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Emerging layered functional materials and their characterization

In layered systems different electronic degrees freedom compete to result self organized textures with ranging scales. In such cases the functional properties are described by these defects. Thematic meetings are common, however, EMRS is an ideal platform to discuss different functional properties originated from their layered structural topology.


Various metal oxides/chalcogenides/pnictides with layered structure exhibit interesting and useful functional properties including high temperature superconductivity and giant thermoelectric performance. In these cases, the structural topology is deeply related to strong fluctuations of valence state and local bond. The layered structure topology of the functional materials is important also due to the fact that electronic degrees of freedom in the layered structures have strongly fluctuating character and the phases with charge (or magnetic order), coexisting with metallic (or superconducting) states can provide an effective mechanism for non-conventional phenomena. On the applied side, layered structures permit to have tuning of properties through external parameters as doping/intercalation, pressure, strain, electric and magnetic fields. This is of key interest for developing new materials through ‘control and manipulation’ for desired properties. Here, the focus is to discuss recent advances in the layered functional materials including emerging superconductors in which inhomogeneous ground state playing important role. In particular quantitative characterization of these materials is a challenging task requiring space and time resolved experimental tools applied under extreme conditions (e.g. pressure, electric and magnetic fields). The symposium welcomes contributions based on theoretical, experimental and applied aspects of the physics, chemistry, materials science on the hot topics given below.

Hot topics to be covered by the symposium:

  • Physics and chemistry of layered thermoelectric materials
  • Defect chemistry of layered pnictides and chalcogenides
  • Layered ferroelectrics and quantum paraelectrics
  • Pressure induced phases in layered materials
  • Intrinsic structure of layered functional materials
  • Emerging layered functional materials

List of invited speakers:

  • Peter Abbamonte, Illinois
  • Neven Barisic, Vienna
  • Sara Barja,San Sebastián
  • Atsushi Fujimori, Tokyo
  • Masaki Fujita, Sendai
  • Hidetoshi Fukuyama, Tokyo
  • Alexei Gruverman, Lincoln
  • Shintaro Ishiwata, Tokyo
  • Jun Sung Kim, Pohang
  • Kazutaka Kudo, Okayama
  • Chul-Ho Lee, Tsukuba
  • Morgan Trassin, Zurich
  • Jouko Nieminen, Tampere
  • Tetsuji Okuda, Kagoshima
  • Eugenio Paris, Zurich
  • Cedomir Petrovic, Brookhaven
  • Wilfrid Prellier, Caen
  • Valerio Scagnoli, Zurich
  • Shik Shin, Tokyo
  • Julia Stähler, Berlin
  • Yoshihiko Takano, Tsukuba
  • Taishi Takenobu, Nagoya
  • Hidenori Takagi, Stuttgart
  • Kensei Terashima, Okayama
  • Di Yi, Stanford
  • Takayoshi Yokoya, Okayama

List of scientific committee:

  • Yuji Aoki, TMU Tokyo, Japan
  • Arun Bansil, Northeastern U., Boston, USA
  • Leonardo De Giorgi, University of Zurich, Switzerland
  • Laszlo Forro, EPFL Lausanne, Switzerland
  • Takuro Katsufuji, Waseda University, Tokyo, Japan
  • Changyoung Kim, Seoul National University, Korea
  • Yoji Koike, Tohoku University, Sendai, Japan
  • Christos Panagopoulos, Nanyang University, Singapore
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Transition metal dichalcogenides-I : to be announced
Authors : Taishi Takenobu
Affiliations : Nagoya University

Resume : 2D layered materials have attracted much attention for exploring new electronic, optoelectronic, and photonic applications. Particularly, direct bandgap and unique electronic structure in monolayer transition metal dichalcogenides (TMDCs) provides a platform for exploring novel optoelectronic functionalities and devices. One of the most interesting properties of TMDCs is topological features, such as a non-centrosymmetric two-dimensional crystal, strong spin-orbit interaction, non-zero Berry curvature and resulting spin-valley coupling. Actually, circularly polarized light emission has been demonstrated. However, the fabrication of TMDC light-emitting devices are still limited, and this fundamental barrier has made investigating electroluminescence (EL) properties of TMDCs inevitably difficult. To overcome this issue, we developed the electrochemical method to dope both holes and electrons, and proposed a simple approach to form p-n junction universally in TMDCs [1-8]. Here, we apply this method into various forms of TMDCs and performed robust circularly polarized EL emission, arising from spin-valley coupling in TMDCs. Our approach paves a versatile way for using TMDCs in discovering new functional optoelectronic devices. [1] Nano Lett. 12, 4013, (2012). [2] ACS Nano. 8, 923 (2014). [3] APL 103, 23505 (2013). [4] ACS Nano. 8, 8582 (2014). [5] Adv. Mater. 28, 4111 (2016). [6] APL 109, 201107 (2016). [7] Adv. Mater. 29, 1606918 (2017). [8] Adv. Mater. just accepted (2018).

Authors : S. Barja, S. Wickenburg, B. Schuler, Z.-F. Liu, S. Refaely-Abramson, D. Y. Qiu, Y. Zhang, A. Pulkin, H. Ryu, M. M. Ugeda, C. Hwang, Z.-X. Shen, S.-K. Mo, M. B. Salmeron, F. Wang, M. F. Crommie, D. F. Ogletree, S. G. Louie, O. Yazyev, J. B. Neaton, A. Weber-Bargioni
Affiliations : Centro de Física de Materiales UPV/EUH-CSIC, 20018 San Sebastián, Spain & Donostia International Physics Center, 20018 San Sebastián, Spain & Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA; Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA; Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA; Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA; Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA; University of California at Berkeley, Berkeley, California 94720, USA; National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093 P.R.China; École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland; Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; Centro de Física de Materiales UPV/EUH-CSIC, 20018 San Sebastián, Spain & Donostia International Physics Center, 20018 San Sebastián, Spain; Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; Stanford Institute of Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA; Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; University of California at Berkeley, Berkeley, California 94720, USA; University of California at Berkeley, Berkeley, California 94720, USA; University of California at Berkeley, Berkeley, California 94720, USA; Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA; University of California at Berkeley, Berkeley, California 94720, USA; École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland; Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA; Molecular Foundry, Lawrence Berkeley National Laboratory, 94720 Berkeley, California, USA;

Resume : Intrinsic properties of 2D transition metal dichalcogenide semiconductors are highly sensitive to the presence of defects in the crystal structure, due to the confined nature of the electronic wavefunctions. Understanding the defect electronic structure at the atomic scale will enable unprecedented control over material functionality. In this talk I will present two different examples of structural defects in single layers of MoSe2. Using 4K scanning tunneling microscopy and spectroscopy, and non-contact atomic force microscopy we visualize and directly correlate in 2D MoSe2 the morphology and electronic properties of structural defects with atomic resolution. In the first example, we identified linear defects in from of mirror twin boundaries in MoSe2, which form 1D metal channels embedded in the surrounding semiconductor. At low temperatures these 1D metallic states open a band gap at the Fermi level of 100 meV together with periodic beatings in the local density of states, both characteristic of charge density waves [1]. In the second example, we suggest that the most abundant defect is an O-passivated Se-vacancy, instead of bare Se vacancy as commonly reported. This atomic defect forms an atomically confined Type I hetero junction with the surrounding pristine MoSe2 and lacks of any in-gap-states. The presence of in-gap-states will be decisive for the different optoelectronic and catalytic properties of the defect. References: [1] S. Barja, et al., Nature Physics, 12, 751, (2016)

Authors : A. Gruverman1, T. Li1, A. Lipatov2, H. Lu1, H. Lee3, J. Woo Lee3, Engin Torun4, L. Wirtz4, C. B. Eom3, J. Íñiguez5, and A. Sinitskii2
Affiliations : 1Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588 2Department of Chemistry, University of Nebraska, Lincoln, NE 68588 3Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706 4Physics and Materials Science Research Unit, University of Luxembourg, L-1511 Luxembourg 5Department of Materials Research and Technology, Luxembourg Institute of Science and Technology, 5 Avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg

Resume : Switchability of ferroelectric polarization enables control of a number of polarization-dependent electronic, mechanical, optical, and other functional properties, and provides a basis for development of advanced ferroelectric-based devices with enhanced functionality. Although polarization reversal is typically realized via application of an electric field, recently it has been shown that mechanical stress and chemical environment can also be used as external stimuli for polarization control. Here, we report a breakthrough finding of optically-induced switching of polarization in the hybrid electronic structures composed of ultrathin ferroelectric BaTiO3 and 2D molybdenum disulfide (MoS2) accompanied by a strong electroresistance effect. This scientifically exciting and potentially technologically important effect is attributed to the interplay between the photo-generated carriers and screening charges at the MoS2/BaTiO3 interface, which leads to destabilization of the existing polarization state and facilitates ferroelectric switching. Specifically, a two-step process, which involves formation of intra-layer excitons during light absorption followed by their decay into inter-layer excitons, results in the accumulation of the positive charges at the interface forcing the reversal of polarization from the upward to the downward direction. Theoretical modeling of the MoS2 optical absorption spectra with and without the applied electric field provides quantitative support for the proposed mechanism. What is especially important is that the observed optical electroresistance effect is not contingent on any specific property of the ferroelectric barrier. This means that it should be present in any hybrid structure composed of a ferroelectric and photo-absorbing narrow-band semiconductor. The obtained results are critical for fundamental understanding of the mechanism of the cross-interface coupling between the intrinsic properties of constitutive materials in hybrid ferroelectric/semiconductor structures. These results also open a possibility for optical control of the electronic transport in memory and logic devices composed of 2D materials and ultra-thin ferroelectrics allowing reduced operation power.

10:30 Coffee Break    
Transition metal dichalcogenides-II : to be announced
Authors : Kyoo Kim,Junho Seo, Eunwoo Lee, K. -T. Ko, B. S. Kim, Bo Gyu Jang, Jong Mok Ok, Jinwon Lee, Youn Jung Jo, Woun Kang, Ji Hoon Shim, C. Kim, Han Woong Yeom, Byung Il Min, Bohm-Jung Yang, and Jun Sung Kim
Affiliations : Department of Physics, POSTECH, Pohang 37673, Korea; Max Planck POSTECH Center for Complex Phase Materials, POSTECH, Pohang 37673, Korea; Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 37673, Korea; Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea; Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Korea; Center for Theoretical Physics (CTP), Seoul National University, Seoul 08826, Korea; Department of Chemistry, POSTECH, Pohang 37673, Korea; Department of Physics, Kyungpook National University, Daegu 41566, Korea; Department of Physics, Ewha Womans University, Seoul 120-750, Korea

Resume : Topological semimetals, new states of matters whose low energy electronic structure possesses several band contact points or lines, are generally expected to exhibit intriguing topological responses. Up to now, most of the studies on topological semimetals, especially those with nodal lines, are limited to non-magnetic materials with time-reversal symmetry. However, magnetic materials can also be endowed with a topological band structure in which the interplay of magnetism and band topology can generate novel correlated topological phenomena. Here we propose a van der Waals (vdW) material Fe3GeTe2 as a candidate for a ferromagnetic (FM) nodal line semimetal. In this system, the spin degree of freedom is fully quenched by the large FM polarization, while the line degeneracy is protected by crystalline symmetries connecting two different orbitals in neighboring layers. This orbital-driven nodal line is tunable by changing the spin orientation due to spin-orbit coupling (SOC) and can produce large Berry curvature, leading to a large anomalous Hall current. This turns out to make Fe3GeTe2 having the largest anomalous Hall angle and anomalous Hall factor among metallic ferromagnets, demonstrating that FM topological semimetal possesses a great potential for various spin and orbital-dependent electronic functionalities.

Authors : Shintaro Ishiwata
Affiliations : Department of Applied Physics, University of Tokyo

Resume : Transition-metal dichalcogenides with layered structure have been extensively studied for their wide variety of electronic properties such as superconductivity, large thermoelectric effect, and giant magnetoresistance effect. Among these layered transition-metal dichalcogenides, 1T'-MoTe2 has attracted recent attention because of a topological band structure characteristic to a type-II Weyl semimetal [1] and superconducting behavior in the polar phase [2]. The point to be noted here is that 1T'-MoTe2 is a rare system showing a temperature dependent polar-to-nonpolar transition with metallic conductivity. In this talk, we will show our recent discoveries on the unusual enhancement of thermopower in Nb-substituted MoTe2 at low temperatures, which is located near the critical region between the polar and nonpolar phases [3]. Since this behavior cannot be explained by the band structure calculations, it is presumable that the polar structural instability plays an important role on the enhancement of thermopower. In fact, we have recently demonstrated that MoTe2 under high pressures, which destabilize the polar structural distortion, shows the anomalous enhancement of thermopower, and thereby the giant thermoelectric power factor [4]. Such a significant influence of the structural instability on thermopower might arise from inelastic electron-phonon scattering enhanced by the polar structural instability, which would pave a novel route to improving thermoelectric efficiency. References [1] M. Sakano et al., Phys. Rev. B 95, 121101(R) (2017) [2] H. Takahashi et al., Phys. Rev. B 95, 100501(R) (2017). [3] H. Sakai et al., Science Adv. 2, e1601378 (2016). [4] H. Takahashi et al., in preparation.

Authors : Andrea Camellini (1), Carlo Mennucci (2), Eugenio Cinquanta (3-4), Christian Martella (5), Andrea Mazzanti (3), Alessio Lamperti (5), Alessandro Molle (5), Francesco Buatier de Mongeot (2), Giuseppe Della Valle (3-4), Margherita Zavelani-Rossi (1-4)
Affiliations : (1) Dipartimento di Energia, Politecnico di Milano, via G. Ponzio 34/3, I-20133 Milano, Italy; (2) Dipartimento di Fisica, Università di Genova, via Dodecaneso 33, I-16146 Genova, Italy; (3) Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, I-20133 Milano, Italy; (4) IFN-CNR, Piazza L. da Vinci 32, I-20133 Milano, Italy; (5) IMM-CNR, Unit of Agrate Brianza, via C. Olivetti 2, I-20864 Agrate Brianza, Italy;

Resume : Two-dimensional transition metal dichalcogenides (TMDs) represent very promising layered materials due to their potentially useful electronic and optical characteristics. Their functional properties can be tailored by manipulating the spatial profile, i.e. nanopatterning [1]. So far, the effect of nanopatterning has been considered only in terms of Raman modes, photoluminescence or absorption [2] whereas morphology induced anisotropy has been largely unexplored. We present a detailed analysis of the linear and non-linear transient optical properties of a few layer MoS2 sheet, deposited by highly conformal chemical vapor deposition, onto a SiO2 substrate with a uniaxial nano-corrugation [3]. Our study of the morphology dependent behavior is based on polarization resolved optical extinction, ultrafast pump-probe experiments and full-wave numerical simulations. We disclose a rich scenario, with: (i) a giant optical anisotropy in the spectral region of the excitonic peaks (A, B, C, D); (ii) a polarization dependence of the ultrafast relaxation dynamics in the region of C exciton; (iii) a key-role of the out-of-plane component of MoS2 permittivity that has been overlooked in literature. Our findings demonstrate that morphology manipulation by substrate nanopatterning is a viable tool for tailoring the opto-electronic properties of TMDs. [1] C. Martella et al, Adv. Mater. 2018, 30, 1705615 [2] C. Martella et al, Adv. Mater. 2017, 29, 1605785 [3] A. Camellini et al, ACS Photonics submitted

Authors : Hyangsook Lee1,2, Yeonchoo Cho3, Eunha Lee1*, and Hyoungsub Kim2*
Affiliations : 1. Analytical Engineering group, Platform Technology Laboratory, Samsung Advanced Institute of Technology, Suwon 16678, Republic of Korea 2. School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea 3. Device Lab., Samsung Advanced Institute of Technology, Suwon 16678, Republic of Korea

Resume : The crucial role of anion vacancies on the phase stability of synthetic metal chalcogenides was elucidated by demonstrating the environment-dependent, reversible phase transition (2H ↔ 1T/1T’) of the MoS2 films synthesized by chemical vapor deposition (CVD). The origins of the S-vacancy-controlled phase transition of the synthetic MoS2 films were supported by various transmission electron microscopy (TEM) studies as well as density functional theory calculations. Under high-vacuum conditions, transition to a metastable 1T/1T’ phase was induced by weak electron irradiation during a TEM observation and explained by the detachment of molecules chemisorbed on the S-vacancy sites, and the subsequent electron-delocalization (charge-transfer) process. In addition, a spontaneous backward transition to a 2H phase could be triggered by exposing the sample to air, which induced electron localization by re-adsorption of ambient molecules on the S-vacancy sites.

Authors : F. Huw Davies, S. P. Hepplestone
Affiliations : Department of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom

Resume : The band gap is of paramount importance to almost all of the electronic and optical properties of materials. By controlling the size and structure of the band gap, we can control both the transport properties and the optical interactions. The ability to energetically control electrons in solid-state devices is pivotal in the fields of sensing, renewable energy [1], information processing and communications technology [2]. Our insights offer the potential of engineering not just the band gap, but the electronic dispersion itself; making it far more versatile than strain engineering of band gaps. Transition metal dichalcogenides (TMDC) is a field of growing research interest in recent years, but TMDC heterostructures have only recently become experimentally viable [3]. We have done a large scale first principles study of TMDCs using density functional theory. These TMDC heterostructures demonstrate weak inter-layer interactions. Due to this, the band structures of individual layers of a heterostructure have a high fidelity to their isolated counterparts. A layered heterostructure will therefore, have a dispersion which consists of an overlay of its components' band structures. We determine that the electronic dispersions of TMDC heterostructures can be tailored by layer composition, and demonstrate a wide range of potentially attainable band structures. 1. Phys. Rev. B 92, 075439 2. Nat. Comm, 8, 15251 3. ACS Nano, 8, 9550

12:45 Lunch Break    
Superconductivity in layered Chalcogenides & Pnictides : to be announced
Authors : Yoshihiko TAKANO
Affiliations : 1. National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, 305-0047, Japan 2. University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8577, Japan

Resume : Data-driven material science (materials informatics, materials genome initiative, chemometrics, and so on) recently brings remarkable results in the field of medicine and et al.. On the other hand, a search for new functional materials of thermoelectric and/or superconducting materials have been still conducted through a carpet-bombing type experiment depending on the experience and inspiration of researchers. We have exhaustively searched the candidates of new thermoelectric and superconducting materials by first-principle calculation as a guideline which is a specific band structure of “flat band” near fermi energy, such as multivalley, pudding, and topological-type structures. If such kinds of flat band approach to the fermi energy, the thermoelectric properties of electrical conductivity and Seebeck coefficient would be enhanced. If the flat band crosses the fermi energy, superconductivity would be appeared due to high density of state (DOS) near the fermi energy In my presentation, I will talk about successful demonstration of the discovery of new superconductor under high pressure by data-driven materials research.

Authors : Cedomir Petrovic
Affiliations : Condensed Matter Physics, Brookhaven National Laboratory

Resume : Superconductivity and charge-density-wave (CDW) are traditionally viewed as Fermi surface instabilities due to electron-phonon coupling whereas arguments have been made both for their cooperation and competition [1-3]. In copper oxides dome of high superconducting Tc exists at a rather complex phase diagram where disorder and various forms of symmetry breaking orders are present, such as spin-density, CDW or nematic order [4]. In this talk I will discuss emergence of superconductivity from the standpoint of disorder in doped two-dimensional (2D) CDW conductors ZrTe3 and 2H-TaSe2-xSx [5-10]. Superconducting domes in these materials exist in the absence of magnetism and could be of interest as an example of non-magnetic limit of copper oxide phase diagram. This work offers new 2D van der Waals bonded bulk single crystals for atomic layer engineering and possible tuning of cooperative phenomena at the nanoscale [11-12]. References: [1] G. Grüner, Rev. Mod. Phys. 60, 1129 (1988), [2] T. Kiss et al., Nature Physics 3, 720 (2007), [3] S. V. Borisenko et al., Phys. Rev. Lett. 102, 166402 (2009). [4] E. Fradkin et al., Rev. Mod. Phys. 87, 457 (2015), [5] Xiangde Zhu et al., Phys. Rev. Lett. 106, 246404 (2011), [6] Hechang Lei et al., Europhys. Lett. 95, 17001 (2011), [7] Xiangde Zhu et al., Sci. Rep. 6, 26974 (2016), [8] Lijun Li et al., NPJ Quantum Materials 2, 11 (2017). [9] M. Hoesch et al., arXiv:1712.03379, [10] A. M. Ganose et al., arXiv:1712.06551. [11] Xiaxiang Xi at al, Phys. Rev. Lett. 117, 106801 (2016), [12] Xiaoxiang Xi et al., Nature Physics 12, 139 (2016),

Authors : Kazutaka Kudo
Affiliations : Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan

Resume : Alkaline-earth platinum pnictides exhibit a variety of hexagonal structures that are characterized by honeycomb networks, such as CaPtxP2-x, SrPtAs, and BaPtSb with an AlB2-, a KZnAs-, and a SrPtSb-type structures, respectively. SrPtAs exhibits superconductivity at 2.4 K, as we reported [1]. Superconductors with honeycomb networks have attracted interest since the theoretical predictions of exotic superconductivity in SrPtAs [2]. In order to examine the predictions, novel compounds should be developed. BaPtAs has been known to crystallize in the cubic LaIrSi-type structure. We discovered novel hexagonal structures of BaPtAs with ordered PtAs honeycomb networks, namely, SrPtSb- and YPtAs-type structures [3]. Both phases exhibited superconductivity at 2.8 and 2.1-3.0 K, respectively [3]. BaPtSb also exhibited superconductivity at 1.64 K [4]. Inversion symmetry is broken in the SrPtSb-type, whereas it is preserved in the YPtAs-type. Our discovery provides opportunities not only for the experimental examination of the predicted superconductivity but also for further studies on exotic states that result from the strong spin-orbit interaction of Pt under broken inversion symmetry. This work was conducted in collaboration with Prof. M. Nohara. [1] Y. Nishikubo, K. Kudo, and M. Nohara, J. Phys. Soc. Jpn. 80, 055002 (2011). [2] J. Goryo et al., Phys. Rev. B 86, 100507(R) (2012). [3] K. Kudo et al., submitted. [4] K. Kudo et al., J. Phys. Soc. Jpn. 87, 063702 (2018).

Authors : Masashi Tanaka, Masahiro Ohkuma, Masanori Nagao, Masaki Mito, Hiroyuki Takeya, Yoshihiko Takano
Affiliations : Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata, Kitakyushu 804-8550, Japan; MANA, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan; University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 400-8511, Japan

Resume : ?F-free? CeOBiS2 single crystals have successfully grown, thoroughly eliminating a concern about F-contamination by using a high-purity CsCl flux. The obtained crystals have a plate-like shape with a size of 1.0-3.0 mm in the well-developed plane. Single crystal X-ray structural analysis clearly revealed that the CeOBiS2 crystallizes with a space group P4/nmm (with lattice parameters of a = 4.0189(6) Å, c = 13.573(2) Å). The bond valence sum estimation and X-ray photoelectron spectroscopy analysis showed that the chemical state of Ce was in the mixed valence of Ce3+ and Ce4+. The single crystals show superconductivity with zero resistivity at ~1.3 K. This is the first conclusive evidence of superconductivity driven by Ce valence fluctuation in surely non-doped CeOBiS2. [1] The superconducting transition temperature was enhanced up to ~3.8 K by applying hydrostatic pressure. The enhancement of Tc was also confirmed by magnetization measurements using miniature diamond anvil cell and piston cylinder type high pressure cell. [2] References [1] M. Tanaka et al. J. Alloys. Compd. 722 (2017) 467-473 [2] M. Ohkuma, M. Tanaka et al. to be published in 2018.

15:45 Coffee Break    
Layered thermoelectrics and related systems : to be decided
Authors : Tetsuji Okuda
Affiliations : Graduate school of science and engineering, Kagoshima University, Kagoshima 890-0065, Japan

Resume : Delafossite CuCrO2 has magnetic Cr layers which form an S = 3/2 antiferromagnetic (AF) triangular lattice. The compound shows an AF transition at the Néel temperature (TN) from a paramagnetic state to a non-collinear 120 degree Néel state. While a slight substitution of nonmagnetic Mg2+ ions for Cr3+ ions much increases the electric conductivity, it slightly increases the TN and dramatically enhances the magnetization around TN. [1] Since a chemical disorder introduced by an isovalent substitution (for example, a substitution of nonmagnetic Al3+ ions for Cr3+ ions) simply disturbs the AF transition, the promotion of Néel state by the Mg substitution seems to come from the enhancement of spin fluctuation around TN caused by the doping of itinerant hole. [2,3] In this talk, I will show some detailed experimental results and will discuss the origin of the nontrivial promotion of the 120 degree Néel state from the view point of the coupling between itinerant hole and localized spin. References 1. T. Okuda, N. Jufuku, S. Hidaka, and N. Terada, Phys. Rev. B 72, 144403 (2005). 2. T. Okuda, Y. Beppu, Y. Fujii, T. Onoe, N. Terada, and S. Miyasaka, Phys. Rev. B 77, 134423 (2008). 3. T. Okuda, R. Kajimoto, M. Okawa, and T. Saitoh, Int. J. Mod. Phys. B 27, 1330002 (2013).

Authors : C.H.Lee(1), H.Kunioka(1), K.Kihou(1), H.Nishiate(1), Y.Mizuguchi(2), T.Hasegawa(3), S.Kawamura(4), M.Nakamura(4), K.Nakajima(4), and K.Suekuni(5)
Affiliations : (1) National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan; (2) Tokyo Metropolitan University, Japan; (3) Hiroshima University, Japan; (4) Japan Atomic Energy Agency (JAEA), Japan; (5) Kyushu University, Japan

Resume : Suppressing lattice thermal conductivity (kL) as low as possible is essential to achieve high thermoelectric performance. One of effective methods is to use the rattling which is large anharmonic vibration of atoms. Typically, rattling atoms locate in oversized atomic cages of caged compounds like clathrates and skutterudites. Recently, we found several new thermoelectric materials that contain rattling atoms without oversized atomic cages. In LaOBiSSe that exhibits low kL, Bi atoms vibrate largely toward out of Bi(S,Se)-plane [1,2]. In tetrahedrites that exhibit extremely low κL ~ 0.5 W/mK with ZT = 1.0 at 673 K, Cu atoms rattle perpendicular to the S3-triangle [3]. In common, rattling atoms have a planar coordination in both compounds. The question is, then, what is the driving force for the occurrence of rattling in a planar coordination? To solve the problem, we investigated crystal structures and phonon dynamics of tetrahedrites and LaOBi(S,Se). We found that the amplitude of Cu rattling increases with decreasing S3-triangle area. Rattling modes were observed to lie around E = 3 meV by inelastic neutron scattering. The rattling energy decrease with decreasing the S3-triangle area and finally damped demonstrating an enhancement of anharmonicity. In LaOBi(S,Se), phonon energy of Bi rattling modes decreases with substituting S by Se that has large atomic size. The results suggest that chemical pressure is essential for the appearance of rattling under a planar coordination in contrast to caged compounds where free space is essential. [1] Y. Mizuguchi et al., J. Appl. Phys. 119, 155103 (2016). [2] C. H. Lee et al., Appl. Phys. Lett. 112, 023903 (2018). [3] K. Suekuni, C. H. Lee et al., Adv. Mater. (2018).

Authors : Thorsten Schmitt 1, Daniel E. McNally 1, Xingye Lu 1, Jonathan Pelliciari 1, Sophie Beck 2, Marcus Dantz 1, Muntaser Naamneh 1, Tian Shang 1, Marissa Medarde 3, Christof W. Schneider 4, Vladimir N. Strocov 1, Ekaterina V. Pomjakushina 3, Claude Ederer 2, Milan Radovic 1
Affiliations : 1 Photon Science Division, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland; 2 Materials Theory, ETH Zürich, Wolfgang-Pauli Strasse 27, CH-8093 Zürich, Switzerland; 3 Laboratory for Scientific Developments and Novel Materials, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland; 4 Laboratory for Multiscale Materials Experiments, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Resume : Controlling transport and magnetic properties in the ultra-thin limit is a prerequisite towards design of more complex heterostructures, where emergent phenomena have been widely predicted. Meanwhile, thickness-driven metal-insulator transitions (MIT) in thin films have been reported in recent years in a broad swathe of correlated 3d and 5d transition metal oxides. Resonant inelastic X-ray scattering (RIXS) is a photon-in / photon-out spectroscopic probe of the electronic structure of condensed matter with atomic and orbital sensitivity. It is a unique tool for studying excitations in correlated transition metal oxides, being directly sensitive to lattice-, charge-, orbital- and spin-degrees of freedom. Bulk CaVO3 is a correlated paramagnetic metal but with decreasing thickness it evolves towards an insulating state. Thick films (>20 u.c.) are metallic, ultrathin films (≤ 6 u.c.) are insulating, while a 10 u.c. CaVO3 film exhibits a clear temperature-driven MIT. We present x-ray absorption spectroscopy and RIXS experiments at the V L3-edge of metallic and insulating CaVO3 films, as a function of both thickness and temperature. Our experiments reveal that the thickness-driven MIT is accompanied by local moment formation driven by charge redistribution and very large reduction in the electronic bandwidth accompanied by decreasing V-O hybridization. Our results suggest that the thickness-driven MIT results from Mott-like correlations that can host charge or spin ordering.

Poster session : to be decided
Authors : P.V. Galiy a), T.M. Nenchuk a), I.R. Yarovets' a), Ya.M. Buzhuk a), O.R. Dveriy b)
Affiliations : a) Electronics and Computer Science Dept., Ivan Franko National University of Lviv, 50 Dragomanov Str., 79005 Lviv, Ukraine; b) Chair of Electromechanics and Electronics, National Academy of Land Forces, 32 Geroiv Majdanu Street, Lviv, 79012, Ukraine

Resume : Formation of metallic self-assembled nanostructures on the corrugated layer structure of In4Se3 crystal [1] is a key to provide new functionality for 2D atomic thin systems, e.g., InSe based field-effect devices [2]. XRD data show that Ag intercalation doesn't affect initial crystal symmetry, but only changes a lattice parameter normal to the layer, while b and c ones in the layer plane are almost stable for all studied Ag concentrations. Besides, the a lattice parameter grow almost linear in the range of 0-3 at.% averaged Ag bulk concentrations and its value saturates at higher Ag concentrations. Such limit in Ag bulk concentration, suitable for excellent In4Se3 layered structure formation yet, is obtained for up to 35% of silver in the melt, when above this value a 2D to 3D phase transition in the crystal structure occurs. The real concentrations of silver in the interlayer gaps are much higher and were evaluated using surface-sensitive methods, because after splitting the content of the interlayer gaps becomes (100) surface. AES data of Ag value are in the range of 15-20 at.%. STS analysis on nanoscale according to ratio of metallic and semiconductor points shows 12-24%. The local density of states contain additional peak next to the Fermi level in the tip-surface range of biases correlating to energy gap of In4Se3. [1] K. Fukutani, et al., Phys.Rev.B 93 (2016) 205156. [2] D.A. Badurin, et al., Nat. Nanotechnol., 12 (2017) 223.

Authors : Renata Karpicz, Ignas Čiplys, Irena Kulszewicz-Bajer
Affiliations : Center for Physical Sciences and Technology, Sauletekio ave. 3, LT-10257 Vilnius, Lithuania Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszaw, Poland

Resume : Molecular switches can be commonly applied to control different functions and properties of materials which can be used in organic electronics (as for example in new memory elements based on single molecule) or in biology to manipulate the biological systems. Photochromism of dimethyldihydropyrene derivatives is a reversible transformation under UV and visible light irradiation between two, open-ring hexatriene and closed-ring cyclohexadiene, isomers with different spectroscopic properties. Usually, the optical-active derivatives of diarylethene were fluorescent in the open-ring state and non-fluorescent in the closed-ring form. New dimethyldihydropyrene derivatives were synthesized, and their optical properties as well as excited state dynamics were investigated in the solutions. We focus on the emissive properties of dimethyldihydropyrene derivatives with the possibility to switch them between fluorescent and non-fluorescent states. During the first 100-300 ps after excitation under visible light the closed-ring cyclohexadiene isomer were opened. Reverse transformation took place through intermediate stage during several picoseconds.

Authors : Aexander Ryabchikov, Igor Stepanov, Denis Sivin, Olga Korneva, Peter Ananin, Sergey Dektyarev
Affiliations : National Research Tomsk Polytechnic University, Tomsk, Russia

Resume : This paper presents the results of the formation of deep modified layers in AISI 420 alloy steel using a high-intensity repetitively pulsed nitrogen ion beam with a current density up to 0.25 A/cm2. An arc generator with a hot cathode provided the DC nitrogen plasma flow. A plasma immersion approach was used for high-frequency, short-pulse very intense nitrogen ion beam formation. A grid hemisphere with radii of 7.5 cm was immersed in the plasma. Negative bias pulses with an amplitude of 1.2 kV, a pulse duration of 4 μs, and a pulse repetition rate of 10 kHz were applied to the grid. The substrates were implanted at the temperature of 500 ˚C and various processing times ranging from 20 to 120 minutes with 1.2 keV nitrogen ions using a very-high current density up to 0.25 A/cm2 ion beams. The work explores the surface morphology, elemental composition, and mechanical properties of deep-layer modified AISI 420 alloy steel after low ion energy, very-high-intensity nitrogen ion beam implantation.

Authors : A.I. Ryabchikov*, I.V. Lopatin**, Y.H. Aakhmadeev**, D.O. Sivin*, O.S. Korneva*, O.V. Krysina**
Affiliations : * National Research Tomsk Polytechnic University, Tomsk, Russia; ** Institute of High Current Electronics,Siberian Branch Russian Academy of Sciences, Tomsk, Russia

Resume : The results of experiments on low-energy implantation of nitrogen ions in titanium alloy Grade-2 are presented. The treatment by a pulsed beam of nitrogen ions obtained by means of a ballistic ion focusing system was performed. The ion focusing system consisted of a box-shaped case with a rectangular bottom. One side of the box by a metal grid in the form of a half-cylinder was covered. The specimens were located on the collector of the system, which was placed inside the box in the focal plane of the grid. Pulse negative bias on the box and collector was applied. The ions were extracted from the nitrogen plasma source of the arc discharge with a thermionic cathode of extended configuration. It was shown that the treatment of specimens of titanium alloy Grade-2 in such a system leads to their saturation with nitrogen. However, intense ion etching occurs on the surface of specimens. Thus, not only the phase composition of the near-surface layers of the samples changes, but also their surface profile.

Authors : Raminta Mazetyte, Urte Bubniene, Vidmantas Gulbinas, Arunas Ramanavicius and Renata Karpicz
Affiliations : Center for Physical Sciences and Technology, Saulėtekio al. 3, LT-10222 Vilnius, Lithuania Department of Physical Chemistry, Faculty of Chemistry, Vilnius University, Naugarduko Str. 24, LT-03225 Vilnius, Lithuania

Resume : Glucose oxidase (GOx) composites with conducting polymers (e.g. polypyrrole) are an excellent nano-biomaterial suitable for the design of bioelectronic devices such as biosensors and biofuel cells. The active part of the biosensor is a GOx enzyme immobilized on the surface of the electrode. When constructing an enzymatic biosensor, one of the most important aims is to determine properties of an enzyme immobilization. Spectroscopic properties of GOx, flavin adenine dinucleotide (FAD) and composites of these compounds with polypyrrole (Ppy) were investigated in this research. During this study, it was found that the Ppy, which formed composites with FAD and GOx, facilitated the removal of FAD molecules from GOx and twice reduced the fluorescence decay rate. The FAD composite with Ppy effectively quenched the FAD fluorescence and FAD could not freely unfold. The intramolecular electron transfer takes place between adenine and isoalloxazine moieties over the first 5 ps seconds after the excitation.

Authors : S Behera(1*), D Chakrabarti(1), R Mitra(1), Md. Basiruddin Sk(2),
Affiliations : Department of Metallurgical and Materials Engineering, Indian institute of Technology Kharagpur, 721302, India.

Resume : A nanoindentation test is one of the most developed technique used to measure the mechanical properties in a sub-micron range. In order to achieve the desired mechanical properties for structural application, samples of high carbon steel have been subjected to different austenitization (850-1150 degree centigrade) temperatures and hot deformation at different temperatures. In the present work, the indentation size effect of different samples has been studied by performing nanoindentation study at imposed depths. Tensile tests of the materials have been carried out in order to determine empirical correlations between nanoindentation test results and tensile properties. Overall,the specimen which is having lower deformation temperature can only achieve the highest required tensile properties and hardness. Keywords: Pearlitic steel; nanoindentation test; tensile test; hot deformation; hardness.

Authors : Cezariusz Jastrzebski1, Daniel J. Jastrzebski2, Karolina Pietak2, Wojciech Gebicki1
Affiliations : 1 Faculty of Physics, Warsaw University of Technology; Koszykowa 75, 00-662 Warsaw, Poland 2 Faculty of Chemistry, Warsaw University of Technology; Noakowskiego 3, 00-664 Warsaw, Poland

Resume : Tin sulphide (SnS) belongs to a group of materials with very high application potential in the field of photovoltaics, nano- and optoelectronics. It belongs to materials with a layered structure, although a relatively large value of van der Waals forces does not allow to obtain very thin 2D layers in the process of micro-mechanical exfoliation as in the case of other sulphides (MoS2, GaS, SnS2, etc.). An energy gap with a value at room temperature in the range of Eg = 1.08 ÷ 1.70 eV [1], p-type conductivity and a high absorption coefficient of 104 - 105 cm-1 [2] make this material as one the most interesting for applications in the area of photovoltaic cells. In addition, tin sulfide is an ecological (non-toxic) material, it is cheap to manufacture and built from generally available elements. A very large structural anisotropy in the orthonormal phase is extremely important in the case of SnS, which makes the material as intensively tested for thermoelectric applications [3]. The presence of minima in the conduction band leads to a strong selectivity of light absorption and emission in SnS depending on the polarization [4]. For this reason, this material is also considered as interesting for spintronic applications - in the field of so-called valley electronics (valleytronic). SnS samples for the study were prepared in CVT method. Raman temperature measurements have been made in the range from 80 K to 450 K. Different wavelengths of the laser beam were used as an excitation. Enhancement Raman scattering for 1064 nm excitation wavelength was obserwed. Polarizing measurements were made in the range of 0-360o. The analysis of the results was made to correlate the structural SnS anisotropy with the phonon processes observed in Raman scattering. The first order temperature coefficients were determined for different Raman modes. Samples of different thicknesses in nanometric scale were applied. 1. Lee A. Burton, Diego Colombara, Ruben D. Abellon, Ferdinand C. Grozema, Laurence M. Peter, Tom J. Savenije, Gilles Dennler, Aron Walsh. Synthesis, Characterization, and Electronic Structure of Single - Crystal SnS, Sn2S3, and SnS2. Chemistry od Materials. 2013, Tom 25 2. T. Raadik, M.Grossberg, J.Raudoja, R.Traksmaa, J.Krustok. Temperature-dependent photoreflectance of SnS crystals. Journal of Physics and Chemistry of Solids. miesięczne, 2013, Tom 74, p.12 3. Asfandiyar, Tian-RanWei, Zhiliang Li, Fu-Hua Sun, Yu Pan, Chao-FengWu, Muhammad Umer Farooq, HuaichaoTang, Fu Li, Bo Li & Jing-Feng Li, Scientific Reports | 7:43262 | DOI: 10.1038/srep43262 4. Shuren Lin, Alexandra Carvalho, Shancheng Yan, Roger Li, Sujung Kim, Aleksandr Rodin, Lídia Carvalho, Emory M. Chan, Xi Wang, Antonio H. Castro Neto & Jie Yao, Accessing valley degree of freedom in bulk Tin(II) sulfide at room temperature, Nature Communications 9, 1455 (2018), doi:10.1038/s41467-018-03897-3

Authors : Feng Yan1, Konstantinos Spyrou2, Petra Rudolf1, Dimitrios Gournis2
Affiliations : 1Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands 2Department of Material Science and Engineering, University of Ioannina, Ioannina, Greece

Resume : Chloridazon has been a widely used especially in sugarbeet cultivation during the past decades since it was considered to be a relatively non-harmful herbicide. UV-induced degradation leads to the formation of the relevant desphenyl- counterparts i.e. desphenyl-chloridazon and methyl-desphenyl-chloridazon. Even if accumulation of these residues in natural waters is far from alarming, a low-cost effective and environmental friendly detergent, capable of binding chloridazon and its degradation products as to reduce their concentration in water even further below set limitations is desirable. Here we show that pillared clay, prepared by cation exchange of sodium with copper complexed, cage-shaped polyhedral oligomeric silsesquioxane (Cu2+_POSS) could be a promising candidate for this purpose. X-ray diffraction and HRTEM evidenced a homogeneous layered structure with interlayer spacing enlarged by ~4.6 Å with respect to the pristine clay, which corresponds to the diameter of POSS. Exposure of this pillared smectite clay to chloridazon and its metabolites in water, the results showed that POSS intercalation significantly improved the adsorption capacity.

Authors : Sandrina Fernandes, Vladminir Havranek, Anna Mackova
Affiliations : Nuclear Physics Institute, Academy of Sciences of the Czech Republic Hlavní 130, 25068 Husinec-Rez, Czech Republic

Resume : Several ceramic insulators, such as boron nitride are being investigated for high temperature applications in microelectronics and optoelectronic devices, irradiation monitoring and neutron dosimetry in high-dose nuclear and reactor environments, fissile materials sensing and neutron therapy. A study to evaluate the radiation induced damage in hexagonal boron nitride grown by chemical vapor deposition by using 14N ion beam irradiation at room temperature and with fluences up to 1x10^16 is going to be presented. The chemical contaminants present in h-BN were assessed by particle-induced X-ray emission (PIXE) and the relative concentration of 11B and of 10B was analyzed with the time of flight spectrometer (TOF) technique. The samples were characterized in terms of the structural (Raman spectroscopy, positron annihilation, AFM), mechanical (nanoindentation) and optical properties change.

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Excitonic Insulators-I : to be decided
Authors : Hidenori Takagi
Affiliations : Max Planck Institute for Solid State Research and Department of Physics, University of Tokyo

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).

Authors : Takashi Mizokawa
Affiliations : Waseda University

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.

Authors : Julia Stähler
Affiliations : Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, Germany

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).

10:30 Coffee Break    
Excitonic Insulators-II : to be decided
Authors : Shik Shin, Kozo Okazaki, Takeshi Suzuki
Affiliations : Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

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.

Authors : Akitoshi Nakano, Kento Sugawara, Shinya Tamura, Naoyuki Katayama, Kazuyuki Matsubayashi, Taku Okada, Yoshiya Uwatoko, Kouji Munakata, Akiko Nakao, Hajime Sagayama, Reiji Kumai, Kunihisa Sugimoto, Naoyuki Maejima, Akihiko Machida, Tetsu Watanuki and Hiroshi Sawa
Affiliations : Department of Applied Physics, Nagoya University, Nagoya, 464-8603, Japan; University of Electro-Communications, Chofu, Tokyo 182-8585, Japan; Institute for Solid State Physics (ISSP), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan; Comprehensive Research Organization for Science and Society (CROSS), Tokai, Ibaraki 319- 1106, Japan; Photon Factory, IMSS, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan; Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Hyogo 679-5198, Japan; Synchrotron Radiation Research Center (SRRC), National Institutes for Quantum and Radiological Science and Technology (QST), 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan.

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

Authors : H.Fukuyama
Affiliations : Tokyo University of Science

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.

12:30 Lunch Break    
Oxide thin films and multilayers : to be decided
Authors : PRELLIER Wilfrid

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.

Authors : T. Katsufuji, T. Saiki, S. Okubo, Y. Katayama, K. Ueno
Affiliations : Department of Physics, Waseda University; Department of Physics, Waseda University; Department of Physics, Waseda University; Department of Basic Science, University of Tokyo; Department of Basic Science, University of Tokyo

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.

Authors : Morgan Trassin
Affiliations : Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 4, 8093 Zurich, Switzerland

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)

Authors : Ma Tao, Li Yi
Affiliations : Institute of Chemical Materials, CAEP, Mianyang 621900, China; Institute of Chemical Materials, CAEP, Mianyang 621900, China

Resume : Reactive multilayer films are the attractive option for miniaturization and multi-functionalization of energetic devices. Laser ignition of energetic materials is considered as one of the ways to solve the safety of energetic devices. Reactive multilayer films consist of rapid alternating layers and the films exhibit rapid exothermic reactions at high temperature. Laser-ignited reactive multilayer films can react and generate sufficient heat at a rapid rate to induce further layer intermixing via thermally induced atomic diffusion. In this work, we seek to study laser initiation and self-propagating reactions of multilayer films. Al/Ni multilayer films were prepared by DC magnetron sputtering. The propagating reactions were ignited by a single pulse irradiation. The laser initiation thresholds were determined and the microstructures were observed using SEM. High speed photography was used to analyze the propagation rate. The final product was characterized by XRD. The results show that all measured critical initiation threshold fluences of multilayers are higher than the typical ablation threshold for metals under laser irradiation. With decreasing bilayer thickness, the initiation threshold decreases and the propagation velocity increases. Meanwhile, with the increase of laser pulse duration, the initiation threshold of Al/Ni multilayer films is decreasing. Keywords: multilayer films; laser ignition; self-propagation

15:45 Coffee Break    
Layered superconductors-I : to be decided
Authors : Atsushi Fujimori
Affiliations : Department of Physics, University of Tokyo

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.

Authors : Neven Bari¨ić1,2
Affiliations : 1)Institute of Solid State Physics, TU Wien, Wiedner Hauptstraße 8, 1040 Wien Austria 2)Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, HR-10000, Zagreb, Croatia

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.

Authors : Y. Koike, T. Kawamata, T. Adachi, H. Fukazawa, Y. Kohori
Affiliations : Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan; Department of Engineering and Applied Sciences, Sophia University, Tokyo 102-8554, Japan; Department of Physics, Chiba University, Chiba 263-8522, Japan

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)

Authors : O. Ivashko (1), M. Horio (1), N. E. Shaik (2), X. Lu (3), C. G. Fatuzzo (2), M. Dantz (3), D. E. McNally (3), D. Destraz (1), E. Paris (3), Y. Tseng (3), P. G. Freeman (4), W. Wan (5), N. B. Christensen (5), T. Kurosawa (6), N. Momono (6,7), M. Oda (6), H. I. Wei (8), C. Adamo (9), M. R. Beasley (9), C. Lichtensteiger (10), M. Gibert (1), K. M. Shen (8), C. Monney (1), H. M. Rønnow (2), J. Tomczak (11), T. Schmitt (3), and J. Chang (1)
Affiliations : (1) Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland; (2) Institute for Condensed Matter Physics, École Polytechnique Fedérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland; (3) Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland; (4) Jeremiah Horrocks Institute for Mathematics, Physics and Astronomy, University of Central Lancashire, PR1 2HE Preston, United Kingdom; (5) Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark; (6) Department of Physics, Hokkaido University, Sapporo 060-0810, Japan; (7) Department of Applied Sciences, Muroran Institute of Technology, Muroran 050-8585, Japan; (8) Department of Physics, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA; (9) Department of Applied Physics, Stanford University, Stanford, CA 94305, USA; (10) Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest Ansermet, 1211 Geneva, Switzerland; (11) Institute of Solid State Physics, Vienna University of Technology, A-1040 Vienna, Austria.

Resume : The interplay between superconductivity and spin excitations has an important role in cuprate superconductors. While it is well-known that this family of superconductors has a d-wave symmetry, the mechanism of Cooper pair formation is yet to be understood. It has been proposed that antiferromagnetic spin excitations may play a role [1]. Studying the evolution of spin excitation for different regimes and doping is therefore an important issue in order to reveal the optimal conditions for high-temperature cuprate superconductors. Here, we present a systematic Resonant Inelastic X-ray Scattering (RIXS) study of the spin excitations in La1.88Sr0.12CuO4 [2] and the related parent compound La2CuO4. Spin excitation spectra are analysed using a dispersion function derived from a Hubbard model [3]. We show that, for La1.88Sr0.12CuO4, the extracted data can be explained only if invoking a two-orbital model in which a second-nearest hopping integral is enhanced as a consequence of the reduction of the crystal field splitting between the dx2-y2 and dz2 orbitals [4]. The importance of dz2 orbital confirmed by our recent Angle Resolved Photoemission Spectroscopy (ARPES) study [5], is investigated by XAS and RIXS in further detail, on the parent compound La2CuO4. The obtained results are discussed in the context of superconductivity optimisation [6]. References [1] D. J. Scalapino et al., Rev. Mod. Phys. 84, 1383 (2012) [2] O. Ivashko et al., Phys. Rev. B 95, 214508 (2017) [3] J.-Y. P. Delannoy et al., Phys. Rev. B 79, 235130 (2009) & B. Dalla Piazza, et al., Phys. Rev. B 85, 100508(R) (2012) [4] H. Sakakibara et al., Phys. Rev. Lett. 105, 057003 (2010) & H. Sakakibara et al., Phys. Rev. B 85, 064501 (2012) [5] C. E. Matt et al., Nat. Comm. 9, 972 (2018) [6] O. Ivashko et al., in preparation

Authors : Nitya Ramanan1, Himal Bhatt2, Wojciech Olszewski1,3, Carlo Marini1, Laura Simonelli1 and M.N. Deo2
Affiliations : 1Alba Synchrotron, Carrer de la llum 2-26, Cerdanyola del valles, 08290 Barcelona, Spain. 2High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India. 3Faculty of Physics, University of Bialystok, 1L K. Ciołkowskiego Str., 15-245, Bialystok, Poland.

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.

Authors : M. Y. Hacisalihoglu* 1; 2; 3, E. Paris 1, B. Joseph 4, A. Provino 5; 6, G. Lamura 5, R. Cimberle 5, A. Martinelli 5, P. Manfrinetti 5; 6, G. Aquilanti 4, K. Ozturk 3, P. Dore 1, M. Putti 5; 7, and N. L. Saini 1,
Affiliations : 1 Roma La Sapienza University, Department of Physics, 00185 Roma, Italy; 2 Recep Tayyip Erdogan University, Department of Physics, 53100 Rize, Turkey; 3 Karadeniz Technical University, Department of Physics, 61080 Trabzon, Turkey; 4 Elettra, Sincrotrone Trieste, Strada Statale 14, Km 163.5, Basovizza, Trieste, Italy; 5 CNR-SPIN, Corso Perrone 24, I-16152 Genova, Italy; 6 Department of Chemistry University of Genova, via Dodecaneso 31, I-16146 Genova, Italy; 7 Department of Physics University of Genova, via Dodecaneso 33, I-16146 Genova, Italy;

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)

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09:00 Plenary session    
12:30 Lunch Break    
Layered superconductors-II : to be announced
Authors : Jouko Nieminen 1,2), Timo Saari 1) and Arun Bansil 2)
Affiliations : 1) Department of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland; 2) Department of Physics, Northeastern University, Boston, Massachusetts, USA

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.

Authors : L. Simonelli
Affiliations : CELLS – ALBA Synchrotron, Barcelona, Spain

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).

Authors : Yoshikazu Mizuguchi, Rajveer Jha
Affiliations : Department of Physics, Tokyo Metropolitan University

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).

15:30 Coffee Break    
Spectroscopy of layered and related systems : to be announced
Authors : Takayoshi Yokoya, Kensei Terashima, Takanori Wakita, Takayuki Muro, Tomohiro Matsushita, Toyohiko Kinoshita, Yuji Muraoka
Affiliations : Research Institute for Interdisciplinary Science(RIIS), Okayama University, Okayama 700-8530, Japan, Japan Synchrotron Radiation Research Institute, Spring-8, Sayo, Hyogo 679-5198, Japan

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

Authors : V. Scagnoli 1,2
Affiliations : 1,Laboratory for Mesoscopic Systems, Department of Materials, ETH Zurich, Switzerland 2,Laboratory for Multiscale Materials Experiments, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

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)

Authors : Xiaohui Zhao; Yang Liu; Ziliang Liu; Wanli Zhang
Affiliations : State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China

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.

Authors : Satoshi Demura(1), Naoki Ishida(2), Hideaki Sakata(2)
Affiliations : (1) Nihon University; (2) Tokyo University of Science

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.

17:30 Graduate students awards    
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Layered functional materials-I : to be announced
Authors : Joung Real Ahn
Affiliations : Department of Physics, Sungkyunkwan University, Republic of Korea

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.

Authors : Changyoung Kim
Affiliations : Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea and Department of Physics and Astronomy, Seoul National University (SNU), Seoul 08826, Republic of Korea

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.

Authors : Yuji Aoki, Rajveer Jha, Shota Onishi, Ryuji Higashinaka, Tatsuma D. Matsuda
Affiliations : Department of Physics, Tokyo Metropolitan University, Tokyo, Japan

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).

Authors : Torben Dankwort1, Christine Koch2, Andriy Lotnyk3, Ulrich Ross4, Anna-Lena Hansen2, Marco Esters5, Dietrich Häußler1, Hanno Volker6, Alexander von Hoegen6, Matthias Wuttif6, David C. Johnson5, Wolfgang Bensch2, Lorenz Kienle1
Affiliations : 1 Institute for Materials Science, Kiel University, Kaiserstrasse 2, 24143, Kiel, Germany; 2 Institute of Inorganic Chemistry, Kiel University, Max-Eyth-Strasse 2, 24118, Kiel, Germany; 3 Leibniz Institute of Surface Engineering (IOM), Permoserstraße 15, 04318 Leipzig, Germany; 4 Institute of Materials Physics, University of Göttingen, Friedrich-Hund-Platz, 37077 Göttingen, Germany; 5 Department of Chemistry and Material Science Institute, University of Oregon, Eugene, OR, 97403, United States; 6 Institute of Physics, RWTH Aachen University, Sommerfeldstrasse 14, 52056, Aachen, Germany;

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.

10:45 Coffee Break    
Layered functional materials-II : to be announced
Authors : Di Yi
Affiliations : Geballe Laboratory for Advanced Materials and Department of Applied Physics, Stanford University

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.

Authors : E. Paris1, D.E. McNally1, Y. Tseng1, E. Paerschke2, V.N. Strokov1, K. Rolfs1, T. Shang1, E. Pomjakushina1, M. Medarde1, C. Schneider1, K. Wohlfeld3, M. Radovic1, T. Schmitt1
Affiliations : 1Photon Science Division, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland; 2 IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany; 3 Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, PL-02093 Warsaw, Poland;

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.

Authors : K. Terashima(1), E. Paris(2), E. Salas-Colera(3), L. Simonelli(4), B. Joseph(5), H. Fujiwara(6), K. Horigane(1), K. Kobayashi(1), R. Horie(1), J. Akimitsu(1), E. Golias(7), D. Marchenko(7), A. Varykhalov(7), T. Wakita(1), Y. Muraoka(1), N. L. Saini(2), and T. Yokoya(1)
Affiliations : (1)Research Institute for Interdisciplinary Science, Okayama University, Japan; (2)Dipertimento di Fisica, Universita di Roma "La Sapienza", Italy; (3)Spanish CRG BM25 Spline, ESRF - The European Synchrotron, France; (4)CELLS - ALBA synchrotron Radiation Facility, Spain; (5)ELETTRA, Sincrotrone Trieste, Italy; (6)Graduate School of Natural Science and Technology, Okayama University, Japan; (7)Helmholtz-Zentrum Berlin fur Materialien und Energie, Germany

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).

12:30 Lunch Break    
Layered functional materials-III : to be announced
Authors : R. Farris (1), P. Alippi (2), M. B. Maccioni (1), A. Urru (1,3), F. Ricci (4), A. Filippetti (1), J. Iniguez (5), V. Fiorentini (1)
Affiliations : 1. U Cagliari, 2. CNR-ISM, 3. SISSA, 4. U Louvain-l.-N., 5. LIST

Resume : Layered perovskites AnBnO3n 2 are extremely interesting materials exhibiting combined multiferroicity, magnetoelectricity, and (in some cases) metallicity. I will review example of ferroelectric and multiferroic metals, and of multiferroic magnetoelectrics in this class [1-4], and report on the calculation of thermoelectric coefficients in LaTiO3.5 and SrNbO3.5, which show some promise due to small electrical and thermal conductivity along the stacking axis [5]. Refs: [1] A. Filippetti, V. Fiorentini, F. Ricci, P. Delugas, and J. Iniguez: Prediction of a native ferroelectric metal, Nature Comm. 7, 11211 (2016). [2] M. Scarrozza, M. B. Maccioni, G. M. Lopez, and V. Fiorentini, Topological multiferroics, Phase Trans. 88, 953 (2015) [3] M. Scarrozza, A. Filippetti, and V. Fiorentini: Ferromagnetism and orbital order in a topological ferroelectric, Phys. Rev. Lett. 109, 217202 (2012); Multiferroicity in vanadium-doped LaTiO: insights from first principles, Eur. Phys. J. 86, 128 (2013) [4] A. Urru et al., to be published [5] R. Farris et al., to be published

Authors : Deepak Kumar[1], Wilfrid Prellier[1], Adrian David[1], Arnaud Fouchet[1], Olivier Copie[2].
Affiliations : [1] Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, Normandie Universite, 6 Bd Marechal Juin, F-14050 Caen Cedex 4, France. [2] Institut Jean Lamour UMR 7198 CNRS-Université de Lorraine F-54506 Vandœuvre-lès-Nancy, France.

Resume : The transition-metal oxides with ABO3 type perovskite structure show a strong coupling between spin-orbital and lattice degrees of freedom, implying the importance of lattice distortion in tuning the electronic and magnetic properties of ABO3 perovskites. With the recent experimental activities [ O. Copie et. al. Adv. Mater. 1604112, (2017)] in mind, where authors reveal a pathway to tune the Neel temperature (TN) of PrVO3 (PVO) thin films in the range of 30 K via chemical strain engineering i.e. by monitoring the concentration of oxygen vacancies in PVO thin films. In here, we present an experimental study of magnetic response to the residual strain in PVO thin films by growing PVO films on a variety of single crystal substrates and by varying PVO film thickness on a single crystal substrate. PrVO3 in bulk, being a strong anti-ferromagnet shows TN ~ 130 K [1]. Our study reveals that the lattice distortion via misfit strain pave the way towards tuning the anti-ferromagnetic ordering temperature in PrVO3 thin films, resulting in a non-trivial evolution of the Neel temperature (TN) with substrate pseudo-cubic lattice parameter and PVO film thickness. We demonstrate that the strain produced via mechanical strain engineering, tune TN of PVO films in the range of 90 K. Moreover, we also show that PVO thin films show low temperature ferromagnetic behavior along with the insulating properties irrespective of the substrate. This work is supported by LABEX, ANR and Region Normandie. [1] Sage M H et. al. Phys. Rev. B 76 195102 (2007).

Authors : Authors : Merve Aksoy 1, U?ur Ünal 1,2,3
Affiliations : Affiliations : Merve Aksoy 1 ; U?ur Ünal 1,2,3 1 Graduate School of Science & Engineering, Koc University, 34450, Sar?yer, Istanbul, Turkey ; 2 Department of Chemistry, Koc University, 34450, Sar?yer, Istanbul, Turkey. 3 Koç University Surface Science & Technology Center (KUYTAM), 34450, Sar?yer, Istanbul, Turkey.

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.


Symposium organizers
Naurang SAINISapienza Università di Roma

Dipartimento di Fisica - P. le Aldo Moro 2 00185 Roma - Italy

+39 0649914387
Siddharth S. SAXENACambridge University

Quantum Matter Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
Takashi MIZOKAWAWaseda University

Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan

+81 3 5286 3230