IMR Workshop 2014

September 30 (Tue)

Talk Time Author Title Chair

　 13:20-13:30 - Opening (M. Fujita)

M. Mori

1 13:30-13:55 S. Uchida Multilayer effect in high-Tc cuprates

2 13:55-14:20 S. Tajima Comparative study of the superconducting gap in electronic Raman scattering andARPES of Bi2Sr2 CaCu2Oz

3 14:20-14:45 K. M. Suzuki Impurity substitution effects on magnetic correlation in La2-xSrxCu1-yMyO4 (M =Fe, Al)

　14:45-15:05 - Break 　

4 15:05-15:30 Y. J. Uemura Converting FeAs superconductors into ferromagnetic semiconductors

Y. Koike5 15:30-15:55 T. Hanaguri Electronic state of FeSe studied by STM/STS

6 15:55-16:20 H. Mukuda Superconducting transition temperature andre-emergence of antiferromagnetic order in LaFe(As1-xPx)(O1-yFy)

　16:20-16:40 - Break 　

7 16:40-17:05 T. Sasagawa Crystal growth and anisotropic properties of various iron-based superconductors

T. Yoshida8 17:05-17:30 A. Iyo Recent discovery of new superconductors including pnictogen atoms

9 17:30-17:55 A. Fujimori ARPES studies of Fe pnictides in the antiferromagnetic-orthorhomic phaseand the superconducting phase

　18:30-20:30 - Banquet (T. Yoshida) 　

October 1 (Wed)

　Time Author Title Chiar

10 9:00-9:25 T. Arima Magnetic-field dependence of directional dichroism in CuB2O4

S. Shamoto11 9:25-9:50 Y. Taguchi Combining multiple degrees of freedom to enhance magnetocaloric effect

12 9:50-10:15 M. Azuma Pb2+/4+ charge glass and intermetallic charge transfer in PbCrO3

　10:15-10:35 - Break 　

13 10:35-11:10 U. Bovensiepen Non-equilibrium electronic structure of transient, laser-excited states in Bi-2212

J. Mizuki

14 11:00-11:25 H. Okamoto Ultrafast photoinduced transitions to metallic states in half-filled Mott insulators

15 11:25-11:50 H. Wadati Ultrafast dynamics studied by time-resolved x-ray diffraction

16 11:50-12:15 T. Tohyama Nonequilibrium electron dynamics in strongly correlated electron systems

　12:15-13:50 - Lunch 　

17 13:50-14:15 M. Greven New insights into the cuprate phase diagram from neutron, X-ray and transport studies of HgBa2CuO4+δ

Y.J. Kim18 14:15-14:40 S. Wakimoto Neutron and resonant inelastic x-ray scattering study of magnetic excitations inhole-doped La2-xSrxCuO4

19 14:40-15:05 K. Ishii Spin and charge excitations in electron-doped cuprates

15:05-15:25 - Break 　

　

20 15:25-15:50 O. P. Sushkov Implications of resonant inelastic x-ray scattering data for theoretical models ofcuprates

T. Tohyama21 15:50-16:15 K. Kuroki Realistic band structure approaches to unconventional superconductors

22 16:15-16:40 M. Ogata Superconductivity and flux state in the Hubbard model

　16:40-17:00 - Break 　

23 17:00-17:25 H. Yamase Ising spin nematic fluctuations near spin-density-wave phase

M. Fujita24 17:25-17:50 A. Q. R. Baron Dynamical anomalies of high temperature superconductors

25 17:50-18:15 　 　

October 2 (Thu)

　Time Author Title Chair

26 9:00-9:25 H. Takagi Exotic magnetism produced by strong spin-orbit coupling in complex Ir oxides

H. Wadati27 9:25-9:50 Y. -J. Kim Spin, orbital, and spin-orbit excitations in iridates probed with RIXS

28 9:50-10:15 Y. Yamaji Emergent topological states in iridium oxides

　10:15-10:35 - Break 　

29 10:35-11:00 T. Yoshida Photoemission and inverse photoemission study of the correlated electron systemSrVO3

C. Ulrich　30 11:00-11:25 H. Kumigashira Unusual behavior of the subbands in strongly-correlated oxide quantum wellstructures

31 11:25-11:50 Y. Okada Imaging coherence of two dimensional electronic liquid on SrVO3 surface

　11:50-13:30 - Lunch 　

32 13:30-13:55 S. Seki Dynamics of magnetic skyrmions

O. P. Sushkov　33 13:55-14:20 Y. Nambu Electric-field driven motion of skyrmion lattices in the chiral magnet Cu2OSeO3

34 14:20-14:45 W. Koshibae A theoretical design of skyrmion device

　14:45-15:00 - Break 　

35 15:00-15:25 C. Ulrich Spin wave dispersion in the helical spin ordered system SrFeO3-δ and CaFeO3

K. Ishii36 15:25-15:50 Y. Kousaka Chiral magnetic soliton lattice in inorganic chiral materials

　15:50-16:00 - Closing (Ishii)

Magnetic-field dependence of directional dichroism in CuB2O4

T. Arima1,2, S. Toyoda1, N. Abe1, S. Kimura3

1Department of Advanced Materials Science, University of Tokyo, Kashiwa 270-8561, Japan2RIKEN Center for Emergent Matter Science, Wako 351-0198, Japan

3Institute for Materials Science, Tohoku University, Sendai 980-8577, Japan

Simultaneous breaking of space inversion and time reversal symmetries in a magnetic mattercan give rise to the linear magneto-electric coupling, i.e., magnetic (electric)-field induced electric(magnetic) polarization. Considering the linear response to alternative-current external fields, amatter with the linear magneto-electric effect is expected to exhibit novel optical responses. Whenan off-diagonal term of the tensor of the linear magneto-electric effect is non-zero, electromagneticwaves propagating in opposite directions may show different optical constants from each other.

It has been reported that copper metaborate CuB2O4 shows extraordinarily large directionaldichroism and directional birefringence in the canted antiferromagnetic phase between 9 K and 21K [1,2]. In this phase, S=1/2 moments on Cu2+ ions coordinated by oxygen squares are arranged inan antiferromagnetic way. One can regard each CuO4 cluster as a toroidal moment.

The optical absorption corresponding to the intratomic d-d excitation between xy and x2-y2

at Cu sites for the light beam of 1.404 eV with the oscillating electric field perpendicular to thec-axis changes by a factor of three with the reversal of the propagation vector. The amount of thedirectional dichroism is dominated by the magnetization direction and the propagation vector of thelight. From the microscopic point of view, however, the absorption intensity of the intratomic d-dexcitation may depend on the Cu-spin direction but not on the magnetization.

The spin directions in the canted antiferromagnetic phase are modified by the application ofa magnetic field. We performed a measurement of directional dichroism of CuB2O4 in magneticfields up to 15 T. We observed a splitting of the 1.404-eV absorption line, which was attributed tothe Zeeman splitting of the excited state. The g-values are estimated to be 2.8 and 2.2 for themagnetic fields parallel to the a- and c-axes, respectively. The directional dichroism at thehigher-lying peak of the doublet is enhanced by increasing the magnetic field. These results areexplained by assuming the spin-orbit coupling of about 100 meV at the Cu site coordinated by anoxygen square.

The optical absorption measurement in high magnetic fields was performed at High FieldLaboratory for Superconducting Materials, Institute for Materials Research, Tohoku University.

[1] M. Saito, K. Taniguchi, T. Arima, J. Phys. Soc. Jpn. 77, 013705 (2008).[2] M. Saito, K. Ishikawa, K. Taniguchi, T. Arima, Appl. Phys. Express 1, 121302 (2008).

Pb2+/4+ Charge Glass and Intermetallic Charge Transfer in PbCrO3

Runze Yu1, Masaichiro Mizumaki2, Tetsu Watanuki3, Takashi Mizokawa4, Kengo Oka1†, Hajime Hojo1, HyunjeongKim5, Akihiko Machida3, Kouji Sakaki5, Yumiko Nakamura5, Akane Agui3, Daisuke Mori6, Yoshiyuki Inaguma6, MartinSchlipf7‡, Konstantin Z. Rushchanskii7, Marjana Ležaić7, Masaaki Matsuda8, Jie Ma8, Stuart Calder8, Masahiko Isobe9§

and Masaki Azuma1

1Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, 226-8503,Japan

2Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan3Quantum Beam Science Center, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan

4Department of Complexity Science and Engineering, University of Tokyo, Kashiwa, Chiba 277-8561, Japan5National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki

305-8565, Japan6Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima, Tokyo, 171-8588, Japan

7Peter Grünberg Institut, Forschungszentrum Jülich, D-52425 Jülich and JARA, Germany8Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

9Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan †Present address: Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27

Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan‡Present address: Department of Computer Science, University of California Davis, Davis, CA 95616, USA

§Present address: Department of Quantum Materials, Max-Planck-Institute for Solid State Research, Heisenbergstr.1,D-70569 Stuttgart, Germany

Charge degree of freedom of transition metal ions gives rise to various fascinating propertiesof transition-metal compounds, such as superconductivity or magnetoresistance. The ordering ordisproportionation of charges in systems with integer or half integer charge number per atom, suchas Pr0.5Ca0.5MnO3

or CaFeO3, causes metal-insulator transitions. These can be regarded ascrystallization of charges. The insulating state tends to have a glassy nature when randomly locateddopants are introduced. A charge cluster glass state owing to geometric frustration withoutrandomness was recently found in an organic compound θ-(BEDT-TTF)2RbZn(SCN)4 and isattracting significant attention.

We report that the charge glass state is realized in a perovskite compound PbCrO3, whichhas been known for almost 50 years, without any obvious inhomogeneity or frustration. PbCrO3 hasa valence state of Pb2+

0.5Pb4+0.5Cr3+O3 with Pb2+-Pb4+ correlation length of three lattice-spacings at

ambient condition. A pressure induced melting of charge glass and simultaneous Pb-Cr chargetransfer causes insulator to metal transition and ~ 10% volume collapse in Pb2+Cr4+O3.

1) W. S. Xiao, D. Y.Tan, X. L. Xiong, J. Liu, and J. Xu, Proc. Natl. Acad. Sci. 107, 14026-14029 (2010).2) R. Yu et al., under review.

Dynamical Anomalies of High Temperature Superconductors

Alfred Q.R. Baron baron@spring8.or.jp

Materials Dynamics Laboratory, RIKEN SPring-8 Center, RIKEN

Research and Utilization Division, SPring-8/JASRI 1-1-1 Kouto, Sayo, Hyogo 679 JAPAN

The talk will present recent work regarding high-temperature superconductors, and related materials, with a focus on dynamical anomalies. Specific issues to be addressed include [1] the changing linewidth of the bond-stretching mode of optimally doped de-twinned YBa2Cu3O7-δ. (YBCO) with temperature. In particular the linewidth increases from 12 to 18meV in the a* direction and from 7 to 20 meV in the b* direction. Analysis suggests this is due to energy-localized electron-phonon coupling. We will also discuss [2] the observation of phonon modes in de-twinned samples of SrFe2As2 . Splitting of modes is clearly observed when magnetic order sets in, but while magnetic calculations give better agreement than non-magnetic calculations, they, in mot parts of the Brillouin zone measured, actually get the sign of the splitting wrong. We will report on progress to measure the Fermi-surface of optimally doped de-twinned YBCO using Compton scattering [3]. A brief report will also be given on the status of the new phonon instrument at SPring-8 [4] used in some of the work above. [1] A.Q.R. Baron, D. Ishikawa, H. Uchiyama, T. Fukuda, T. Masui, N. Murai, R. Heid, S. Miyasaka, S. Tajima, et al.,

In preparation. [2] N. Murai, T. Fukuda, H. Uchiyama. S. Tsutsui, D. Ishikawa, T. Kobayashi, H. Nakamura, M. Machida, M.

Nakajima, S. Miyasaka, S. Tajima and A. Q.R. Baron , work in progress. [3] T-H Chuang, K. Lee, M. Itou, Y Sakurai, S. Miyasaka, S Tajima and A.Q.R. Baron, work in progress. [4] A.Q.R. Baron, The RIKEN Quantum NanoDynamics Beamline (BL43LXU): The Next Generation for Inelastic X-

Ray Scattering, SPring-8 Information Newsletter 15 (2010) 14-19, http://user.spring8.or.jp/sp8info/?p=3138

Non-equilibrium electronic structure of transient, laser-excited states in Bi-2212

S. Freutel1, J. D. Rameau2, M. Ligges1, Y. Yoshida3, H. Eisaki3, G. D. Gu2, P. D. Johnson2, and U. Bovensiepen1

1University Duisburg-Essen, Faculty of Physics, 47048 Duisburg, Germany 2Brookhaven National Laboratory, Upton, New York 11973, USA

3National Institute of Adv. Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan

Optical excitations in solid materials typically decay on femto- to picosecond time scales

due to elementary interactions which lead to a redistribution of the excess energy among the electronic, the lattice, and the spin subsystem, before final dissipation. In femtosecond time-resolved experiments ultrafast dynamical changes are analyzed in order to shed light on the nature and the dynamics of the superconducting state in high-Tc materials. Femtosecond optical and THz spectroscopy are powerful methods here [1-3]. Recently, a transient dynamic coherence was reported even far above Tc [4]. Femtosecond time- and angle-resolved photoemission spectroscopy (tr-ARPES) [5] probes the electronic structure directly in the presence of optical excitations and opens new opportunities to investigate the influence of collective excitations on the single particle spectral function. Tr-ARPES has been employed to analyze the response of the electronic structure to optical excitations in optimally doped Bi-2212 below Tc [6,7]. In this talk, we discuss the transient electronic structure above Tc at 100 K. We analyze an increased electronic mass and quantify an effective hole photo-doping [8]. We further discuss changes of the relaxation times in the vicinity of the 70 meV kink energy above the Fermi energy originating from coupling to a Boson mode and recent results obtained for underdoped Bi-2212.

Figure 1: Schematic representation of the tr-ARPES experiment on cuprates (left) and transient changes of the ARPES intensity in the presence of the infrared optical excitation.

This work was supported by the Deutsche Forschungsgemeinschaft through Sfb 616 and BO 1823/2, by the Mercator Research Center Ruhr through through Grant No. PR-2011-0003, and by the European Union within the seventh Framework Program under Grant No. 280555 (GO FAST).

1) J. Demsar, B. Podobnik, V. V. Kabanov, Th. Wolf, D. Mihailovic, Phys. Rev. Lett. 82, 4918 (1999). 2) S. Dal Conte, C. Giannetti, G. Coslovich, F. Cilento, D. Bossini, T. Abebaw, F. Banfi, G. Ferrini, H. Eisaki,

M. Greven, A. Damascelli, D. Van Der Marel, F. Parmigiani. Sc ience 335, 1600 (2012). 3) R. Matsunaga, N. Tsuji, H. Fujita, A. Sugioka, K. Makise, Y. Uzawa, H. Terai, Z. Wang, H. Aoki,

R. Shimano, Science online, (2014), DOI: 10.1126/science.1254697. 4) W. Hu, S. Kaiser, D. Nicoletti, C. R. Hunt, I. Gierz, M. C. Hoffmann, M. Le Tacon, T. Loew, B. Keimer,

A. Cavalleri, Nature Materials 13, 705 (2014). 5) U. Bovensiepen, P. S. Kirchmann, Laser Photonics Rev. 6, 589 (2012). 6) R. Cortés, L. Rettig, Y. Yoshida, H. Eisaki, M. Wolf, U. Bovensiepen,

Phys. Rev. Lett. 107, 097002 (2011). 7) C. L. Smallwood, J. P. Hinton, C. Jozwiak, W. Zhang, J. D. Koralek, H. Eisaki, D.-H. Lee, J. Orenstein,

A. Lanzara, Science 336, 1137 (2012). 8) J. D. Rameau, S. Freutel, L. Rettig, I. Avigo, M. Ligges, Y. Yoshida, H. Eisaki, J. Schneeloch,

R. D. Zhong, Z. J. Xu, G. D. Gu, P. D. Johnson, U. Bovensiepen, Phys. Rev. 89, 115115 (2014).

ARPES studies of Fe pnictides in the antiferromagnetic-orthorhomic phase and the superconducting phase

Atsushi FujimoriDepartment of Physics, University of Tokyo, Tokyo 113-0033, Japan

Interplay between antiferromagnetism, orthorhombic distortion, and superconductivity is theunique and essential feature of Fe-based superconductors. In particular, the nature of the“nematic”/pseudogap phase above the magneto-structural transition temperature TS [1] has beencontroversial whether it is due to spin nematic or orbital ordering/fluctuations. As for the orbitalordering, both ferro-orbital and antiferro-orbital ordering/fluctuations have been discussed. To gainmore insight into this issue, we have performed temperature-dependent ARPES studies of banddispersions in undoped BaFe2As2 and its isovalent Ru-substituted compounds. The Fermi surfacetopology of BaFe2As2 in the antiferromagnetic-orthorhombic (AFO) phase was found to be fullyconsistent with the Shubnikov-de Haas result [2]. The anisotropic band dispersions betweenthe Γ-X and Γ-Y directions, which were also observed in a previous ARPES study not only belowTS but also above it [3], suggest that ferro-orbital ordering persists above TS. On the other hand, theDirac cone, which results from band folding due to the AFO order, was also found to persist aboveTS, indicating that the stripe-type AF fluctuations survive above TS or antiferro-orbital orderingexists below TS [4] and persists above it.

In the superconducting state, such interplay between antiferromagnetism, orthorhombicdistortion, and orbital ordering is expected to result in a complex pairing mechanism, which will bereflected on the superconducting gap anisotropy. We have studied the gap anisotropy of theisovalent-substituted systems BaFe2(As,P)2, SrFe2(As,P)2, and Ba(Fe,Ru)2As2 as well as theelectron-doped Ba(Fe,Co)2As2. The gap anisotropy on the electron Fermi surfaces (FSs) [5] wasfound to depend on the system as well as on the chemical composition. The gap on the hole FSs wasreduced around the Z point (i.e., near the Brillouin zone boundary) in some compounds but not inothers. In Ba(Fe,Ru)2As2, two hole FSs exhibited the same shrinkage of the gap around the Z point.In order to interpret the material-dependent, complicated gap anisotropy, fluctuations of both spinand orbital have to be considered for the pairing mechanism [6].

*This work has been done in collaboration with L. Liu, K. Koshiishi, H. Suzuki, J. Xu, K. Okazaki, S. Ideta,T. Yoshida, T. Shimojima, Y. Ohta, S. Shin, M. Hashimoto, D. Lu，Z.-X. Shen，H. Anzai, A. Ino, M. Arita,H. Namatame, M. Taniguchi, H. Kumigashira, K. Ono, T. Kobayashi, S. Miyasaka, S. Tajima, S. Kasahara,T. Terashima, T. Shibauchi, Y. Matsuda, S. Ishida, M. Nakajima, Y. Tomioka, T. Itoh, K. Kiho, C.-H. Lee, A.Iyo, H. Eisaki, and S. Uchida.1) S. Kasahara et al., Nature 486, 382 (2012); T. Shimojima et al., Phys. Rev. B 89, 045101 (2014). 2) M. Yi et al., PNAS 106, 117001 (2011).3) T. Terashima et al., Phys. Rev. Lett. 107, 176402 (2011).4) H. Konani et al., Phys. Rev. B 84, 024528 (2011)5) T. Yoshida et al., arXiv:1301.4818.6) T. Saito et al., Phys. Rev. B 88, 045115 (2013).

New insights into the cuprate phase diagram from neutron, X-ray and transport studies of HgBa2CuO4+δ

Martin Greven University of Minnesota, Minneapolis, MN 55414, USA

I will review our extensive collaborative effort to understand the properties of the simpletetragonal cuprate superconductor HgBa2CuO4+δ, with particular focus on recent neutron scattering[1,2], X-ray scattering [3] and charge transport [4-6] experiments that reveal an unusual magneticresponse, charge-density-wave correlations and Fermi-liquid behavior below optimal doping. Thecomparison with the properties of cuprates that feature a higher degree of disorder and/or lowerstructural symmetry leads to new insights into the phase diagram of the cuprates.

*This work has been supported by the US Department of Energy, Office of Basic Energy Sciences.

1) Y. Li et al., Phys. Rev. B 84, 224508 (2011). 2) M. K. Chan et al., arXiv:1402.4517.3) W. Tabis et al., arXiv:1404.7658.4) N. Barišić et al., Proc. Natl. Acad. Sci. USA 110, 12235 (2013).5) N. Barišić et al., Nature Phys. 9, 761 (2013).6) M. K. Chan et al., arXiv:1402.4472.

Electronic states of FeSe studied by STM/STS

T. Hanaguri1, T. Watashige2, Y. Kohsaka1, K. Iwaya1, T. Machida1, S. Kasahara2,D. Watanabe2, Y. Mizukami2, T. Mikami2, Y. Kawamoto2, S. Kurata2, T. Shibauchi2,Y. Matsuda2, A. Böhmer3, T. Wolf3, P. Adelmann3, C. Meingast3, H. v. Löhneysen3

1RIKEN CEMS, 351-0198, Wako, Japan2Department of Physics, Kyoto University, 606-8502, Kyoto, Japan

3Institut für Festkörperphysik, Karlsruhe Institute of Technology, D-76021, Karlsruhe,Germany

Although fully-gapped behavior has been observed in various iron-based superconductors,there is growing evidence that some materials show nodal superconductivity. It is important toclarify the nature of nodal iron-based superconductors to elucidate the pairing mechanism. FeSe isone of such nodal superconductors and is suitable to examine the electronic state using modernsurface-sensitive spectroscopic techniques such as ARPES and STM/STS, because its cleavedsurface is electronically neutral. FeSe has another interesting aspect in that superconductivity occursin an orthorhombic phase which might be related to the orbital ordering.

We performed low-temperature STM/STS on high-quality single crystals of FeSe toinvestigate the superconducting gap and the electronic states in the orthorhombic phase. STMtopograph of cleaved surfaces exhibits clear Se atomic lattice with small amount of defects and twinboundaries. Previous STM/STS studies on MBE-grown thin films revealed V-shaped tunnelingspectra, indicating the presence of gap nodes [1]. We found that the nodes are strongly affected bythe twin boundaries. With approaching to the twin boundary, V-shaped spectra gradually change toU-shaped ones. Interestingly, in the area between the twin boundaries separated by about 30 nm, thegap node is completely lifted and there appears a finite gap over ±0.4 meV. This unusualtwin-boundary effect will give us a hint to elucidate the superconducting-gap structure.

We also found quasi-particle interference (QPI) patterns in the Fourier-transformedspectroscopic images. Unidirectional electron- and hole-like QPI branches are clearly identified andthey disperse in orthogonal directions. In both branches, effective Fermi energies estimated from theband dispersions are as small as the superconducting gap amplitude, indicating that FeSe is in theBCS-BEC crossover regime. Interestingly, superconducting-gap amplitude is spatially modulated inthe same manner as the QPI pattern near the Fermi energy. This may suggest that even small changein the electronic state can affect superconductivity in FeSe.

1) C. -L. Song et al., Science 332, 1410 (2011).

Spin and charge excitations in electron-doped cuprates

Kenji Ishii1

1SPring-8, Japan Atomic Energy Institute, Sayo Hyogo 679-5148, Japan

The evolution of electronic (spin and charge) excitations upon carrier doping is one of the important issues in superconducting cuprates as well as in strongly correlated electron systems. However the knowledge of the excitations in energy-momentum space and their electron-hole asymmetry is still fragmentary. While low-energy (< 0.1 eV) spin excitations have been studied by inelastic neutron scattering (INS), Cu L3-edge resonant inelastic x-ray scattering (RIXS) became a complementary tool for the spin excitations, especially for their high-energy part, recently [1,2] and experimental data on various copper oxides are accumulating in a last few years. Regarding the charge degree of freedom, the excitations above 1 eV have been clarified by Cu K-edge RIXS [3,4], lower energy range is rarely explored. Since electron dynamics in the sub-eV range (0.1-1 eV) is predominantly governed by nearest-neighbor hopping integral (t) and spin exchange interaction (J), experimental observation of the electronic excitations in the sub-eV range is crucial for verifying theoretical models describing the electronic structure and thereby explaining the superconductivity.

Here I will present our RIXS and INS work on the electron-doped cuprates (Nd,Pr,La)2-xCexCuO4 [5]. Cu L3-edge RIXS and INS revealed that high-energy spin excitations shift to higher energy upon electron doping. This is in distinct contrast to the hole-doped case, where spectral distribution of the magnetic excitations broadens but keeps its energy position almost unchanged upon doping, namely, the spin excitation at high energy is a remnant mode of the parent antiferromagnetic insulator. On the other hand, excitation spectra of the electron-doped cuprates lose localized nature of the parent compound and electrons acquire an itinerant character in the doped metallic state. Above the magnetic excitations, an additional dispersive feature is observed in the Cu L3-edge RIXS spectra near the Brillouin zone center. Because its peak positions are found to follow the dispersion of the particle-hole charge excitations observed in the K-edge RIXS spectra, the dispersive feature is ascribed to the same charge origin. *This work was performed in collaboration with M. Fujita, T. Sasaki, M. Minola, G. Dellea, C. Mazzoli, K. Kummer, G. Ghiringhelli, L. Braicovich, T. Tohyama, K. Tsutsumi, K. Sato, R. Kajimoto, K. Ikeuchi, K. Yamada, M. Yoshida, M. Kurooka, and J. Mizuki. 1) M. Le Tacon et al., Nat. Phys. 7, 725 (2011). 2) M. P. M. Dean et al., Nat. Mater. 12, 1019 (2013). 3) K. Ishii et al., Phys. Rev. Lett. 94, 207003 (2005). 4) S. Wakimoto et al., Phys. Rev. B 87, 104511 (2013). 5) K. Ishii et al., Nat. Commun. 5, 3714 (2014).

Recent discovery of new superconductors including pnictogen atoms

A. Iyo1, Y. Yanagi1,2, H. Kito1, T. Kinjo1,3, T. Nishio1,3, S. Ishida1, N. Takeshita1, K. Oka1 T. Yanagisawa1, I. Hase1, H. Eisaki1, Y. Yoshida1

1 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan 2 IMRA Material R&D Co., Ltd., 2-1, Asahi-machi, Kariya, Aichi, 448-0032, Japan

3 Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan

Since the discovery of iron-based high-Tc superconductors, a compound containing pnictogen atoms is attracting much attention as a candidate of a new superconductor. Recently, we have found various new superconductors containing Bi, P or Sb as follows.

Ba2Bi3 contains planar anionic Bi ribbon nets with four- and three-bonded Bi separated by cationic Ba layers (Fig.1 (a)). Ba2Bi3 is found to be a superconductor with a Tc of 4.4 K. From the analysis of ρ(T), the Debye temperature ΘD and electron-phonon coupling constant λe-p are derived as 75.9 K and 1.0, respectively, indicating that Ba2Bi3 is a superconductor in the strong-coupling regime. We have succeeded in synthesizing a series of intermetallic ternary phosphide chalcogenide superconductors, AP2-xXx (A = Zr, Hf; X = S, Se) using high-pressure technique. These materials have a PbFCl-type structure (Fig.1 (b)) when x is greater than 0.3. Tc changes systematically with x, yielding dome-like phase diagrams. The maximum Tc is achieved at approximately x = 0.7, at which point the Tc is 6.3, 5.5, 5.0 and 4.6 K for ZrP2-xSex, HfP2-xSex, ZrP2-xSx and HfP2-xSx, respectively.

A Au-Sb-Te ternary system crystalizes into a simple cubic structure (α-Po-type) (Fig.1 (c)) when it is quenched from high temperature under high pressure. We found that Au0.125Sb0.75Te0.125 (AuSb6Te) that are reported to be semiconductors above 20 K, is superconductors with a Tc of 6.7 K. The maximum Tc of 8.1 K is achieved for Au0.15Sb0.85. This Tc value is the highest among materials with the α-Po-type structure under ambient pressure.

(a) Ba2Bi3� (c) Au-Sb-Te alloy�(b) ZrP2-xSx�

Au,Sb,Te�Ba�

Bi�

P�

Zr�P,S�

Figure 1: Crystal structures of (a)Ba2Bi3, (b)ZrP2-xSx and (c)Au-Sb-Te alloy. * This work was partially supported by the Strategic International Collaborative Research Program (SICORP) of the Japan Science and Technology Agency (JST) and KAKENHI (Grant No. 26400379) from Japan Society for the Promotion of Science (JSPS). 1) A. Iyo et al., Supercond. Sci. Technol. 27, 72001 (2014). 2) H. Kito et al., J. Phys. Soc. Jpn. 83, 074713 (2014). 3) A. Iyo et al., Supercond. Sci. Technol. 27, 25005 (2014).

A theoretical design of skyrmion device

W. Koshibae1, Y. Kaneko1, J. Iwasaki2, and N. Nagaosa1,2

1 RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan

2 Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

The key to develop the magnetic memory devices is nothing more than the control technique of

the magnetic texture by external fields. The recent studies reveal that skyrmion, the swirling magnetic texture, is driven by a much smaller electric current density than that for the domain wall motion, and hence, the potential application of the skyrmion has attracted much attention. To utilize the skyrmion for device applications, the technique for creation (write), annihilation (eliminate) and motion (transport) must be established. We theoretically study the creation, annihilation and current-driven motion of skyrmion in the chiral and dipolar magnets in two dimension by numerically solving Landau-Lifshitz-Gilbert equation,

d H d

dt dt

⎛ ⎞∂= − × +α ×⎜ ⎟∂⎝ ⎠

r rr r

r

n nn nn

, (1)

where nr is the normalized vector along magnetic moment, H is the Hamiltonian describing the magnetic system, and α denotes the Gilbert damping constant. By the numerical study, we explore the optimal condition to control the skyrmion in the ferromagnetic background.

The topology of the skyrmion is characterized by the skyrmion number,

21

4skN d rx y

⎛ ⎞∂ ∂= ⋅ ×⎜ ⎟π ∂ ∂⎝ ⎠

∫ r rr

n nn , (2)

which gives a wrapping number of a sphere by the magnetic moments. For the perfect ferromagnetic state nr=(0,0,+1), Nsk=0 and a skyrmion in the ferromagnetic background gives Nsk=−1. Because of the difference in topology, Nsk, the skyrmion cannot be reached from the perfect ferromagnetic state within the continuous deformation of the magnetic texture. As a result, the skyrmion carries a (meta-) stability and is protected by a potential barrier. To overcome the barrier, an energy being large enough to destroy the magnetic ordering is needed. However, the spatial discontinuity gives a favorable environment to change the topology of magnetic texture and the stability is reduced. For example, the skyrmion is created rather easily at the edge of a magnet in comparison to the deep inside of the system. Also the laser irradiation can induce the hot spot where the skyrmions are nucleated. We show the numerical results of the real-time dynamics in the magnetic textures induced by external stimuli and discuss the creation, annihilation and current-driven motion of skyrmion(s) for the theoretical design of the skyrmion memory devices.

Chiral Magnetic Soliton Lattice in Inorganic Chiral Materials

Y. Kousaka1,2,9, K. Ohishi3, J. Suzuki3, H. Ninomiya4, Y. Matsumoto4, S. Ohara4, H. Hiraka5, J. Zhang5,6, P. Miao5,7, S. Torii5, T. Kamiyama5,7, J. Campo8, K. Inoue1,2, and J. Akimitsu9

1Graduate School of Science, Hiroshima University, Higashihirosima 739-8526, Japan 2Center for Chiral Science, Hiroshima University, Higashihirosima 739-8526, Japan

3Research Center for Neutron Science and Technology, CROSS, Tokai, 319-1106, Japan

4Graduate School of Engineering, Nagoya Institute and Technology, Nagoya 466-8555, Japan 5Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan

6Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049 7Graduate University for Advanced Studies (Sokendai), Hayama 240-0115

8CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain 9Department of Physics and Mathematics, Aoyama-Gakunin University, Sagamihara 252-5258, Japan

The concept of chirality, meaning left- or right-handedness, plays an essential role in

symmetry properties of nature at all length scales from elementary particles to cosmic science. In material sciences, it is very important to understand the chirality in molecules, crystals and magnetic structures both from theoretical and experimental viewpoints. Recently, large attention has been paid to the relationship between crystallographic chirality and that of magnetic structure, because the sense of a helical spin structure depends on the right- or left-handed chiral crystallographic structure that allows an asymmetric Dzyaloshinskii-Moriya (DM) interaction due to spin-orbit interaction. Kishine et al. theoretically proposes that the chiral helimagnetic compounds form a chiral magnetic soliton lattice [1]. However, there have been few experimental results due to the difficulty to synthesize the suitable materials to realize such research. In chiral helical magnetic structures, the pitch angle, mainly determined by the ratio of exchange interaction and DM interaction, is usually very small. As a result, the helimagnetic period can be hundreds of angstroms. Therefore sometimes the angle resolution of thermal neutron diffraction experiments is not high enough to separate fundamental Bragg peaks and magnetic satellite peaks. As consequence, some compounds with chiral helimagnetic ordering may be easily misinterpreted as ferromagnetic ordering.

Firstly, we will present a unique crystallization technique to make a single crystallographic chirality in inorganic compounds. By adapting our crystallization technique, we succeeded in obtaining the mm-ordered enantiopure single crystals. Secondly, we will present neutron diffraction works performed at BL08 (Super HRPD) and BL15 (TAIKAN) in the Materials and Life Science Experimental Facility (MLF) of J-PARC. By means of super high-resolution powder neutron diffraction in Super HRPD, we observed very long periodical magnetic satellite peaks in some of ferromagnetic T1/3MS2 (T = transition metal, M = Nb and Ta) compounds. Magnetic structure analysis indicates the magnetic structure forms helimagnetic structure. We performed small and wide angle polarized neutron scattering in TAIKAN, and observed chiral imcommensurate magnetic structures in MnSi, which is a chiral magnet with cubic symmetry, and YbNi3Al9, which is a new rare earth based chiral magnetic compound. The difference in magnetic satellite intensity between up- and down-spin neutron indicates that chiral helimagnetic ordering Moreover, under an applied magnetic field, we succeeded in observing higher harmonics as an evidence of the chiral magnetic soliton lattice.

*This work is supported by a Grant-in-Aid for Scientific Research (C) (No. 26400368), Exploratory Research (No. 26108719), and Scientific Research (S) (No. 25220803) from MEXT of the Japanese Government. 1) J. Kishine et al., Prog. Theor. Phys. Suppl. 159, 82 (2005).

Unusual behavior of the subbands in strongly-correlated oxide quantum wellstructures

Hiroshi KUMIGASHIRA1

1Photon Factory, Institute of Materials Structure Science, High Energy Accelerator ResearchOrganization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan

The quantum confinement of strongly correlated electrons in artificial structures hasheralded the possibility of controlling and/or designing the extraordinary physical properties ofstrongly correlated oxides [1]. After the discovery of metallic quantum well (QW) states in SrVO3

ultrathin films grown onto SrTiO3 substrates [2] as well as cleaved surfaces of oxidesemiconductors SrTiO3 [3] and KTaO3 [4], the creation and control of the QW states in stronglycorrelated oxides become a subject of great interest; not only for promising technologicalapplications in future oxide electronics [1], but also for understanding of the fundamentallow-dimensional physics in strongly correlated electron systems [5].

The observed metallic QW states in SrVO3 ultrathin films exhibit characteristic behaviorsuch as an “unusual mass enhancement”, which depends on the subbands : The subband dispersionbecomes considerably narrower as the bottom energy of the subbands approaches the Fermi level(EF) [2]. Such anomalous subband-dependent mass enhancement has not been observed inconventional metallic QW structures based on metals having nearly free-electron-like sp bands,suggesting the importance of underlying strongly correlated electronic states in the SrVO3 QW.

In order to address the origin of the anormalous mass enhancement in the QW subbands, wehave performed in-situ angle-resolved photoemission spectroscopy (ARPES) measurements onSrVO3 ultrathin films and analyzed the line shape of ARPES spectra in detail. The line-shapeanalysis reveals that the strength of the electron-electron correlation in each subband increases asthe subband bottom energy approaches EF, indicating the importance of electron correlation in theQW states. Taking into account the characteristic orbital-selective quantization in SrVO3 QW [2], itis reasonable to conclude that the anomalous enhancement of electron-electron correlation mainlyarises from the quasi-one-dimensional electronic structure of quantized V 3d t2g states, where thedensity of states diverges at the band bottom.

References1) H. Y. Hwang et al., Nature Mater. 11, 103 (2012).2) K. Yoshimatsu et al., Science 333, 319 (2011). 3) A. F. Santander-Syro et al., Nature 469, 189 (2011).4) P. D. C. King et al., Phys. Rev. Lett. 108, 117602 (2012).5) M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1998).

Realistic band structure approaches to unconventional superconductors

Kazuhiko Kuroki

Department of Physics, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043, Japan

In recent studies of unconventional superconductivity, more focus has been paid to the electronic

band structure of the actual materials. In these approaches, first principles band calculation is performed, and an effective tightbinding model that reproduces the realistic band structure is constructed exploiting such methods as the maximally localized Wannier orbitals. By applying many body theories to the effective models, we can analyze the material dependence of the pairing symmetry and/or the superconducting transition temperature. In the present talk, I will focus on our recent studies on the iron-pnictide[1] and the cuprate superconductors[2] along this line.

As for the iron-pnictides, much focus has been put on the Fermi surface configuration from the early stage of the study, where the presence of disconnected electron and hole Fermi surfaces are characteristic of this high Tc family. In addition to this, however, we have recently found that the real space motion of electrons (i.e.,hopping integrals between Fe sites), which can be largely different among materials (Fig.1) even when the Fermi surface looks similar, is also quite important and strongly affects the spin fluctuations and Tc. This view provides understanding for the occurrence of high Tc superconductivity in materials in which the nesting between electron and hole Fermi surfaces seems to be ill-conditioned.

For the cuprates, we have constructed two orbital models that considers Cu 3dx2-y2 and Cu 3d3z2-r2 orbitals for various single layer and bilayer cuprates. This gives us an explanation of the Tc vs. the Fermi surface shape trend found experimentally for the cuprates.

An interesting contrast between the cuprates and the iron-pnictides is noticed. Namely, in the cuprates, dx2-y2 and d3z2-r2 orbitals work destructively against each other to degrade superconductivity, while in the iron-pnictides, dxz, dyz, and dxy orbitals work coherently to enhance superconductivity. This contrast puts the iron-pnictides in a special position among various multiorbital systems.

Figure 1: The electron doping rate dependence of the nearest (t1) and the next nearest neighbor (t2) hoppings in LnFeAs(O,H or F) . (a) The bond angle is hypothetically varied for Ln=La. (b) Calculation results for the actual materials Ln=La and Sm. Notice the strong variation of t1 against the doping rate x, suggesting that the rigid band picture is not valid. 1) K. Suzuki et al., Phys. Rev. Lett. 113, 027002 (2014). 2) H. Sakakibara et al., Phys. Rev. B 89, 224505 (2014).

Superconducting transition temperature and re-emergence of antiferromagnetic order in LaFe(As1-xPx)(O1-yFy)

H. Mukuda1, F. Engetsu1, T. Shiota1, K. T. Lai2, M. Yashima1, Y. Kitaoka1, S. Miyasaka2, and S. Tajima2

1Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan2Graduate school of Science, Osaka University, Osaka 560-0043, Japan

Superconducting transition temperature (Tc) of LaFe(As1-xPx)(O1-yFy) exhibits a nonmonotonicvariation with x[1], as shown in Fig.1. Previous 31P-NMR studies on these compounds haverevealed that the antiferromagnetic spin fluctuations (AFMSFs) at low energies are markedlyenhanced around x=0.6 for y=0.05 and x=0.4 for y=0.1, where Tc exhibits respective peaks at 24 Kand 27 K[2], as shown in Fig. 2. This result, however, brings about anotherquestion: Why are AFMSFs enhanced despite thefact that the lattice parameters of the compounds arefar from those of the AFM mother compoundLaFeAsO. Recently, we revealed the re-emergentphase of the homogeneous AFM order (AFM2) inx=0.6 of LaFe(As1-xPx)O, which is not linked withthe original AFM order phase (AFM1) of the mothercompound LaFeAsO[3,4,5], as plotted in Fig. 1. TheAFM2 order homogeneously develops belowTN=35K at x=0.6. The presence of AFM2 may berelated with unexpected nonmonotonic variation ofTc in LaFe(As1-xPx)(O1-yFy). These resultsdemonstrate that the AFMSFs at low energy areresponsible for the increase in Tc for LaFe(As1-xPx)(O1-yFy). However, from a systematic comparison ofAFMSFs with a series of(La1-zYz)FeAsOδcompounds in which Tc reaches 50K for z=0.95, we revealed that a moderatedevelopment of AFMSFs causes the Tc to increaseup to 50 K under the condition that the local latticeparameters of FeAs tetrahedron approaches those ofthe regular tetrahedron[6]. These results suggestthat the Tc of Fe-pnictides exceeding 50 K ismaximized under an intimate collaboration of theAFMSFs and other factors originating from theoptimization of the local structure.

[1] K. T. Lai et al., JPS Conf. Proc. 1, 012104 (2014).[2] H. Mukuda et al., Phys. Rev. B 89, 064511 (2014).[3] H. Mukuda et al., J. Phys. Soc. Jpn. 83, 083702 (2014). [4] K. T. Lai et al., Phys. Rev. B 90, 064504 (2014).[5] S. Kitagawa et al., J. Phys. Soc. Jpn 83, 023707 (2014) .[6] H. Mukuda et al., Phys. Rev. Lett. 109, 157001 (2012).

Figure 1: Phase diagram of LaFe(As1-xPx)(O1-yFy) [1-4]

Figure 2: Relationship between AFM spin fluctuations at low energies and Tc

Electric-field driven motion of skyrmion lattices in the chiral magnet Cu2OSeO3

Yusuke Nambu1

1Institute of Multidisciplinary Research for Advanced Materials,Tohoku University, Sendai 980-8577, Japan

Topologically protected spin texture has been of great interest in condensed matter physics.One prominent example is a magnetic skyrmion, a vortex-like spin texture in which electronic spinspoint in all directions of the solid angle [1]. The skyrmion lattice has attracted much attentiontoward potential technological applications such as magnetic information storage and processingdevices. The skyrmions have been so far observed by neutron scattering in the reciprocal space andmicroscopy techniques in the real space in the itinerant magnets such as MnSi, Fe1-xCoxSi, FeGe andMn1-xFexGe. Recently the insulating multiferroic magnet Cu2OSeO3 has been reported to alsopossess the skyrmion lattice [2]. Detailed investigation on the magnetic field/temperature phasediagram was performed, where two different phases, named SkX(1) and SkX(2), were identified[3]. These two phases are related to each other by 30 degrees rotation in the skyrmion-lattice plane.

In the itinerant magnets showing the skyrmion-lattice formation, conduction electrons of anultralow current density are enough to drive the skyrmion motion [4]. For the first insulatingmaterial, on the other hand, a study of electric field effects is interesting. Given a loss due to theJoule heating in an insulator is negligible when applying an electric field, the control may be donewithout thermal energy dissipation, and hence would provide a significant advantage for futuretechnological applications. We have recently carried out small angle neutron scattering experimenton BL15-TAIKAN at J-PARC, and have observed electric-field driven rotation of skyrmion lattices.In this talk, we will show our preliminary results of the skyrmion rotation up to a few decadesdegrees in the SkX(2) phase. Comparison with competing work [5] from the view of amagnetoelectric coupling, and interesting dynamics for the formation of the skyrmion lattice willalso be discussed.

This work was done through collaboration with Daiki Higashi, Daisuke Okuyama, Taku JSato, Shinichiro Seki, Naoto Nagaosa and Yoshinori Tokurta.

1) N. Nagaosa and Y. Tokura, Nat. Nanotech. 8, 899 (2013).2) S. Seki, X.Z. Yu, S. Ishiwata and Y. Tokura, Science 336, 198 (2012).3) S. Seki et al., Phys. Rev. B 85, 220406(R) (2012).4) See e.g. F. Jonietz et al., Science 330, 1648 (2010).5) J.S. White et al., J. Phys. Cond. Mat. 24, 432201 (2012).

Superconductivity and flux state in the Hubbard model

Masao Ogata1, Hisatoshi Yokoyama2, and Shun Tanmura2

1Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

2Department of Physics, Tohoku University, Aoba-ku, Sendai 980-8578, Japan

With high-Tc cuprates in mind, the properties of correlated dx2-y2-wave superconductivity

and flux states are studied in the two-dimensional t-t'-U Hubbard model on a square lattice, using a variational Monte Carlo method [1]. We find that it is crucial to take account of the doublon-holon- binding correlations for describing the correlated superconductivity and the normal flux states as "doped Mott insulators". The U/t, t'/t, and δ (doping rate) dependences of relevant quantities are systematically calculated. In the dx2-y2-wave superconductivity, a sharp crossover occurs at Uco/t from a conventional BCS- type superconductivity to a kinetic-energy-driven superconductivity for any values of t'/t (Fig. 1 left). (Uco/t is smoothly connected to the Mott transition point at half filling as δ →0.) In the weak coupling region, d-wave superconductivity correlation function is very small. In the strong-coupling region (U > Uco), the ground state behaves as that in the t-J model, namely, only the doped holes are charge carriers and the doublons always form bound states with holons. In this case, the δ-dependence of the superconductivity correlation function shows a dome-shape (Fig. 1 right) which is consistent with experiments.

As a candidate for the pseudogap state of cuprates, we also study a staggered flux state in a similar scheme [2]. In the trial wave function, a configuration-dependent phase factor, which was recently shown to be indispensable to treat a current-carrying state, is introduced in addition to the ordinary phase factors. We find that the staggered flux state is markedly stabilized as a normal state in the underdoped region and for the strong-coupling region. The stability of this state as a function of t'/t is quantitatively consistent with the pseudogap features of hole-doped cuprates. It is also shown that a spin current (flux or nematic) state is never stabilized in the same parameter regime.

Figure 1: Superconductivity correlation function as a function of U/t (left) and the doping rate δ (right). [1] H. Yokoyama, M. Ogata, Y. Tanaka, K. Kobayashi, and H. Tsuchiura, J. Phys. Soc. Japan, 82, 014707 (2013). [2] H. Yokoyama, S. Tamura, and M. Ogata, in preparation. ogata@phys.s.u-tokyo.ac.jp

Ultrafast photoinduced transitions to metallic states in half-filled Mott insulators

Hiroshi Okamoto and Tatsuya MiyamotoDepartment of Advanced Materials Science, Univ. of Tokyo, Kashiwa 277-8561, Japan

Phase controls of solids by photoirradiations are now being extensively studied. This phenome-non is called a photoinduced phase transition (PIPT). When we aim to realize a PIPT in a sub-pstime scale, a key strategy is to use photoinduced changes of electronic structures via purely elec-tronic processes. Good target materials for this purpose are Mott insulators. It is well known that theelectronic structures of Mott insulators exhibit large variations with respect to chemical carrier dop-ing. In fact, in various kinds of perovskite-type transition metal oxides, Mott-insulator to metal tran-sitions are observed by chemical carrier doping. It is expected that similar phase transitions can bedriven by photoirradiations. Photoinduced Mott insulator to metal transitions were reported in 1DMott insulators such as halogen-bridged transition metal compounds, [Ni(chxn)2Br]Br2 [1] and[Pd(en)2Br](C5-Y)2H2O [2] and an organic molecular compound, ET-F2TCNQ [3,4]. Especially, inthe latter, a Drude-like response was clearly observed by a photoirradiation, which was interpretedby the concept of the spin-charge separation characteristic of 1D strongly correlated electron sys-tems. A representative of the filling-control Mott transitions is observed in perovskite-type cuprates.In this talk, we report photoinduced Mott insulator to metal transitions and photocarrier dynamics intypical half-filled Mott insulators of the cuprates, Nd2CuO4 and La2CuO4, investigated by femtosec-ond pump-probe (PP) spectroscopy [5,6].

The PP spectroscopy with the time resolution of ∼200 fs and ∼40 fs revealed that in Nd2CuO4,metallic state is generated with low excitation photon density less than 0.01 photon/Cu and decayswithin 40 fs via the rapid photocarrier recombination. Residual photocarriers are localized by theeffect of charge-spin coupling, exhibiting two mid-gap absorptions due to a particle and a hole.La2CuO4 also shows a photoinduced Mott-insulator to metal transition, while it exhibits thedifferent charge dynamics from those in Nd2CuO4; the larger threshold excitation photon density forthe formation of metallic state, the higher energies of the mid-gap absorptions, and the slowerrecombination of polaronic carriers. These behaviors can be explained by the larger charge-phononcoupling strength in La2CuO4 than in Nd2CuO4 [5,6].

In order to measure the initial dynamics of spin and charge degrees of freedom in thosephotoinduced transitions, we have recently developed the PP system with the time resolution of 10fs and applied it to Nd2CuO4. The results reveal two important dynamics; in the weak excitationcase, the reflectivity at the gap transition region instantaneously decreases, which is followed by theadditional decrease of reflectivity with the time constant of ∼20 fs. Possible origin of this additionalcomponent is the destruction of the spin order. In the strong excitation case, a metallic state isphotogenerated just after the photoirradiation and the decay time of the metallic state is about 20 fs.Such an ultrafast decay of the metallic state suggests that the excess energy generated by thephotocarrier recombination is rapidly transferred to the spin system by emission of magnons via thecharge-spin coupling. This work has been done in collaborations with Y. Matsui, H. Yada, H. Matsuzaki, Y. Tokura, A.Sawa, B.-S. Li, and T. Ito.

References[1] S. Iwai et al., PRL 91, 057401 (2003). [2] H. Matsuzaki et al., PRL 113, 096403 (2014).[3] H. Okamoto et al., PRL 98, 037401 (2007). [4] S. Wall et al., Nat. Phys. 7, 114 (2011). [5] H. Okamoto et al., PRB 82, 060513(R) (2010). [6] H. Okamoto et al., PRB 83, 125102 (2011).

Crystal Growth and Anisotropic Properties

of Various Iron-based Superconductors

Takao Sasagawa and Takao Katagiri

Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan

By a self-flux technique in an evacuated double quartz tube, single crystals of various

iron-based superconductors, including those having thick blocking layers such as Sr2VFeAsO3 [Fig.

1(a)] and Ca5(Mg,Ti)4Fe2As2O11, were successfully grown. From the angular dependence of the

in-plane resistivity at various temperatures and fields, the anisotropy parameter ( = Hc2//ab

/ Hc2//c

)

in the obtained crystals was quantitatively evaluated by using the anisotropic Ginzburg-Landau

theory. Figs. 1 (b) and (c) show the results obtained in the Sr2VFeAsO3 crystal.

Figure 1: (a) Crystal structure of Sr2VFeAsO3. (b) Angular dependence of the in-plane resistivity at various

fields at T/Tc = 0.8. (c) Scaling plot of the data in (b) based

on the anisotropic Ginzburg-Landau theory.

The obtained values of for Sr2VFeAsO3 and

Ca5(Mg,Ti)4Fe2As2O11 amount to 20 and 160, which

are comparable to the cuprate superconductors

(La,Sr)2CuO4 and Bi2Sr2CaCu2Oy, respectively. On the

analogy of Cu-based high-Tc superconductors, it

appears that the thickness of the blocking layers plays a

fundamental role for the material-dependent anisotropy

in the iron-based superconductors. As the simplest

empirical rule, we find that the logarithm of the

anisotropy parameters is proportional to the number of

the blocking layers.

Figure 2: Anisotropy parameter for various

iron-based superconductors as a function of

the number of the blocking layers.

Dynamics of magnetic skyrmions

Shinichiro Seki1,2

1 RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan2 PRESTO, Japan Science and Technology Agency (JST), Tokyo 102-0075, Japan

Magnetic skyrmion is a topologically stable particle-like object, which appears as nanometer-scale vortex-like spin texture in a chiral-lattice magnet[1]. In metallic materials (MnSi, FeGe,Fe1−xCoxSi etc), electrons moving through skyrmion spin texture gain a nontrivial quantumBerry phase, which provides topological force to the underlying spin texture and enables thecurrent-induced manipulation of magnetic skyrmion[2]. Such electric controllability, in additionto the particle-like nature, is a promising advantage for potential spintronic device applications.Recently, we newly discovered that skyrmions appear also in an insulating chiral-lattice mag-net Cu2OSeO3[3,4]. We find that the skyrmions in insulator can magnetically induce electricpolarization through the relativistic spin-orbit interaction, which implies possible manipulationof the skyrmion by external electric field without loss of joule heating[5]. The present findingof multiferroic skyrmion may pave a new route toward the engineering of novel magnetoelectricdevices with high energy efficiency. In this talk, the latest experimental and theoretical resultson the dynamical aspect of magnetoelectric skyrmions, such as electrically-active magnetic ex-citation and associated unique magneto-optical response, will also be discussed[6-8].

Figure 1: Schematic illustration of skyrmion spin texture.

* This work supported by the Grant-in-Aid for Challenging Exploratory Research (Grant No. 26610109)by the MEXT of Japan, and PRESTO program by JST.1) S. Muhlbauer et al., Science 323, 915 (2009).2) F. Jonietz et al., Science 330, 1648 (2010).3) S. Seki et al., Science 336, 198 (2012).4) S. Seki et al., Phys. Rev. B 85, 220406(R) (2012).5) S. Seki et al., Phys. Rev. B 86, 060403(R) (2012).6) Y. Onose et al., Phys. Rev. Lett. 109, 037603 (2012).7) M. Mochizuki and S. Seki, Phys. Rev. B 87, 134403 (2013).8) Y. Okamura et al., Nature Communications 4, 2391 (2013).

Implications of resonant inelastic x-ray scattering data fortheoretical models of cuprates

Wei Chen1, Oleg P. Sushkov2,1 Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse 1, D-70569 Stuttgart,

Germany2 School of Physics, University of New South Wales, Sydney 2052, Australia

There are two commonly discussed points of view in theoretical description of cupratesuperconductors: (i) Cuprates can be described by the modified t J model; (ii) overdopedcuprates are close to the regime of normal Fermi liquid (NFL). We argue that recent resonantinelastic x-ray scattering data challenge both points. While the modified t J model describeswell the strongly underdoped regime, it fails to describe high energy magnetic excitations whenapproaching optimal doping. This probably indicates failure of the Zhang-Rice singlet picture.In the overdoped regime the momentum-integrated spin structure factor S(ω) has the sameintensity and energy distribution as that in an undoped parent compound. This implies thatthe entire spin spectral sum rule is saturated at ω ≈ 2J , while in an NFL the spectral weightshould saturate only at the total bandwidth which is much larger than 2J .

Impurity substitution effects on magnetic correlation in La2-xSrxCu1-yMyO4 (M = Fe, Al)

K. M. Suzuki1, T. Adachi2, M. A. Baqiya3, H. Guo4, I. Kawasaki4, M. Abdel-Jawad4, S. Yoon4, I. Watanabe4 and Y. Koike3

1Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 2Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan

3Department of Engineering and Applied Sciences, Sophia University, Tokyo 102-8554, Japan 4Advanced Meson Science Laboratory, RIKEN Nishina Center, Wako 351-0198, Japan

The so-called stripe correlation of spins and holes has been studied intensively in order to clarify its relationship with the appearance of the high-Tc cuprate superconductivity. Impurity substitution is one of crucial ways to study the stripe correlation, because substituted impurities tend to slow down the spin fluctuations, leading to the formation of the static stripe order [1-3]. Here we summarize our work on impurity effects on the magnetic correlation and introduce our recent results of magnetic Fe3+- and nonmagnetic Al3+-substituted La2-xSrxCu1-yMyO4 (M = Fe, Al) revealed by muon-spin-relaxation (μSR) and magnetic susceptibility experiments.

It has been found that the 5% Fe substitution induces double successive magnetic transitions in the overdoped regime of La2-xSrxCu1-yFeyO4, as shown in Fig. (a). While the magnetic transition at higher temperatures is a spin-glass transition of Fe3+ spins due to the RKKY interaction, the magnetic transition at lower temperatures is the transition to the stripe order. Furthermore, the stripe order develops both in the underdoped and overdoped regimes and disappears at the hole concentration per Cu, p, of ~ 0.30 where the superconductivity disappears in pristine La2-xSrxCuO4. The 3% Al substitution has also been found to induce the transition to the stripe order in a wide range of p in La2-xSrxCu1-yAlyO4 as shown in Fig. (b). The magnetic transition temperature decreases with hole doping, and disappears at p ~ 0.30. A similar result has been obtained in Zn-substituted La2-xSrxCu1-yZnyO4 [4], where the Cu-spin correlation is developed by nonmagnetic Zn2+ substitution up to p ~ 0.30. Therefore, it has been concluded that, regardless of the type of impurities, the development of the stripe correlation is observed up to p ~ 0.30, suggesting an intimate relation between the stripe correlation and the appearance of the high-Tc superconductivity. Results of Ni and Ga substitution will also be discussed.

[1] T. Adachi et al., Phys. Rev. B 78, 134515 (2008).

[2] M. Fujita et al., J. Phys. Chem. Solids 69, 3167 (2008).

[3] K. M. Suzuki et al., Phys. Rev. B 86, 014522 (2012).

[4] Risdiana et al., Phys. Rev. B 77, 054516 (2008).

Fig.: (a) Hole concentration per Cu, p, dependence of Tg, defined as local maximum in magnetic susceptibility, for La2-xSrxCu1-yFeyO4 with y = 0.05. (b) p dependence of the magnetic transition temperature defined by μSR, TN, for La2-xSrxCu1-yAlyO4 with y = 0.03.

Combining multiple degrees of freedom to enhance magnetocaloric effect

Y. Taguchi1

1RIKEN Center for Emergent Matter Science (CEMS) , Wako 351-0198, Japan

Magnetocaloric effect is the phenomenon that temperature (entropy) change of a magneticmaterial is induced when a magnetic field is applied adiabatically (isothermally). Since this effectcan be used as an environmentally-benign, highly-efficient refrigeration technique, it has recentlyattracted much attention. Therefore, materials showing a large entropy change at the magnetictransition are desirable for such purposes.

One of the strategies to enhance the magnetocaloric effect is to explore materials exhibitingfirst-order magnetic transitions to gain entropy release not only from magnetic degree of freedombut also from lattice degree of freedom. However, details on how the cross-coupling betweenmagnetism and structure enhances the magnetocaloric response have remained elusive. In thispresentation, we will discuss the intimate interplay between magnetic and lattice degrees of freedomin two material systems, namely, Fe1-xMnxV2O4 spinel material with spin and orbital degrees offreedom [1] and MnCo1-xZnxGe intermetallic alloys [2], and demonstrate the important role ofmagnetostructural coupling in enhancement of magnetocaloric effect, pointing to the generalstrategy to design the high-performance magnetocalorics.

The work has been done in collaboration with D. Choudhury, T. Suzuki, D. Okuyama, D.Morikawa, K. Kato, M. Takata, K. Kobayashi, R. Kumai, H. Nakao, Y. Murakami, M. Bremholm,B. B. Iversen, T. Arima, and Y. Tokura. The work was supported by Funding program for WorldLeading Innovative R&D on Science and Technology (FIRST) on “Quantum Science on StrongCorrelation” from JSPS.

1) D. Choudhury, T. Suzuki, D. Okuyama, D. Morikawa, K. Kato, M. Takata, K. Kobayashi, R. Kumai, H. Nakao, Y. Murakami, M. Bremholm, B. B. Iversen, T. Arima, and Y. Tokura, and Y. Taguchi, Phys. Rev. B89,104427 (2014).2) D. Choudhury, T. Suzuki, Y. Tokura, and Y. Taguchi, submitted.

Comparative study of the superconducting gap in Raman scattering and ARPESof Bi2Sr2CaCu2Oz

Nguyen Trung Hieu, Kiyohisa Tanaka, Takahiko Masui, Shigeki Miyasaka, and Setsuko TajimaDepartment of Physics, Osaka University, Osaka 560-0043, Japan

One of the puzzles in high Tc superconducting cuprates is the gap structure. Although thed-wave gap symmetry has been established, an ideal d-wave structure is observed only near theoptimal doping. When the pseudogap opens at T > Tc, the pair-breaking peak below Tc shows anunusual behavior, deviating from a d-wave k-dependence of ∆(k). Both of Raman scattering andARPES give two different energy scales linked to the gap near the nodal and anti-nodalregions[1,2]. However, the proposal for the interpretation is different in Raman scattering andARPES.

In order to obtain a unified picture, we have examined whether these two spectroscopyresults are reconciled with each other by directly comparing the data on the same crystals. Ourchallenge was to reproduce Raman scattering spectra from the ARPES data and compared themwith the measured Raman spectra on the same Bi2Sr2CaCu2Oz crystals. Here we focus on thesuperconducting state because the ARPES can provide the information only for the occupied statewhich is not enough to calculate the normal state Raman spectra.

Developing the analytical method based on the Kubo formula[3], we have succeeded incalculating Raman scattering spectra from the ARPES data. The obtained Raman spectra wellreproduce the experimental ones from under- to over-doped regime. During this comparisonprocedure, we found that the ARPES intensity near the anti-nodal region loses its weight and lesscontributes to the superconductivity when going toward underdoping. This electronic change affectsthe low ω behavior in B2g Raman spectra which reflects the nodal gap.

This study enabled us to understand the electronic state in a unified picture, revealing that i)the doping dependence of the B2g gap in Raman spectra can be explained even assuming the dopingindependent d-wave slope in the nodal region, ii) the anti-nodal gap energy in B1g Raman spectra isalways lower than that of ARPES, and its difference becomes larger in the underdoped regime. Thelatter fact is supposed to result from a strong effect of the pseudogap, depending on thespectroscopic techniques.

*This work supported by a Grant-in Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology in Japan [KAKENHI Grants No.24340083]. The authors thank T. P. Devereaux for his useful comment on the Raman calculation.

1) K. Tanaka et al., Science 314 (2006) 1910. I. M. Vishik et al., PNAS 109 (45), 18332-18337 (2012).2) M. Le Tacon et al., Nat. Phys. 2, 537 (2006). S. Blanc et al., PRB 82, 144516 (2010).3) T. P. Devereaux et al., Rev. Mod. Phys. 79, 175 (2007).

Exotic magnetism produced by strong spin-orbit coupling in complex Ir oxides

Daigorou Hirai1, Tomohiro Takayama2 & Hidenori Takagi1,2

1Department of Physics, University of Tokyo, Tokyo 113-0033, Japan

2Max-Planck-Institute for solid state research, Heisenbergstrasse 1, Stuttgart 70569, Germany

In 5d Iridium oxides, a large spin-orbit coupling of ~0.5 eV, inherent to heavy 5d elements, is notsmall as compared with other relevant electronic parameters, including Coulomb U, transfer t andcrystal field splitting D, which gives rise to a variety of exotic magnetic ground states. In thelayered perovskite Sr2IrO4, spin-orbital Mott state with Jeff=1/2 is realized due to the novel interplayof those energy scales [1-3]. Despite the strong entanglement of spin and orbital degrees offreedom, Jeff=1/2 iso-spins in Sr2IrO4 was found to be surprisingly isotropic, very likely due to asuper-exchange coupling through almost 180° Ir-O-Ir bonds [4]. The temperature dependence ofin-plane magnetic correlation length of Jeff=1/2 iso-spins, obtained from inelastic x-ray resonantmagnetic scattering, was indeed well described by that expected for two-dimensional S=1/2Heisenberg antiferromagnet [4]. Such Jeff=1/2 2D Heisenberg magnet was recently shown to betailored using SrIrO3/SrTiO3 super-lattice structure [5].

When Jeff=1/2 iso-spins interact with each other through 90° Ir-O-Ir bonds, very anisotropic bonddependent ferromagnetic coupling is expected, unique to strong SOC system. Complex Ir oxideswith honeycomb and more recently identified hyper-honeycomb lattices [7], where x-, y- and z- 90°Ir-O-Ir bonds are realized, may be candidates for quantum spin liquid expected for the Kiatevmodel. Very likely due to the superposition of additional magnetic couplings not included in theKitaev model [8], in reality, a long range magnetic ordering emerges at low temperatures in thosecompounds. Hyper-honeycomb β-Li2IrO3, though eventually show a marginal ordering, appears tobe located at bthe critical vicinity to the Kitaev spin liquid.

In this talk, we focus on those exotic magnetisms in complex Ir oxides.

1) B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).2) B. J. Kim et al., Science 323, 1329 (2009).3) S.Fujiyama et al., Phys. Rev. Lett. 112, 016405 (2014).4) G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).5) S. Fujiyama et al., Phys. Rev. Lett. 108, 247212 (2012).6) J. Matsuno et al., submitted.7) T.Takayama, A. Kato et al., submitted.8) A.Kitaev, Annals of Physics 312 2 (2006).

Nonequilibrium Electron Dynamics in Strongly Correlated ElectronSystems

T. TohyamaDepartment of Applied Physics, Tokyo University of Science, Tokyo 125-8585, Japan

One of the outstanding contemporary challenges in condensed matter physics is to under-stand the dynamics of interacting quantum systems exposed to an external perturbation. Ad-vanced pump and probe techniques with few femtosecond time resolution and broadband THzspectroscopy were developed to drive the system out of equilibrium and measure its nonequilib-rium physical properties. There are several topics in terms of nonequilibrium photo dynamicsin strongly correlated electron systems. Firstly, we theoretically examined the initial relaxationprocesses after photo irradiation caused by interplay between charge, spin, and lattice degreesof freedom, based on a Hubbard-Holstein chain [1] and a t-J-Holstein square lattice [2]. Wefound that the relaxation is predominantly controlled by the lattice degree of freedom in thechain, while it is by the spin degree of freedom in the square lattice. Second topics we have ex-amined is photo-induced change of states exemplified by a change from spin-density-wave stateto charge-density-wave state in the extended one-dimensional Hubbard model [3]. We clarifiedconditions for photo irradiation to have such a change. The third topic is directly related toexperimental observation of Mott insulator to metal transition due to photo excitation. Weexamined the time-dependent optical conductivity in the extended one-dimensional extendedHubbard model and compared the calculated results with experimental data in one-dimensionalorganic materials [4]. Implications of these results will be discussed.[1] H. Matsueda, S. Sota, T. Tohyama, and S. Maekawa, J. Phys. Soc. Jpn. 81 (2012) 013701.[2] L. Vidmar, J. Bonca, T. Tohyama, and S. Maekawa, Phys. Rev. Lett. 107 (2011) 246404.[3] H. Lu, S. Sota, H. Matsueda, J. Bonca, and T. Tohyama, Phys. Rev. Lett. 109, 197401(2012).[4] H. Lu, and T. Tohyama, unpublished.

Multilayer Effect in High-Tc Cuprates

Shin-ichi Uchida1,2

1Department of Physics, University of Tokyo, Hongo 7-3-1, Tokyo 113-0033, Japan2National Institute of Advanced Industrial Science and Technology (AIST), Umezono1-1-1

, Tsukuba 305-8568, Japan

The number of CuO2 planes, n, in a unit cell is one of the old-known factors

influencing Tc values. Tc rises with increasing n up to 3 but turns to decrease for n ≧ 4

The decrease of Tc certainly arises from the charge imbalance, difference in the hole

density between outer CuO2 planes (OP) and inner planes (IP) [1]. Although various

scenarios have been proposed [2-5], the mechanism of a systematic rise in Tc with n is

not fully understood yet.

The ARPES measurement on the trilayer Bi2223 with Tc = 110 K gives evidence

that a multilayer effect is at work in Bi2223 which simultaneously enhances the pairing

interactions in both OP and IP probably via a strong coupling between them [6].

Connected with this, the study of the infrared optical response of the Hg-based multilayer

family (n = 1 - 5) reveals a distinctively large Josephson coupling between nearest CuO2

planes within a unit cell for n = 2 and 3 [7]. The result shows that the interlayer

Josephson coupling strength within a multilayer is a parameter that controls the increase

of Tc up to n = 3 and the subsequent decrease of Tc for n > 3.

Thus, the interlayer pair hopping (intra-multilayer tunneling) remains a possible

scenario to explain the enhancement of Tc in the multilayer cuprates. Based upon this

scenario it is demonstrates that there is a possibility of Tc enhancement to 150 K or even

higher when the multilayer structure is optimized by reducing the charge imbalance

between OP and IP or/and by quenching the structure realized at high pressures.

[1] H. Kotegawa et al., J. Phys. Chem. Solids 62, 171 (2001).

[2] S. Chakraverty et al., Eur. Phys. J. B 5, 337 (1998).

[3] A.J. Leggett, Phys. Rev. Lett. 83, 392 (1999).

[4] L. Jansen and R. Block, Physica A 289, 165 (2001).

[5] S. Johnston et al., Phys. Rev. B 82, 064513 (2010).

[6] S. Ideta et al., Phys. Rev. Lett. 104, 227001 (2010).

[7] Y. Hirata et al., Phys. Rev. B 85, 054501 (2012).

Converting FeAs superconductors into ferromagnetic semiconductors

Yasutomo J. Uemura

Physics Department, Columbia University, New York, USA

In the mid 1990's Hideo Ohno and co-workers succeeded in substituting Mn into Ga site of a III-Vsemiconductor GaAs by using Molecular Beam Epitaxy (MBE), opening a field of diluted magneticsemiconductors (DMS). Since then, ferromagnetic (Ga,Mn)As as been extensively studied with respect topossible applications to spin sensitive electronics (spintronics) devices. Substitution of Mn2+ and Ga3+,however, led to limitations as (a) very small chemical solubility which prohibits availability of bulkspecimens; and (b) simultaneous spin and charge doping leading only to p-type systems.

Collaborative efforts by the group of Chanqing Jin (IOP), Fanlong Ning (Zhejiang U) and the presentspeaker have succeeded in synthesizing new DMS systems Li(Zn,Mn)As ([1] ferromagnetic Tc up to 50 K)and (Ba,K)(Zn,Mn)2As2 ([2] Tc up to 200 K), and (La,Ba)(Zn,Mn)AsO ([3] Tc up to ~ 50 K). These systemshave similar/identical crystal structures with those of FeAs superconductors LiFeAs, (Ba,K)Fe 2As2, andLaFeAsO with a very good matching of lattice parameters. Bulk specimens of these new DMS systms havealready enabled MuSR and NMR measurements, while future developments of single crystals may allowproduction of n-type ferromagnets, bipolar transistors, and multilayer/interface junctions of variouscombinations of lattice-matched companion compounds of semiconductor (without Mn), ferromagnet(1-15% Mn), antiferromagnet (100% Mn) and superconductor.

In this talk, I will describe our materials development and MuSR studies on these novel [1-3] DMS systems,and compare them with the results from traditional (Ga,Mn)As systems [4].

[1] D. Zheng et al. Nature Communications, 2, 422 (2011)[2] K. Zhao et al., Nature Communications, 4, 1422 (2013)[3] C. Ding et al., Phys. Rev. B 88, 041102(R) (2013)[4] S.R. Dunsiger et al., Nature Materials, 9, 299-303 (2010).

Spin wave dispersion in the helical spin ordered system SrFeO3-δ and CaFeO3

C. Ulrich1,2, G. Khaliullin3, D. Efremov3,5, M. Reehuis4, A. Maljuk3,5, A. Ivanov6, K. Schmalzl6, Ch. Niedermayer7, K. Hradil8, and B. Keimer3

1School of Physics, University of New South Wales, Sydney, NSW 2052, Australia2Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia

3Max-Planck-Institut f ür Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany4Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany

5Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden (IFW), Dresden, Germany6Institut Laue-Langevin, 156X, 38042 Grenoble, France

7Labor für Neutronenstreuung, Paul-Scherrer-Institut, CH-5232 Villigen, Switzerland8Forschungs-Neutronenquelle Heinz Maier-Leibnitz, FRMII, 85748 Garching, Germany

In the ferrates SrFeO3-δ and CaFeO3, spin and charge degrees of freedom play an intriguing role.Their detailed interplay results in various electronic and magnetic phases, for example asconsequence of charge order [1]. The ferrates are isoelectric to the Jahn-Teller distorted manganitesystem and exhibit also colossal magnetoresistance effects. But in contrast, the ferrates show ahelical instead of a collinear spin structure [2]. Oxygen doping has a dramatic effect on theelectronic properties of the ferrates since the oxygen deficiencies order systematically, leading todifferent well defined crystallographic phase with different electronic properties, e.g.metal-insulator transitions or charge order [1-3]. Remarkably, our elastic and inelastic neutronscattering experiments have revealed an almost universal magnetic behavior for all the differentelectronic phases. The spin wave dispersion is comprising upward- and downward-dispersingbranches in the form of an hour glass. Such a dispersion is common to compounds with metallicand charge-ordered insulating ground states and closely resembles the extensively studied, universaldispersion of spin excitations in layered copper oxides such has high temperature superconductors.The helical spin arrangement is a consequence a competition between long range double-exchangeand short range superexchange interactions [4]. Our theoretical calculations were able toconvincingly reproduce the helical spin structure and the experimentally obtained spin wavedispersion in the ferrates SrFeO3-δ and CaFeO3.

[1] A. Lebon, P. Adler, C. Bernhard, A.V. Boris, A.V. Pimenov, A. Maljuk, C.T. Lin, C.Ulrich, and B. Keimer, Phys Rev. Lett. 92, 037202 (2004).[2] M. Reehuis, C. Ulrich, A. Maljuk, Ch. Niedermayer, B. Ouladdiaf, A. Hoser, T. Hofmann, and B. Keimer, Phys. Rev. B 85, 184109 (2012).[3] J. P. Hodges, S. Short, J. D. Jorgensen, X. Xiong, B. Dabrowski, S. M. Mini, and C. W. Kimball,Journal of Solid State Chemistry 151, 190 (2000).[4] P.-G. De Gennes, Phys. Rev. 118, 141 (1960).

Ultrafast dynamics studied by time-resolved x-ray diffraction

H. Wadati1,2, P. Beaud3, U. Staub3, T. Tsuyama1,2, N. Pontius4, C. Schußler-Langeheine4,M. Nakamura5, S. Chakraverty5, H. Y. Hwang5,6, M. Kawasaki1,5 and Y. Tokura1,5

1 Department of Applied Physics and Quantum-Phase Electronics Center,University of Tokyo, Hongo, Tokyo 113-8656, Japan

2 Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan3 Swiss Light Source, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

4 Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH, 12489 Berlin, Germany5 RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan6 Department of Applied Physics, Stanford University, Stanford, California 94305, USA

X-rays from synchrotron radiation (SR) have time structures related to the SR pulsewidth of several 10 ps. X-ray free electron laser (XFEL) creates intense ultra-short (fs) x-ray pulses, enabling much more detailed study of the dynamics of the materials. We recentlyperformed a time-resolved x-ray diffraction and scattering study in a pump-probe setup by usingXFEL in LCLS (USA) and by using SR in BESSY (Germany). The pump light is Ti:sapphirelaser (800 nm), and the probe is XFEL or SR. Figure 1 (a) shows the time evolution of theintensity of the superlattice reflection (2 1/2 0) in charge and orbital ordered Pr0.5Ca0.5MnO3

thin films. One can see clear oscillations, which correspond to the frequency of phonons.Figure 1 (b) shows the time-resolved x-ray magnetic circular dichroism (XMCD) intensity inferromagnetic BaFeO3 thin films, showing rather slow demagnetization of ∼ 100 ps.

Figure 1: (a) Time evolution of the normalized diffracted x-ray intensity for the (2 1/2 0) reflectionin Pr0.5Ca0.5MnO3 thin films taken at 6.53 keV (off resonance). This superlattice peak is sensitive tothe structural atomic motion. (b) Time evolution of the XMCD intensity in BaFeO3 thin films takenat 710 eV (Fe 2p3/2 edge).

* This work supported by JSPS through the FIRST Program, initiated by the Council for Science and

Technology Policy (CSTP), and by the Ministry of Education, Culture, Sports, Science and Technology

of Japan (X-ray Free Electron Laser Priority Strategy Program).

1) P. Beaud et al., Nature Materials (2014), doi:10.1038/nmat4046.

Neutron and resonant inelastic x-ray scattering study of magnetic excitations in hole-doped La2-xSrxCuO4

Shuichi Wakimoto1

1Quantum Beam Science Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan

Magnetic excitations in the high-Tc cuprates show “hour-glass” type magnetic excitation,

whose origin is under debate. The hour-glass dispersion consists of two energy scales, below the saddle point energy Er of the hour-glass, the excitation stands at incommensurate position around (π, π) with incommensurability δ, and above Er the excitation shows apparently spin-wave-like dispersion. We have performed neutron and synchrotron x-ray scattering to observe magnetic excitations of heavily overdoped La2-xSrxCuO4 (LSCO) with x = 0.25 and 0.30 in a wide energy range to elucidate the relation between magnetic excitation and the superconductivity. Neutron were measured by the chopper spectrometer SEQUOIA at SNS and RIXS were measured using the AXES spectrometer at ESRF.

Neutron study using triple-axis and time-of-flight technique up to 100 meV energy transfer revealed that the spectral weight of the lower part of hour-glass dispersion decreases with superconducting transition temperature Tc. On the other hand, the high energy part (> 100 meV) observed by neutron and resonant inelastic x-ray scattering (RIXS) at Cu-L3 edge was found to have a dispersion relation similar to the spin-wave dispersion of non-doped La2CuO4 although the excitation in the overdoped sample is damped. Thus, the magnetic excitation of LSCO can be distinguished into two regions: low energy part which is highly sensitive to the doping, and the high energy part which is only weakly doping-dependent. We will report details of the above experimental results and discuss the origin of this phenomenon.

Figure 1: Cu-L3 RIXS sprctra of overdoped LSCO. Solid lines are spin wave dispersions of La2CuO4. Red circles indicate peak positions of magnetic excitations determined by neutron scattering.

Emergent Topological States in Iridium Oxides

Youhei YamajiQuantum-Phase Electronics Center, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku,

Tokyo, 113-8656, Japan

Iridium oxides have been intensively studied as a new platform of interplay betweenCoulomb repulsion and spin-orbit couplings [1,2], and resulting novel quantum phases [3].Especially, as possible realizations of Kitaev’s spin liquids [4] and Weyl semimetals [5], honeycomband pyrochlore lattice iridates have attracted much attention. However, there is hardly any clearexperimental confirmation of these quantum phases.

Here we theoretically clarify the ground states of a honeycomb iridate Na2IrO3 based on abinitio electronic structure calculations and propose a possible root to realization of the Kitaev’s spinliquid phase [6]. We also show that magnetic domain walls in pyrochlore iridates R2Ir2O7 (R:rare-earth elements) becomes topologically protected two-dimensional metallic layers as anindispensable footprint of pair-annihilated Weyl electrons [7].

1) B. J. Kim, Hosub Jin, S. J. Moon, J.-Y. Kim, B.-G. Park, C. S. Leem, Jaejun Yu, T.W. Noh, C. Kim, S.-J. Oh, J.-H. Park, V. Durairaj, G. Cao, and E. Rotenberg, Phys. Rev. Lett. 101, 076402 (2008).2) B. J. Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H. Takagi, and T. Arima, Science 323, 1329 (2009).3) W. Witczak-Krempa, G. Chen, Y.-B. Kim, and L. Balents, Annu. Rev. Condens. Matter Phys. 5, 57 (2014).4) J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010).5) X. Wan, A. M. Turner, A. Vishwanath, and S. Y. Savrasov, Phys. Rev. B 83, 205101 (2011).6) Y. Yamaji, Y. Nomura, M. Kurita, R. Arita, and M. Imada, Phys. Rev. Lett. 113, 107201 (2014).7) Y. Yamaji and M. Imada, Phys. Rev. X 4, 021035 (2014).

Ising spin nematic fluctuations near spin-density-wave phase

Hiroyuki Yamase1,2 and Roland Zeyher2

1National Institute for Materials Science, Tsukuba 305-0047, Japan, 2Max-Planck-Institute for Solid State Research, D-70569 Stuttgart, Germany

Employing a general form of a spin-fluctuation spectrum near the spin-density-wave

(SDW) phase, we study a spin-nematic susceptibility in energy and momentum space for various choices of a SDW critical temperature TSDW. Spin nematic temperature TSN depends on TSDW and typically becomes higher than the latter. We find that a temperature difference between TSN and TSDW increases monotonically as TSDW is higher. As a result, low-energy nematic fluctuations extend up to a higher temperature for a higher TSN. These properties originate from the general feature that the imaginary part of a spin-fluctuation bubble has a term linear in energy and its coefficient is proportional to the square of temperature. Consequently, approaching the spin nematic instability from high temperature, the nematic spectral function exhibits a central peak as a function of energy at zero momentum. Furthermore, we find that the nematic spectral function exhibits a diffusive peak around zero momentum and zero energy, and a clear dispersive feature is not seen. We discuss the origin of the nematic phase observed in iron-based superconductors as well as a possible connection to cuprate superconductors.

Figure 1: Typical phase diagram of the spin nematic phase (SN) near the SDW phase in the plane of a control parameter δ and temperature T. Strong low-energy spin nematic fluctuations extend up to a higher temperature for a higher TSN.

SDW

SN fluctuations

strong SNT

!

Spin, orbital, and spin-orbit excitations in iridates probed with RIXS

Young June Kim

Department of Physics, University of Toronto, 60 St. George St., Toronto, Ontario, M5S 1A7,Canada

An overview of electronic excitations in iridate materials measured with resonant inelastic x-rayscattering (RIXS) will be given. Unlike the first-row transition metal compounds, orbital angularmomentum is not quenched for the Ir4+ ions in iridates. The spin-orbit coupling instead stronglycouples spin and orbital moment, giving rise to an unusual spin-orbital entangled ground state.Exotic topological quantum phases are predicted to exist in the presence of such a strong spin-orbitcoupling and electron correlation. Physics of these phases can be explored by studying its excitationspectrum, using momentum-resolving spectroscopic tools such as RIXS. We present RIXS spectraof several iridate materials and discuss their implications.

Photoemission and inverse photoemission study of the correlated electron systemSrVO3

Teppei YoshidaGraduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan

SrVO3 is one of the perovskite-type light transition-metal oxides and is a prototypicalMott-Hubbard-type system with the d1 electronic configuration. Therefore, SrVO3 is an ideal systemto study the fundamental physics of electron correlation and has been extensively studied byphotoemission spectroscopy measurements. Also, the electronic structure of this system has beenstudied by a dynamical mean-field theory (DMFT) calculation, because the system is ideal for therealistic modeling of correlated materials.

Detailed ARPES measurements have been achieved by the growth of high-quality filmsusing the pulsed laser deposition technique. In our ARPES study of SrVO3 with thin-film sample,we have revealed the self-energy with “kink” structure, which is similar to the high-Tc cuprates [1].The extracted self-energy from the ARPES spectral weight is similar to the LDA+DMFTcalculation. This correlation effect is characterized by a mass renormalizaion of the coherentquasi-particle band as well as an incoherent part which reflects localized electrons.

On the other hand, a recent theoretical study by GW approximation has pointed out that sucha simple mass renormalization picture is not valid for the unoccupied electronic states [2]. Thus, wehave investigated the unoccupied electronic structure of SrVO3 by inverse photoemissionspectroscopy. As shown in Figure 1, we have observed at least three characteristic structures nearthe Fermi level. These structures may represent the t2g quasi-particle band and the eg band, similar tothe prediction by GW + DMFT calculation. The present interpretation is different from a previousinverse photoemission study of SrVO3 where the eg band was not taken into account.

Figure 1: Resonant inverse photoemission spectra of SrVO3

*This work is a collaboration with H. Sato, A. Fujimori, H. Kumigashira, M. Oshima, S. Miyasaka, S. Tajima, H. Eisaki, and S. Biermann.1) S. Aizaki et al., Phys. Rev. Lett. 109, 056401 (2012).2) J. M. Tomczak, M. Casula, T. Miyake, and S. Biermann, arXiv:1312.7546.

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