenario is the one where polaron formation takes place at low temperature, thus explaining the change in the electronic transport properties..

Summarizing, the main research theme of my research project will be the study of the nature of polarons in selected materials (starting from TiO2 and HfTe5), by means of Time-Resolved ARPES. By following in real time how the features revealing polaron formation in the band structure are affected after the system is brought out-of-equilibrium by an ultrashort light pulse, and by measuring the timescale on which polaron features relax back to equilibrium, I will extract novel information on the coupling strength of the polaron bound state. The topic of my PhD project will be developed in the frame of the T-Rex laboratory at the Elettra synchrotron, Trieste. Professor F. Parmigiani will be my supervisor. References [1] Damascelli A., Probing the electronic structure of complex systems by state-of-the-art ARPES. Phys. Scripta, T109, 61 (2004). [2] Bostwick A. et al., Quasi particle dynamics in grapheme, Nature Physics 3, 36-40, (2007). [3] Perfetti L. et al., Broken symmetries and photoexcitation of 1T-TaS2, Journ. Of Phys., 148, (2009). [4] Petersen J.C. et al., Clocking the melting transition of charge and lattice order in 1T-TaS2 with ultrafast extreme violet angle-resolved photoemission spectroscopy, Phys. Rev. Lett., 107, 177402 (2011). [5] Fox M., Optical Properties of Solids, Oxford Master Series in Physics, (2010). [6] Moser S. et al., Tunable polaronic conduction in anatase TiO2, Phys.Rev.Lett., 110, 196403 (2013). [7] Perfetti L., Spectroscopic Indications of Polaronic Carriers in the Quasi-One-Dimensional Conductor (TaSe4)2I, Phys. Rev. Lett., 21, (2001). [8] Tournier-Colletta C. et al., Electronic Instability in a Zero-Gap Semiconductor: The Charge-Density Wave in (TaSe4)2I, Phys. Rev. Lett., 110, 236401 (2013). [9] Ranninger J. et al., Two-site polaron problem: Electronic and vibrational properties, Phys. Rev. B, 14, (1992). [10] Hufner S., Photoelectron Spectroscopy, Principles and applications, Springer, (2003).

PHD PROJECT for Chiara Mongardi

Title: The galaxy/Intergalactic Medium (IGM) interplay in the low and high-redshift UniverseSupervisors: Dr. Valentina D'Odorico, Dr. Matteo Viel (INAF-OATS Staff Researchers)

The main goal of this PhD project is to explore and characterize the properties of the intergalactic medium around galaxies in the low and high-redshift Universe, in order to shed light on the life-cycle of gas and the processes of feedback and metal enrichment.

The methodology adopted will be based on:• analysis of hydrodynamical simulations and post-processing of the relevant physical quantities

in a way which will be as close as possible to the procedure adopted with real data;• comprehensive comparison of hydrodynamical results with state-of-the-art data sets, mainly

spectra of quasars obtained both at low redshift (<0.5) with the COS spectrograph onboard HST and at high redshift (~2-3) obtained with high and intermediate resolution spectrographs, in particular UVES and X-shooter at the VLT and HIRES at the Keck Telescope;

• comparison of the absorption systems observed in the QSO spectra and the position of galaxies in the field, when available.

Part of the simulations at high resolution are already in place and both public and proprietary data sets are available for this study. Subgrid physical modelling will be needed in some cases in order to address some properties of the galaxies.

More precisely, we will use observations and simulations to investigate the physical and chemical properties of the IGM adopting several approaches like the “pixel optical depth” technique, the correlation of metal absorption lines and the cross-correlation of absorption lines and galaxies, the detailed analysis of metal absorption systems to derive constraints on ionizing background and star formation history. The PhD student will have the possibility to compare the simulated results with the data from an X-Shooter Large Program, in which we are involved, that is observing the spectra of 100 quasars at z>3.5 and with ~100 QSO spectra taken with COS/HST at 0 < z< 0.4. She will also be involved in the writing of observational proposals to gather more data for her research work (e.g., to follow up at higher resolution the most interesting quasars). The project is meant also as a preparatory work to shape the IGM science cases for the next generation of high resolution spectrographs, ESPRESSO at the VLT and HIRES at the E-ELT. Thanks to the use of simulations with different implementations of feedback mechanisms and a detailed treatment of the physics of the IGM, it will be possible to identify the most constraining observables to be derived from the future observations.

Visiting period outside Italy: It is foreseen that the PhD student will spend a period up to 8 months at the Institute of Astronomy in Cambridge (UK) to work with Prof. Maiolino and Prof. M. G. Haehnelt.

PhD School – XIX cycle – Trieste University

PhD student: Erik RomelliSupervisor: dott. Anna Gregorio

Title: Performance simulations of the on-board instrumentation of the Euclid mission

The proposed PhD project foresees to contribute to the full simulation process of the

performance of the on-board instrumentation of the Euclid mission. The aim of this

simulator is to simulate an observation and produce instrument telemetry data, which

can be processed as if these were real Euclid data.

Euclid is a medium-class ("M-class") mission and is part of ESA's (European Space Agency)

"Cosmic Vision" (2015–2025) scientific program. Euclid is a mission to map the geometry

of the dark Universe. The mission will investigate the distance-redshift relationship and

the evolution of cosmic structures by measuring shapes and redshifts of galaxies and

clusters of galaxies out to redshifts ~2, or equivalently to a look-back time of 10 billion

years. In this way, Euclid will cover the entire period over which dark energy played a

significant role in accelerating the expansion.

Euclid comprises a 1.2m diameter telescope and two scientific instruments: a

VISible-wavelength camera (VIS) and a Near-Infrared camera/SPectrometer (NISP).

The instrument performance is traditionally the result of specifications based on

scientific requirements and can only be confirmed by Instrument Level Tests. Euclid is a

high precision cosmology experiment dealing with a large statistical object sample

obtained from a sky survey.

ESA, in collaboration with the two instruments, is going to develop a simulator to test

the performance of the on-board instrumentation and in general the scientific capability

of Euclid. The performance simulation environment will include a number of software

modules that will include at least the simulated sky, an optical model of the telescope

and the detection system.

During my PhD period I will be involved in the design and implementation of the

simulated sky, with particular attention to the Interplanetary Dust Particles (IDPs)

dynamics in the Solar System and their thermal infrared emission, the Zodiacal Light

Emission (ZLE). These are important aspects dealing with the sky simulation, because the

ZLE signal is significant at the frequencies at which Euclid works.

A similar analysis has already started within the Planck team and needs to be finalized,

tested and validated on real data. During the first part of my PhD program I will focus on

the work performed on Planck, then the work will gradually shift to Euclid.

Universitá degli studi di TriesteScuola di Dottorato di Ricerca in Fisica, XXIX ciclo

Andrea Sterzi

Research Project in PhysicsProject Title: Tr-ARPES on Topological Insulators and SmB6

My research activity will be focused on the physics of the out of equilibrium electronic bandstructure of novel and exotic compounds displaying strong electron-electron correlations effect alongwith topologically protected surface states. In particular, the non-equilibrium properties will be ex-ploited by extensive time-resolved and angle resolved photoelectron spectroscopy (tr-ARPES). Thescope is to shed light on the nature of topologically protected surface states in the presence of strongelectron-electron correlation mechanisms.My PhD research project will be supervised by Prof. Fulvio Parmigiani with the help of Dr. AlbertoCrepaldi and Dr. Federico Cilento (Elettra Sincrotrone Trieste) as co-tutors. Most of the experi-mental activities will be carried on at the T-Rex laboratory1 -Elettra Sincrotrone Trieste-.

The Experimental Technique: tr-ARPES

The first evidence of a photoelectric effect was reported by Hertz2. This discovery had a pro-found impact on the modern physics along with important fallouts in spectroscopy. X-ray core levelphoto-emission (XPS), x-ray photo-electron diffraction (XPD) and angular resolved photo-electronspectroscopy (ARPES), are some examples. In particular ARPES can provide informations on theelectronic properties at equilibrium of an ordered solid by directly probing its band structure. In atypical ARPES experiment, (figure 1), a beam of monochromatized light (with photon energy rang-ing from the UV to the hard X-rays) is absorbed by solid inducing emission of photoelectrons. Byusing an electron energy analyzer (spectrometer in figure 1) the photoelectron kinetic energy Ekin

can be measured along the allowed directions and the electronic momentum K = h/ν in vacuum isfully determined by the modulus K =

√2mEkin/~, and the components K|| and K⊥ as a function

of the polar (θ) and azimutal (φ) emissions angles defined by the experimental geometry3. The k||component is conserved and it matches the one inside the solid, whereas the k⊥ component is notconserved and it must be evaluated by considering also the surface potential. As a result it is possi-ble to map the dispersion E(k), of a crystalline solid, i.e. the relation between the binding energyEb and the momentum k for electrons that propagate inside the material. In particular, from theoccupation of the electronic states in proximity of the Fermi level (EF ) it is possible to infer aboutthe characteristic electronic temperature, the photohole lifetime and the coherence length, hence toextract information on the electronic correlations.Aim of my PhD project is to go beyond the study of the equilibrium properties of materials, byinvestigating also the temporal evolution of photo excited electronic states. That is possible by em-ploying ultrafast laser sources and ad-hoc time-resolved techniquess. In particular, by exploiting thea pump-and-probe approach the tr-ARPES will provide a stroboscopic picture on the femtosecondtime-scale of the evolving excited states in the sample. In other word, the first pulse, (pump), ab-sorbed the sample it will excite the electrons in the system, while the second pulse (probe) "sees"the modification induced by the pump pulse.Combining the use of the pump-and-probe technique and angle-resolved photoemission experimentsit is possible directly monitor the effect of light excitation and measurng the characteristic decaytime of the excited states. In particular, I will measure the temporal evolution of the Fermi-Dirac(FD) distribution and looking for the the expected time evolution of the electronic band structure.

1Time Resolved X-Ray Spectroscopies,www.elettra.trieste.it/labs/t-rex.html2Hertz observed that when light is incident on a sample an electron can absorb a photon and escape from the material

with a maximum kinetic energy Ekin = hν − φ ( where ν is the photon frequency and φ the material work function) and laterinterpreted as a quantum interaction process by Einstein, who was awarded the Noble Prize in Physics in 1921.

3Only the the K|| (K parallel) component is conserved when an electron escapes from the surface of a solid.

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Figure 1: Basic concept and geometry of an ARPES experiment. Image taken from: http://newscenter.lbl.gov/feature-stories/2011/08/24/harpes/

Topological insulators and SmB6

Three dimensional Topological Insulators (TIs) are materials characterized by insulating bulk and asurface conductive electronic structure [2]. The most fascinating aspect of these systems arises fromthe metallic surface states, in particular the characteristic spin texture [5]. The existence of thesemetallic states is not barely a surface property, but it is a consequence of the topology of the bulkband structure. The hallmark of a TI is the linearly dispersing surface state (SS) which is guaranteedto disperse across the band gap separating the bulk valence band (VB) from the conduction band(CB). In addition to this so-called topological protection, the SS has a chiral spin texture whichprovides the electrons protection against backscattering and has great appeal for spintronics appli-cations.A number of interesting phenomena have been proposed to arise from optical coupling to TIs, suchas colossal Kerr rotation, divergent photon absorption, spin transport, and a long-lived SS popula-tion [2]. Therefore, TIs’ properties make these system natural candidates for a new generation ofspintronics and opto-spintronics devices. In this perspective a detailed knowledge of the dynamicalresponse of TIs after optical excitation is needed. Theoretical calculations of conduction electronicstates have already been proposed, but not yet confirmed by experiments.A recent experimental work [6] has shown that tr-ARPES may have a key role in the study of thesesystems, thanks to the combined ability to map the electronic states, both at equilibrium and afteroptical perturbation, in k-space and thanks to its surface sensitivity.Among recent discoveries concerning TIs, one is represented by a new class of materials called KondoInsulators (KIs). This new class of topological insulators has attracted enormous scientific interestbecause it represents the first case of metallic spin polarized surface state dispersing in a gap whichstems from the strong electronic correlations (Kondo effect) [8]. In particular, interest in SmB6 hasbeen recently rekindled since band structure calculations and transport measurements have led to theproposition that electronic states with topological character might exist within the Kondo hybridiza-tion gap, and be responsible for the non-divergent behavior of the resistivity at low temperatures.If this scenario is correct, SmB6 will be the first topological Kondo insulators (TKI). Furthermore,SmB6 represents a very promising alternative to the Bi-based 3D topological insulators (TI’s) [8].Nevertheless, the topologically protected character of the surface state is still subject of intense ex-perimental studies, and so far conflicting results have been reported in the literature [6][7][8].

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The tr-ARPES will give me the opportunity to probe the unoccupied electronic structure above4

the fermi level (EF ) in order to search for new energy band gaps where surface state with topologicalcharacter may disperse. Moreover, by analyzing the relaxation dynamics of excited photoelectronsout of equilibrium, it will be possible to infer about important information on the d -like and f -likenature of the states at EF and possibly to measure their relaxation times. Scattering processes,fundamental for transport properties and technological application, will be finally investigated.

References

[1] A.Damascelli, Probing the Electronic Structure of Complex System by ARPES, Physica Scripta,Vol. T109, 61-74, (2004)

[2] J.A.Sobota et al. Direct Optical Coupling to an Unoccupied Dirac Surface State in the TopologicalInsulator Bi2Se3,(2013)

[3] A.Crepaldi, Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particlesin Bi2Se3, Physical Rev.B 86,205133 (2012).

[4] L.Perfetti et al. Femtosecond dynamics of electronic states in the Mott insulator 1T-TaS2 bytime resolved photoelectron spectroscopy, New Journal of Physics 10 (2008) 053019

[5] P.Hofman, Synchrotron-radiation studies of topological insulators, arXiv: 1210.2672, V 2, (2013)

[6] ,M. Hajlaoui,Ultrafast Surface Carrier Dynamics in the Topological Insulator Bi2Te3, Nanolett.,12, 3532

[7] M.Neupane, Surface electronic structure of the topological Kondo insulator candidate correlatedelectron system SmB6, arXiv: 1312.1979, v2

[8] E.Frantzeskakis,Kondo hybridization and the origin of metallic states at the (001) surface ofSmB6, arXiv:1308-0151,v2

4Conventional ARPES measurements are based on one-photon photoemission (1PPE), therefore conceptually 1PPE canmeasure only occupied states while 2PPE ( two-photons photoemission) grants access to unoccupied states. We want to employboth processes in order to be able to investigate under and upper the fermi level EF .

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Ph.D. in Physics: XXIX cycle

Ph.D. student: Marko Toros Supervisor: Angelo Bassi

Temporary title of the research project: Spontaneous wave function collapse models: origin of the noise.

Spontaneous collapse models have been devised to solve the measurement problem of quantum mechanics, by making sure that the wave function of macroscopic object is always very well localized in space. This is achieved by coupling all physical systems to a noise field. The coupling is nonlinear. For microscopic system, the effect of the noise is negligible; therefore their quantum properties are preserved. On the other hand, an amplification mechanism is built in the model, according to which the effect of the noise scales with the size of the system. So, contrary to microscopic systems, macroscopic ones behave classically. The fact that the noise scales with the size, makes it appealing to link it to gravitational effects. This possibility was first taken into account by Penrose, and more recently by Adler. Here we want to explore the following problem. Given a quantum system with a minimal nonlinear anti-hermitian coupling to a noise: psi’(t) = [-iH + Aw(t)] psi(t) is it possible to derive – with some sort of coarse graining – the collapse equation? This, in more mathematical terms, is the conjecture put forward by Adler. If this proves to be correct, than the next natural step to take is to identify the noise w(t) with the fluctuations of the gravitational field. We will explore also this possibility. Last, we will work out in detail the consequences of the model thus developed, and check

that it agrees with all know facts, and where it departs from standard quantum predictions in an appreciable way. This is crucial in order to set up future experiments aiming at testing the model. The Ph.D. candidate will become familiar with quantum foundations, stochastic processes and stochastic differential equations, models of spontaneous wave function collapse, general relativity. He will acquire broad enough competences to eventually become an experienced researcher.