Emerging layered functional materials and their characterization

Resume : A new generation of candidate excitonic insulator, a layered chalcogenide Ta2NiSe5, was very recently proposed [1,2]. Band structure calculations show that the valence band comprises a hybridized Ni 3d/Se 4p state, while the conduction band contains the Ta 5d state. Since both the top of valence band and the bottom of conduction band are about touching at the Γ point, Ta2NiSe5 is a direct and almost zero-gap semiconductor/semimetal, which gives rise to an ideal playground for excitonic physics. An almost zero-gap semiconductor-to-insulator transition occurs at TC ∼ 328 K, accompanied with a second-order structural transition without any superlattice formation. The optical gap formed below TC is as large as ~0.2 eV, close to the exciton binding energy of 0.2-0.3 eV observed in the sister compound Ta2NiS5 with one electron energy gap EG of ~ a half eV[3]. Those results provide convincing evidences for the formation of excitonic insulating phase in Ta2NiSe5 below Tc. In this talk, we will present the most recent progress in the study of excitonic insulator Ta2NiSe5, including STM observation of excitation gap and understanding of the pressure-induced semimetallic phase with fattened layer above PS~3.0 GPa. [1] Wakisaka, Y. et al. , Phys. Rev. Lett. 103, 026402 (2009). [2] Lu, Y. F. et al., Nat. Commun. 8 14408 (2017). [3] Larkin, T. I. et al., Phys. Rev. B 95, 195144 (2017).

Resume : Semimetals with hole and electron Fermi pockets or semiconductors with a tiny band gap tend to exhibit charge/spin/orbital density wave transitions due to electronic coupling between the Fermi surfaces or between the valence band top and the conduction band bottom. Such charge/spin/orbital instabilities can be described by the theoretical framework of the excitonic insulator which has been established in 1960's [1-3]. The first angle-resolved photoemission spectroscopy (ARPES) study on a possible excitonic insulator system has been done for semimetallic TiSe2 [4]. Although the excitonic coupling between the valence band top and the conduction band bottom has been suggested, the excitonic insulator transition in TiSe2 is not established yet because of the deviation from the semimetal by the extra electrons in the conduction band as well as its large lattice distortion across the transition. Among the various excitonic insulator candidates, Ta2NiSe5 is very unique in that it is semiconducting below and above the transition temperature [5,6]. In addition, the transition is accompanied by a very small lattice distortion in contrast to the large distortion in TiSe2. Indeed, an ARPES study on Ta2NiSe5 suggested the BEC type excitonic insulator transition [7] which has been followed by recent time-resolved ARPES studies [8,9]. On the other hand, semiconducting Ta2NiS5 does not show any transitions probably due to the relatively large band gap. Another interesting point of Ta2NiSe5 and Ta2NiS5 is the smallness of charge-transfer energy which was indicated by Ni 2p core-level XPS spectra. Based on the ARPES and XPS results, we speculate that the hybridization between the Ni 3d and Se 4p(S 3p) orbitals plays an important role to keep the relatively large electron-hole interaction in Ta2NiSe5 and Ta2NiS5. [1] N. F. Mott, Philos. Mag. 6, 287 (1961). [2] D. Jérome, T. M. Rice, and W. Kohn, Phys. Rev. 158, 462 (1967). [3] B. I. Halperin and T. M. Rice, Rev. Mod. Phys. 40, 755 (1968). [4] H. Cercellier et al., Phys. Rev. Lett. 99, 146403 (2007). [5] F. J. Di Salvo et al., J. Less-Common Metals 116, 51 (1986). [6] Y. F. Lu et al., Nat. Commun. 8, 14408 (2017). [7] Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009). [8] S. Mor et al., Phys. Rev. Lett. 119, 086401 (2017). [9] K, Okazaki et al., submitted.

Resume : Excitonic insulating phase is a spontaneously condensed state of excitons due to their binding energies larger than a band gap. Ta2NiSe5 has been considered as a candidate of excitonic insulators based on its characteristic flat band observed by ARPES(angle-resolved photoemission spectroscopy) measurements. In order to investigate the transient electronic state of Ta2NiSe5, we have performed time-resolved ARPES using extreme ultraviolet laser from high harmonic generation. We have observed photo-induced insulator-to-metal transition with coherent phonons in Ta2NiSe5. We would like to discuss the mechanism of the observed photo-induced insulator-to-metal transition coherent phonons.

Resume : Ta2NiSe5 is proposed to support an excitonic insulator phase below Tc ≈ 328 K combined with a structural change. The former occurs in small gap semiconductors with strong electron-hole interaction where excitons form spontaneously and condense into a new insulating ground state. We study the ultrafast electron and lattice dynamics of Ta2NiSe5 by means of time- and angle- resolved photoemission spectroscopy [1] (trARPES) and time-resolved coherent optical phonon spectroscopy [2]. We find that the low temperature structural phase persists even for high excitation densities and the photoinduced structural phase transition is hindered by absorption saturation at Fc = 0.2 mJ cm-2. Below Fc, the band gap shrinks transiently due to photoenhanced screening of the Coulomb interaction. However, above Fc, the band gap transiently widens at the Gamma point and recovers to its equilibrium value after ≈ 1.5 ps. Hartree-Fock calculations reveal that the band gap widening is due to photoenhancement of the exciton condensate density, persisting until interband carrier relaxation occurs. These results demonstrate the possibility to manipulate exciton condensates with light and gain ultrafast band gap control. [1] S. Mor et al., Phys. Rev. Lett. 119, 086401 (2017). [2] S. Mor et al., Phys. Rev. B 97, 115154 (2018).

Resume : The narrow gap semiconductor Ta2NiSe5 has recently been proposed as a candidate for excitonic insulator (EI) accompanying soft phonon mode at Ts =328 K [1,2]. As the formation of excitons is strongly influenced by the carrier density, the electronic state can be investigated by controlling the electronic density state by use of physical pressure. In this context, high-pressure electrical resistivity measurements of Ta2NiSe5 produced a comprehensive pressure–temperature phase diagram that included semiconducting phase, EI phase, semimetal phase, pseudo gap phase, and a further high-pressure superconducting phase [3]. Therefore, in order to elucidate the electronic state on the pressure temperature phase diagram, it is necessary to elucidate the crystal structure of each phase. In this study we investigated the crystal structure of Ta2NiSe5 under a range of pressures, as determined by the complementary analysis of both single-crystal and powder synchrotron X-ray diffraction measurements. The monoclinic ambient-pressure excitonic insulator phase I transforms under a modest pressure to give the semiconducting C-centred orthorhombic phase II. At higher pressures (i.e. >3 GPa), transformation to the primitive orthorhombic semimetal phase III occurs. In the presentation, we will discuss the detail of the crystal structure of each phase and the mechanism of the structural phase transition from phase I to III. [1] Y. Wakisaka, et al., Phys. Rev. Lett. 103, 026402 (2009). [2] A. Nakano, et al., arXiv:1803.00274v1 [3] K. Matsubayashi, Pressure-induced semiconductor-metal transition and superconductivity in Ta2NiSe5. CiNii Articles. Available at https://ci.nii.ac.jp/naid/110009990261/en

Resume : History: Excitonic Insulator or more broadly Excitonic Phase (EP), a partiular state of matter resulting from the coherent condensate of excitons in narrow band semiconductors and semimetals, was theoretically proposed in early 1960's[1-4]. Excitons are pairs of electrons in conduction bands and holes in valence bands via Coulomb interaction, which are of course repulsive between electrons but turns into attractive because of different sign of effective masses between conduction band and valence band. In the case of semiconducting state (band insulators) there is no overlap in energy of electrons between conduction and valence bands to start with and then the result of onset of EP appears just as a renormalization of band dispersion keeping semiconducting state unchanged. In the case of semimetallic state, however, nature of conducting properties of EP depends on some details of band structures. For example, the original considerations for the semimetallic states was carried out on a model [3,5], which has one conduction band and one valence band with isotropic mass tensors. In this case the sizes and shapes of the fermi surfaces of electrons and holes are identical resulting in the perfect "nesting" of Fermi surfaces for which the same formalism as in superconductivity beautifully applies. In this case the EP is insulating at absolute zero because of the gap formation everywhere on the Fermi surface of the original bands, and may properly be called excitonic insulator. The phase diagram for this very special case was proposed on the plane of band gap, Vg ( Vg >0 for semiconductors, Vg< 0 for semimetals), and temperature, T. However the theoretical assumption concerning above-mentioned band structures is not realistic and should critically be kept in mind in comparison with experiments. Another important feature of the EP is the driving force toward this phase. The attractive forces between electrons and holes are direct Coulomb interactions and can be far stronger than the attractive forces toward superconductivity, which are due to electron-phonon interactions (BCS mechanism) or to exchange of some kind (spin, charge, orbital) bosonic fluctuations, which are indirect. Where is Excitonic Phase? There have been several experimental candidates. In 1960's soon after the theoretical proposal, EP was proposed[5] for the possible origin of the anomalous behaviors of ultrasonic attenuations in Bi single crystal under strong magnetic field where some particular pair of Landau levels of electrons and holes crosses. It has been clarified very recently [6], however, that the crossing of the pair of the Landau levels is due to twins and hence no physical effects are to be expected and then another proper pair of Landau levels should be studied. Later 1T-TiSe2 , which has a hole pocket at ?point and three electron pocket at zone boundaries and shows a transition to a phase with superlattices below 200K [7], was originally studied as a model case of charge density wave due to electron-phonon interaction but later there was an indication that the low temperature phase may be EP [8,9,10]. More recently Ta2NiSe5, which has been known for some time [11], has been proposed to be a candidate of EP [12]. In this case system undergoes a phase transition from orthorhombic to monoclinic structures at Tc=328K as temperature is lowered, with semiconductor-type conductivity below 500K which is far above Tc. On the other hand DFT calculations[13] predict semimetallic state with quasi-one-dimensional electronic state with one valence band and two conduction bands both at G point. It has been claimed that the semiconducting behaviors in the temperature region well above Tc indicates that excitonic correlations can be very strong [14, 15]. In this context it is of great importance and interest whether the two conduction bands come into play or the splitting of these bands is large enough leading to only one conduction band playing roles. Very recent ARPES experiment [16] under strong photoexcitation indicates the latter, a surprising finding. If this is the case, which may be due to the formation of bonding state between two Ta chains via Se, Ta2NiSe5 will be a unique example in the study of EP, where one conduction and one valence band forming the condensate in quasi one-dimension leading to a genuine excitonic insulators. Another possible example of EP is charge ordering (CO) often observed in oxides and molecular solids, between orbitals in the same atoms in the former and between different molecules in the latter. Especially in charge-transfer type molecular solids, CO was first predicted theoretically [17] through NMR experiments [18] in one-dimensional systems and turned out ubiquitous [19, 20]. Typically in the case of a-ET2I3 (ET=BEDT-TTF), which is a 3/4-filled layered systems, the conducting state turns into semiconducting state at 150K (metal-insulator (MI) transition) at ambient pressure, but this MI transition disappears above 2MPa[21.]. This MI transition has been identified as the onset of CO [22]. It has been clarified that electrons in this conducting state at high pressure are of massless Dirac (Weyl ) type [23]. If this transition is viewed from high pressure region, the change from gapless Weyl electrons to EP with narrow gap semiconductors can be viewed as "mass (gap) generation of Weyl electrons by excitonic correlations" [24]. Another possible candidate may be FeSe, which is semimetallic and belongs to a family of Fe high temperature superconductors in bulk. If FeSe layers are thin enough, system becomes semiconducting [25] and then EP may be realized near the boundary of superconductivity and semiconductors. For What ? : Thermoelectric Effect A very particular property of EP in the case of semiconducting states is the ?narrow gap semiconductors with strongly temperature dependent band-gap?. This feature results in strong variation of chemical potential with temperature which sensitively depends on off-stoichiometry or doping. These will first of all be reflected in the magnitude and temperature dependence of resistivity. The noteworthy diamagnetism both in TiSe2 [8] and Ta2NiSe5 [11] can also result from such small band-gap as already indicated by theoretical studies [26, 27]. However it is to be noted that careful treatment of interband effects of magnetic field [28, 29] should be properly assessed for diamagnetism as performed in [27]. Moreover Seebeck coefficient, whose electronic contribution is governed by the flow of electron energy relative to the chemical potential, is of great interest in view of recent microscopic calculations based on the linear response theory together with the help of thermal Green function applied to the region beyond Boltzmann transport theory, first of the kind [30, 31]. The Seebeck coefficient and power factor turn out to have strong dependences on the doping rate and chemical potential as clarified by the detailed studies on n-doped and bipolar carbon-nanotubes(CNT) clarifying the importance of band-edge engineering[31]. Acknowledgement The author thanks T. Mizokawa, S. Shin and Y. Iwasa for stimulations and T. Yamamoto, M. Ogata, H. Maebashi, H. Matsuura, A. Kobayashi and Y. Fuseya for informative discussions. References [1]A.N.Kozlov, and L.A.Maksimov, JETP 21(1965)790. [2]D.Jerome,T.M.Rice, and W.Kohn,Phys.Rev.158(1967)462. [3]For a review, B.I.Halperin, and T.M.Rice, Rev.Mod.Phys.40(1968)755; Solid State Physics(Academic Press, 1968)vol.21,p.115. [4]J.Zittarz, Phys.Rev.162(1967)752;164(1967)575;168(1967)605. [5 ] H.Fukuyama and T.Nagai, J.Phys.Soc.Jpn 31(1971)812. [6]Z. Zhu, B. Fauqué, K. Behnia, and Y. Fuseya, arXiv: 1801.07098. [7 ]J.A.Wilson,F.J.DiSalvo and S.Mahajan, Adv. In Phys. 24(1975)117. [8]F.J.DiSalvo,,D.E.Moncton, and Waszczak, Phys.Rev.B 14(1976)4321. [9]M.M. Traum, G. Margaritondo,N.V.Smith,J.E. Row, and F.J.DiSalvo, Phys.Rev.B 17(1978)1836. [10]H.Cercellier,C.Monney, F.Clerc, C.Battaglia,L.Despont, M.G. Garnier, H. Beck, P.Aebi, L. Patthey,H.Berger and L.Forro, Phys.rev.Letters 99(2007)146403. [11]F.J.DiSalve,C.H.Chen,R.M.Fleming, J.V. Waszczak, R.G.Dunn, S.A.Sunshine, and J.Ibers, J.of Less-Common Metals,116(1986)51. [12]Y. Wakisaka, T. Sudayama, K. Takubo, T. Mizokawa, M. Arita, H. Namatame, M. Taniguchi, N. Katayama, M. Nohara, and H. Takagi, Phys. Rev. Lett. 103 (2009) 026402. [13]T.Kaneko, T.Toriyama, T.Konishi, and Y. Ohta, Phys.Rev.B87(2013)035121. [14]K.Sugimoto, S. Nishimoto, T.Kaneko, and Y.Ohta, arXiv:1803.05622v1. [15]T.Yamada, K.Domon, and Y.Ono, J.Phys.Soc.Jpn 85(2016)053703. [16] K. Okazaki, et. al., unpublished and T. Suzuki et. al., unpublished [17]H.Seo and H.Fukuyama, J. Phys. Soc. Jpn. 66(1997)1249. [18]K.Hiraki and K.Kanoda,Phys.Rev.Lett.80(1998)4737. [19]H.Seo, C. Hotta and H. Fukuyama,Chem.Rev.104(2004)5005. [20]H.Fukuayama,Crystals(2012) 875. [21]For a review, K. Kajita, Y. Nishio, N. Tajima, Y. Suzumura, and A. Kobayashi, J.Phys.Soc.Jpn 83(2014)072002. [22]H.Seo, J.Phys.Soc.Jpn 69(2000)805. [23] S.Katayama, A. Kobayashi and Y.Suzumura, J.Phys.Soc.Jpn 75(2006)054705. [24]G.Matsuno and A.Kobayashi, J.Phys.Soc.Jpn 86(2017)014705. [25]J.He et al., Proc.Natl. Acad.Sci. USA 111(2014)18501. [26]K.Sugimoto and Y.Ohta,Phys.Rev.B94(2016)085111. [27]H.Matuura and M.Ogata, J.Phys.Soc.Jpn85(2016)093701. [28]M.Ogata and H. Fukuyama, J.Phys.Soc.Jpn84(2015)124708. [29] M.Ogata,J.Phys.Soc.Jpn 85(2016)064709. [30]M. Ogata and H. Fukuyama, J. Phys. Soc. Jpn 86 (2017) 094703. [31]T. Yamamoto and H. Fukuyama, J. Phys. Soc. Jpn 87 (2018) 024707.

Resume : Transition metal oxides often having a perovskite structure form a wide and technologically important class of compounds. In these systems, ferroelectric, ferromagnetic, ferroelastic, or even orbital and charge orderings can develop and eventually coexist. These orderings can be tuned by external electric, magnetic, or stress field, and the cross-couplings between them enable important multifunctional properties, such as piezoelectricity, magneto-electricity, or magneto-elasticity. Here, will illustrate with different examples of utilisation of oxide films. First, by growing PrVO3 thin films epitaxially on an SrTiO3 substrate, I will show that the role of oxygen vacancies can be rationalized to introduce a chemical strain similar to the so-called mechanical strain (±2%), which in turns produce a nontrivial evolution of Néel temperature in a range of 30 K.. Second, I will show that they can also be used as bio-adaptive surfaces, a field of research which is clearly unexplored. For this, we prepared a series of oxide thin films by the pulsed laser deposition technique, grown mesenchymal stem cells on these surfaces, and studied their adhesion and proliferation. We will discuss the feasibility of different thin films to promote appearance of multicellular structures with a better performance in terms of cell proliferation. These results will confirm the potential of such materials for various applications in electronic or medicine Financial support from ANR and Region Normandie (INCOX project) are acknowledged.

Resume : We developed a new technique of thermoreflectance that can precisely measure the thermal conductivity of thin films. By using this technique, we measured the thermal conductivity of SrVO3-SrTiO3 multilayer thin films normal to the surface, where one d electron exists at the V site but no d electron exists at the Ti site. We found convincing evidence that a large intrinsic thermal resistance exists at the interface between SrVO3 and SrTiO3, even though the phononic properties are quite similar for these two compounds. We propose that the reconstruction of the d orbitals near the interface substantially affects the thermal properties in these multilayer thin films.

Resume : In ferroelectric thin films, the polarization state, i. e. orientation and domain architecture, defines the macroscopic ferroelectric properties such as the switching dynamics. Ferroelectric domain engineering is in permanent evolution from the epitaxial strain tuning to the chemical control on interface atomic termination. Technology promising complex polar flux closure or vortices architecture have been recently demonstrated in ferroelectric heterostructures. However the mechanism involved in the formation of these complex polar states remains unexplored. The optical second harmonic generation process is an efficient and non-invasive tool for thin films ferroic properties probing. Here, we investigate the emergence of the ferroelectric polarization in ultra-thin ferroelectric and multiferroic films and monitor in situ the optical non-linear response of the films during the growth. We find that, the ferroelectric critical thickness and domain state can be addressed in situ during the film deposition. The impact of epitaxial strain, surface termination and electrostatic environment on the polar state of ultra-thin films and multilayers can now be explored in real-time, exempt from substrate contribution. Our work provides direct observation of ferroelectric states during the growth as well as new insights towards further control of ferroelectric-based heterostructure. M. Trassin et al., Adv. Mater 27, 4871 (2015) G. De Luca et al., Nat Commun. 8, 1419 (2017) J. Nordlander et al., Appl. Sci. 8, 570 (2018)

Resume : It has been well known that reduction annealing is necessary to realize superconductivity in the electron-doped cuprates, in addition to the Ce-atom substitution. Recently, however, improved annealing methods were shown to induce superconductivity in bulk crystals with Ce concentrations as low as ~ 5% and in thin films without Ce substitution. We have performed systematic studies of the effects of the improved annealing methods using ARPES [1,2] and core-level spectroscopy [3], and found that the annealing dramatically suppresses antiferromagnetic correlations and increases the electron carrier concentration as well as Tc over a wide electron doping range. The result means that a significant amount of oxygen atoms are removed from the block layer and/or the CuO2 plane, thereby providing the system with a large amount of electron carriers. This work has been done in collaboration with M. Horio, C. Lin, T. Mizokawa, K. Horiba, H. Kumigashira, T. Anzai, M. Arita, H. Namatame, M. Taniguchi, S. Ideta, K. Tanaka, S. Shin, H. Wadati, A. Yasui, E. Ikenaga, Y. Krockenberger, H. Yamamoto, T. Adachi, and Y. Koike. [1] M. Horio et al., Nat. Commun. 7, 10567 (2016). [2] M. Horio et al., arXiv:1801.03702 [3] M. Horio et al., arXiv:1710.09028; to appear in Phys. Rev. Lett.

Resume : We have performed a thorough experimental study of HgBa2CuO4+δ, which in many respects is a model cuprate compound. From the comparison with data for other cuprates we are able to separate universal underlying behavior from compound-specific features. The most remarkable finding is the existence of an underlying Fermi-liquid scattering rate [1] that remains essentially unchanged across the phase diagram [2,3]. Guided by established universalities, and by the knowledge that the cuprates are inherently inhomogeneous, we propose a simple model in which exactly one localized hole per planar copper-oxygen unit is delocalized and becomes itinerant with increasing doping and temperature [4]. The model is percolative in nature, with parameters that are nearly compound- and doping-independent and experimentally constrained. It comprehensively captures pivotal unconventional experimental results, including the temperature and doping dependence of the pseudogap phenomenon, the strange-metal linear temperature dependence of the planar resistivity, and the doping dependence of the superfluid density.

Resume : The high-Tc superconductivity in cuprates has been believed to appear by the doping of holes or electrons into Mott-insulating mother compounds. Recently, however, the mother compounds of electron-doped high-Tc cuprates with the T’-structure have been found to show superconductivity without carrier doping, when excess oxygen at the apical site is adequately removed. Here, our recent works on the electronic state and superconductivity in the undoped high-Tc superconductor T’-La1.8Eu0.2CuO4 (T’-LECO), using polycrystalline bulk samples, are reviewed. From the impurity effects on Tc, it has been found that the superconductivity has d-wave symmetry and is mediated by the spin fluctuation, because the impurity effects are very similar to those in the overdoped (OD) regime of hole-doped high-Tc cuprate T-La2-xSrxCuO4 (T-LSCO) with the T-structure [1]. From the muon spin relaxation measurements, it has been found that the superconductivity coexists with a short-range magnetic order due to a very small amount of excess oxygen. This suggests that T’-LECO is a strongly correlated electron system [2,3]. From the NMR experiment, it has been found that antiferromagnetic spin fluctuations exist in the normal state, which is similar to in the OD regime of T-LSCO. Moreover, the Knight shift and the spin-lattice relaxation rate 1/T1 have revealed that the superconductivity has d-wave symmetry [4]. Our works were performed in collaboration with K. Ohashi, T. Takamatsu, M. Kato, I. Watanabe, S. Ishiyama, M. Goto, S. Kanamaru, M. Hirata, and T. Sasaki. [1] K. Ohashi et al., J. Phys. Soc. Jpn. 85, 093703 (2016) [2] T. Adachi et al., J. Phys. Soc. Jpn. 85, 114716 (2016) [3] T. Kawamata et al., J. Phys. Soc. Jpn. [4] H. Fukazawa et al., Physica C 541, 30 (2017)

Resume : Ru1212 oxides belong to a class of ruthenocuprates which exhibit the remarkable co-existence of superconductivity and magnetism [1-2]. In this category of materials, RuSr2GdCu2O8 has a layered structure comprising of superconducting CuO2 layers sandwiched between a Gd layer on one side and a Sr layer on the other side, followed by a RuO6 layer sandwiched between two Sr layers. This structure is similar to the tetragonal lattice structure of the high Tc superconductor YBaCu3O7[2], with the difference that CuO ribbons, which connect the superconducting CuO2 planes, are replaced by RuO2 planes The apical oxygen connects the CuO2 and RuO6 planes along the c-axis. While the RuO2 layers are responsible for the magnetic ordering in this system, the superconducting properties are brought about by the CuO2 layers. Doping Cu in the RuO2 layers has been seen to bring about an increase in the superconducting transition temperature at the expense of magnetism when the dopant concentration is >20% [3]. The exact structural changes leading to this remain unknown. In this work, we consider the case of 10% Cu doping at Ru sites, with an aim to correlate the structural changes accompanying the magnetic transition in this compound. The factors affecting the magnetic ordering include the Cu-Cu distance, which directly affects the thickness of CuO2 layers, buckling angle of the CuO2 planes, and the Cu-O bondlengths. We have used temperature-dependent X-ray Absorption Fine Structure Spectroscopy at Cu-K edge to deduce the thermal evolution of Cu-O bond distances in the apical and basal directions, the Cu-Cu distances, and thus the change in Cu-O-Cu angle during the magnetic transition. Our results shed light on the magneto-elastic coupling existing in this material and hence provide incentives for design goals. References [1] I. Felner, U. Asaf, Y. Levi, and O. Millo, Phys. Rev. B 55 (1997) R3374; [2] O. Chmaissem, J. D. Jorgensen, H. Shaked, P. Dollar and J. L. Tallon Phys. Rev. B, 67 (2000) 6401; [3] P. W. Klamut, B. Dabrowski, S. Kolesnik, M. Maxwell, and J. Mais, Phys. Rev. B 63 (2001) 224512-1.

Resume : The discovery of superconductivity in fluorine doped quaternary LaFeAs(O_{1-x}F_{x}), has triggered large interest. Superconductivity can also be induced by electron doping with O deficiency or Co or Ni substitution in the Fe site as well as hole doping with Sr substitution to the rare earth site of the parent phase LaFeAsO. Unexpectedly, isovalent substitution of the Mn to the Fe site has a peculiar effect remains a matter of large debate. Small Mn concentration is enough to suppress the structural phase transition as well as the long-range magnetic order without inducing superconductivity. Therefore, in order to understand the peculiar properties of the Mn substitution, determination of the local structure and local atomic disorder of the system take a very important place. So we have studied the local structure of La(Fe_{1-x}Mn_{x})AsO system. Extended X-ray absorption fine structure (EXAFS) measurements at As K-edge and Fe K-edge are used to obtain direct information on the atomic correlations in the FeAs-layer and at La L_{3}-edge on the atomic correlations in the LaO-layer. We find that both Fe-As and Fe-Fe bond-lengths tend to increase with increasing the Mn concentration due to a larger size of this atom with respect to that of Fe. The associated mean square relative displacements (MSRD) reveal a gradual increase indicating increased local disorder in the Fe-As (active) layer with Mn content. La-O bond-lengths and associated mean square relative displacements (MSRD) did not show any significant change indicating Mn substitution has no significant effect on the La-O (spacer) layer. The results reveal that the Fe-As thickness, measured by the As-height (hAs) from the Fe-Fe plane, is correlated with the Néel temperature. The importance of the FeAs layer thickness and interlayer atomic correlations in describing magnetic correlations in La(Fe_{1-x}Mn_{x})AsO is discussed. References 1. Hacisalihoglu M. Y., Paris E., Joseph B., Provino A., Lamura G., Martinelli A., Manfrinetti P., Olivi L., Ozturk K., Dore P., Putti M. and Saini N.L., ?Local Structure and Magnetic Correlations in La(Fe_{1?x}Mn_{x})AsO System?, (To be submitted)

Resume : While pristine single graphene layers (SGL) seem to lack a superconducting phase, superconductivity (SC) in graphene may be induced by proximity effect or metal-decoration in the case of SGL and small twisting angles in the case of bilayer graphene. In the present study, spectroscopic signatures of various symmetries of the SC order parameter (OP) in metal-decorated graphene are examined. As a specific system we investigate CaC6, especially the OP dependent features of its quasiparticle spectra and simulated scanning tunneling spectra (STS). The computations have been performed using Nambu-Gorkov Green's function techniques within the framework of an effective tight-binding Hamiltonian. While various BCS-like anomalous matrix elements have been implemented to the Hamiltonian, our study indicates that p+ip-symmetric nearest-neighbor singlet pairing yields the most enhanced superconducting gap. However, there are several choices of orbital characters of OP matrix elements exhibiting this symmetry. In addition to singlet pairing, we make representative calculations with triplet pairing, and find but tiny differences in simulated STS spectra. To alleviate the difficulty in experimentally determining the symmetry and orbital character of the order parameter, we outline a method to distinguish between different OP characters using spin-polarized STS in the presence of magnetic substitutional impurities.

Resume : Transition-metal dichalcogenides with layered structure possess a wide range of physical properties with large potential in applications. Among these, MoCh2-based systems (Ch = S, Se, Te) have been in focus due to their potential use in electronic devices. The bidimensional nature of MoCh2-based compounds resulted correlated to their peculiar optoelectronic properties. Similar to other layered systems MoCh2-based systems exhibit an electronic band gap, whose value can be controlled by varying the number of layers and can be then optimized for a large variety of different applications. Recently, ammonia-thermal reaction has been used for molecular intercalation in layered MoSe2 producing Mx(NH3)yMoSe2 (M=Li, Na, K, Sr) superconductor with Tc~5 K. This new superconductor, with variable superconducting shielding fraction can be an ideal candidate for tunable transport properties of these materials. We have investigated the local structure of MoCh2 (for Ch = S, Se, Te) and intercalated Nax(NH3)yMoSe2 by means of EXAFS spectroscopy as function of temperature [1, 2]. Bond distances and Debye–Waller factors have been extracted by the analysis of the EXAFS oscillations. We find that the temperature dependence of the Debye–Waller factors is quite well reproduced by the standard correlated Einstein model, thus providing reliable values of the Einstein frequencies and, consequently, of the local force constants. The obtained local force constant looks to rescale linearly with the Mo-Ch interatomic distances, i.e. the thickness of the Mo-Ch layer, except for the MoTe2 case, where the effect of acoustic vibrational modes cannot be neglected. It appears that the Ch-Ch coupling in the structure is reduced by the intercalation or by increasing the Ch covalent radius. The decrease of the Ch-Ch coupling most likely increase the hybridization between Se 4p and Mo 4d, favoring metallicity in the intercalated MoCh2 system and hence superconductivity at low temperature. Refrences: [1] S Caramazza et. al, J. Phys.: Condens. Matter 28, 325401 (2016). [2] L. Simonelli et al, Journal of Physics and Chemistry of Solids 111, 70-74 (2017).

Resume : Since the discovery of BiS2-based superconductor in 2012, many kinds of BiS2-based superconductors have been discovered, and the possibility of unconventional superconductivity has been proposed by recent theoretical and experimental studies. The basic structure of the BiS2-based family is composed of the alternate stacks of insulating (blocking) layers and BiS2 conducting layers. When electron carriers are generated in the BiS2 layers, superconductivity is emerged. To further develop the physics and chemistry field and enhance superconducting, thermoelectric, or other functionalities, development of new BiS2-based compounds is needed. Recently, we synthesized and reveled crystal structure of La2O2Bi2M2S6 with M = BiAg and Pb. The structure can be regarded as a stacks of a La2O2Bi2S4 layer and a M2S2 rock-salt type layer. In this study, we will show crystal structure and physical properties of layered oxychalcogenide La2O2Bi2M2S6 (M = Pb, Ag, Cd, Bi).

Resume : Defect plays a crucial role for emerging and modifying the physical properties of solid. To understand the local structure of defects has not been so easy. High resolution photoemission holography, which is a combination of a measurement of angular dependent core level intensity (photoelectron hologram) and reconstruction of an atomic image from it, has a capability of studying chemical site resolved local structure around a specific atom. In this talk, we present recent results on determination of local structures around dopants in doped diamond, studied by high resolution photoemission holography

Resume : Three dimensional magnetic systems hold the promise to provide higher density devices and new functionality associated with greater degrees of freedom. Over the last years we have worked towards developing methods to fabricate and characterise three dimensional magnetic structures. Specifically, we have fabricated an artificial magnetic buckyball and performed a high spatial resolution tomographic characterisation of the structural and elemental properties [1]. In order to determine the magnetic configuration in such three dimensional systems, we have combined X-ray magnetic imaging with a new iterative reconstruction algorithm to achieve X-ray magnetic tomography [2,3]. In a first demonstration, we determine the three dimensional magnetic nanostructure within the bulk of a soft GdCo2 magnetic micropillar, observing a complex magnetic configuration consisting of vortices and antivortices that form cross-tie and vortex walls. By determining the magnetic structure surrounding singularity points found at the intersections of these magnetic structures we have identified the presence of Bloch points of different types [3]. X-ray magnetic nanotomography will enable to unravel complex three dimensional magnetic structures for a range of magnetic systems with high spatial resolution. [1] C. Donnelly et al, PRL 114, 115501 (2015) [2] C. Donnelly et al, PRB 94, 064421 (2016) [3] C. Donnelly et al, Nature 547, 328-331 (2017)

Resume : Due to excellent conductivity, large Seebeck coefficient and oxidation resistant characteristics, ITO is considered as promising high temperature thermoelectric sensitive material. The current carrier of ITO is induced by oxygen vacancy, which has significant impact on thermoelectric property of ITO. Here we reported the modulation of the current carrier concentration of ITO film with N doping, the microstructure of ITO film was revealed with XPS, and the stability of thermoelectric property of ITO films at high temperature was investigated.

Resume : BiCh2 (Ch=S,Se) based superconductors are attractive materials because they have analogous properties to cuprates and iron-based superconductors. LaO1-xFxBiSe2, which is one of these materials, shows superconductivity when F ion is substituted for O ion. At the F concentration of x=0.5, Fermi surface along (π,π) direction connects. Some theoretical calculations predicted appearance of a charge density wave (CDW) state because of Fermi surface nesting along the (π,π) direction at that F concentration. However, this has not been confirmed by direct observations. Here, we report observation of supermodulation in LaO1-xFxBiSe2 (x=0.1, 0.5) single crystals by scanning tunneling microscopy and spectroscopy. On the surface of the crystal prepared by cleavage, square lattice of Bi atom was observed at a positive bias voltage. Conversely, electronic supermodulation with the period of about 5 times of the lattice constant was observed for both samples at the negative bias voltage along the diagonal directions of Bi square, whose directions correspond to the theoretically predicted directions of CDW. However, the period of the observed supermoduration is inconsistent with the predicted CDW. Furthermore, the observation of the supermodulation in x=0.1 sample is also inconsistent with the theoretical prediction. These results indicate this supermodulation seems not to be described by the predicted nesting picture. We will discuss the detail of the supermodulation observed in LaO1-xFxBiSe2 (x=0.1, 0.5) in the presentation.

Resume : Quantum states of quasiparticles in solids are dictated by symmetry. Thus, a discovery of unconventional symmetry can provide a new opportunity to reach a novel quantum state. Recently, Dirac and Weyl electrons have been observed in crystals with discrete translational symmetry. Here we experimentally demonstrate Dirac electrons in a two-dimensional quasicrystal without translational symmetry. A dodecagonal quasicrystal was realized by epitaxial growth of twisted bilayer graphene rotated exactly 30 degreeC. The graphene quasicrystal was grown up to a millimeter scale on SiC(0001) surface while maintaining the single rotation angle over an entire sample and was successfully isolated from a substrate, demonstrating its structural and chemical stability under ambient conditions. Multiple Dirac cone replicated with the 12-fold rotational symmetry were observed in angle resolved photoemission spectra, showing its unique electronic structures with anomalous strong interlayer coupling with quasi-periodicity. Our study provides a new way to explore physical properties of relativistic fermions with controllable quasicrystalline orders.

Resume : We investigate the hidden Berry curvature in bulk 2H-WSe2 by utilizing the surface sensitivity of angle resolved photoemission (ARPES). The symmetry in the electronic structure of transition metal dichalcogenides is used to uniquely determine the local orbital angular momentum (OAM) contribution to the circular dichroism (CD) in ARPES. The extracted CD signals for the K and -K valleys are almost identical but their signs, which should be determined by the valley index, are opposite. In addition, the sign is found to be the same for the two spin-split bands, indicating that it is independent of spin state. These observed CD behaviors are what are expected from Berry curvature of a monolayer of WSe2. In order to see if CD-ARPES is indeed representative of hidden Berry curvature within a layer, we use tight binding analysis as well as density functional calculation to calculate the Berry curvature and local OAM of a monolayer WSe2. We nd that measured CD-ARPES is approximately proportional to the calculated Berry curvature as well as local OAM, further supporting our interpretation.

Resume : The observation of an extremely large and nonsaturating magnetoresistance (MR) in semimetallic WTe2 [1] has attracted considerable attention in recent years. Angle-resolved photoemission spectroscopy confirmed the existence of Fermi arcs connecting Weyl points of opposite chirality [2], confirming that WTe2 is a type-II Weyl semimetal. In order to investigate the electronic states of such Weyl semimetals, we have grown high-quality single crystals of WTe2 and others with the residual resistivity ratio higher than 10^3 [3], and have measured the electronic transport properties. We discuss the anisotropy of the electrical resistivity measured for different crystallographic directions, slight deviation from the Kohler’s rule, the Fermi surface topologies obtained from Shubnikov–de Haas oscillations, and magnetic impurity effects. [1] M. N. Ali et al., Nature., 514. 205 (2014). [2] Y. Wu, et al., Phys. Rev. B., 94.121113(R) (2016). [3] R. Jha et al., Physica B: Condensed Matter. 536. 68 (2018).

Resume : Layered Ge-Sb-Te compounds, also known as Phase Change Materials (PCMs), can be used for data storage applications as these materials can be easily switched from a high to a low resistive state. Here we are presenting recent advances in manipulating the defect structure (structural vacancies) in situ which are crucial for further understanding of the switching mechanisms. For amorphous Ge6Sn2Sb2Te11 a so far unreported intermediate step during the sheet resistance measurements was observed. Detailed in situ heating transmission electron microscopy (TEM) and in situ X-ray diffraction experiments were conducted, thus yielding a detailed analysis of the micro and nanostructural changes. Three distinct phases were observed, where each phase can be separated by its unique layered structure. As we will demonstrate these nanostructural changes are likely responsible for the unusual electrical behavior during the sheet resistance measurements. Further, in situ TEM measurements demonstrate that the order in certain phases is increased by in-plane movement of so called bi-layer defects. We are also able to show that this type of defect can be initiated by deliberate electron beam irradiation. This procedure allows investigating the dynamical reconfiguration of van der Waals gaps, possibly responsible for the facile switching processes in interfacial PCMs.

Resume : Heterointerfaces of complex oxides provide a rich playground to explore emergent phenomena that are not found in the bulk, which have been largely studied in 3d transition metal oxide (TMO) layered superlattices. With recent advances in the synthesis of 5d TMOs, superlattices based on 3d and 5d TMOs offer new model systems to realize emergent functionalities of complex oxides. In this talk, we demonstrate the emergent magnetism and perpendicular magnetic anisotropy in the layered superlattices, comprised of the colossal magnetoresistive manganite La1-xSrxMnO3 (LSMO) and the paramagnetic metal SrIrO3 (SIO). A weak ferromagnetism and a new spin-orbit state are induced in SIO due to dimensional confinement and interfacial coupling. On the other hand, LSMO shows ferromagnetism in superlattices for all compositions of x (x=0, 0.3, 0.5, 0.8, 1.0). The stabilization of ferromagnetism in both La rich and Sr rich LSMO superlattices points to an interfacial charge transfer which is confirmed by x-ray absorption spectroscopy measurements. Surprisingly, we find a strong interface-induced perpendicular magnetic anisotropy (PMA) for Sr rich compositions, which changes as a function of x. Synchrotron x-ray diffraction and linear dichroism measurements indicate a strong correlation between the PMA and the oxygen octahedral rotations at interfaces. Lastly, we show that the electronic and magnetic properties can be dramatically modulated by an electric field by using ionic liquid gating. Our comprehensive study provides insights into the unique emergent phenomena driven by the 3d-5d heterointerfaces due to the delicate interplay of charge, spin, orbital and lattice degrees of freedom. The emergence of a strong PMA and its modulation also point to the potential of its incorporation in future spintronic applications.

Resume : The combination of electron-electron correlations and high spin-orbit coupling in strontium iridates lead to exotic ground states in which the electronic and magnetic properties are strongly coupled with the lattice degrees of freedom. In the Mott-insulating state of Sr2IrO4, the complex isospin interactions are sensitive to the length and angle of the Ir-O bond, indicating epitaxial strain as a promising route to manipulate the ground state properties. When SrIrO3 is combined with other oxides in the form of a superlattice, the effect of confinement, as well as the lattice distortions due to interlayer coupling, lead to a metal-insulator transition (MIT) with magnetism developing in highly confined samples. In this talk, I will present an investigation of the effect of strain and dimensionality in strontium iridates using a combination of O K and Ir L-edge Resonant Inelastic X-ray Scattering (RIXS). In SrIrO3 we detect signatures of the MIT when the thickness of the iridate layer is reduced below 3 unit cells, with a clear evolution of the low-energy elementary excitations. In Sr2IrO4, we find epitaxial strain to control the magnetic correlations, with the spin-wave dispersion showing an anisotropic softening affecting mainly the (π,0) direction of the reciprocal lattice. By comparison with simulations based on band structure calculations, we assign a dispersive mode at 400 meV to electron-hole pair excitations. We find both energy and bandwidth of this mode to be highly affected by strain, connecting its development to the evolution of the band structure and the Mott insulating gap upon lattice distortions.

Resume : Sr2IrO4 is an antiferromagnetic insulator with TN = 240 K, being considered as a J_eff=1/2 analogue of s=1/2 cuprate superconductors. Indeed, a number of experimental evidences suggest that lightly-doped Sr2IrO4 shares a common nature with pseudogap state in cuprates, such as a symmetry-broken state [1], anisotropic excitation gap [2,3], and spin density wave [4]. In this talk, we will present our recent results on local crystal and electronic structure of lightly-doped Sr2IrO4, studied by EXAFS and ARPES[5]. Regarding the electronic states, we have found that a dispersive state develops inside the Mott gap by La doping, that coexists with lower Hubbard band. E-k dispersion of this in-gap state corresponds to that of calculated one by local density approximation (LDA) but with anisotropic energy shift. By raising temperature, the energy shift of the band tends to be suppressed. Regarding local crystal structures, marked differences have been found between Sr2IrO4 and cuprates. The estimated Einstein frequency of in-plane Ir-O bond is much higher than that of Cu-O bond in cuprates. Unlike in cuprates, Ir-O bond does not show softening accompanied by the formation of pseudogap in the lightly-doped regime. [1] J. Jeong et al., Nat. Commun. 8:15119 (2017). [2] A. de la Torre et al., Phys. Rev. Lett. 115, 176402 (2015) [3] Y. Cao et al., Nat. Commun. 7:11367 (2016) [4] X. Chen et al., Nat. Commun. 9:103 (2018). [5] K. Terashima et al., PRB 96, 041106(R) (2017).

Resume : Dion-Jacobson series with the general formula M[An-1BnO3n 1] with n=3 (M: alkali metal ions, A: alkaline earth metal ions, B: Nb, Ta) are new members of layered perovskite compounds. The alkali metal ions in the interlayer positions proceed ion-exchange reactions (hydrogen ion exchange reaction, etc.) so that the physical property of the compound is tuned by ordering the different cations between the interlayer spaces. The cations in the intralayer position can also be varied by supplying different amount of lanthanides instead of B2 alkaline earth metal so that the luminescence properties can be examined. Beside this, the nanosheet solution of the powders, which are fabricated following hydrogen exchange and exfoliation reactions between the layers are very important because of their large surface area required for high catalytic activity. In this study, triple layered KCa2TaxNb3-xO10 (x=0,1) powders doped with 1% Ru, Ir, Pd were prepared by solid state reaction. Here, noble metal atoms replaced with the Nb or Ta atoms sitting in the octahedral positions. Characterization of the main powders, H-exchanged forms (HCa2TaxNb3-xO10) and nanosheet form of them (Ca2TaxNb3-xO10 - ) was conducted by X-ray Diffractometer, Uv-Vis Absorbance and Diffuse Reflectance spectrometer, X-ray Photoelectron Spectrometer, SEM, STEM and AFM techniques. For the photoelectrochemical measurements, the films of the powder and nanosheets were fabricated. Two different methods were used to deposit the films onto the conductive FTO. The electrophoretic deposition is the technique to load main and H-exchange powders. The other technique, which is Layer by Layer deposition technique, was followed to deposit oxide nanosheets onto FTO to fabricate thin films. XRD, UV-vis absorption, DRS, XPS and SEM techniques were followed to characterize the produced films. The photocurrent generated by irradiation of chopping light is compared for doped and undoped perovskite films and these two method was compared in terms of photoelectrochemical activity. The XRD pattern of the main powders was in a good aggrement with the literature. The protonated form of KCa2TaxNb3-xO10 shows shifting to higher 2? value which proves that the interlayer distance between the layers has decreased and H-ion exchange step was completed. After H-ion exchange reaction, exfoliation reaction is followed with 1:1 TBA / Ca2TaxNb3-xO10- mol ratio. The concentration of TBAOH solution and the mol ratio between Ca2TaxNb3-xO10- / TBA are the parameters that can be tuned to achieve exfoliation step properly. The size, shape and thickness of the nanoplates were examined by getting STEM and AFM images. The effect of doping different noble metals on the band gap value has been examined by drawing Tauc Plot which is also supported by the XPS measurements. The main powders and H-ion exchanged forms were deposited on FTO in iodine-acetone solution by electrophoretic deposition technique. The deposition has occurred on the cathode electrode by applying 20-50 V along 10-30 minute. And, the dip coating technique were conducted to deposit negatively charged nanosheets onto the FTO electrodes by using cationic polymer PDDA. The photocurrent measurement was carried out in 0.1 M K2SO4 solution including 0.1 M methanol. As the light is on, the photocurrent produced has the higher value for the doped films for both techniques.