Research activity information

• Divalent Cation Doping into SrTiO₃ for Enhancing the Photocatalytic Performance of Water Splitting

  Kazutaka Nishiguchi, Tomoaki Takayama, Shogo Takasuka, Yosuke Harashima, Mikiya Fujii, Seiichiro L. Ten-no

  Lead, American Chemical Society (ACS), Mar. 2025, The Journal of Physical Chemistry Letters, 2823 - 2830

  [Refereed]

  Scientific journal


• First-principles Study of Metallic-atom Diffusion in Thermoelectric Material Mg₃Sb₂

  Masayuki Ochi, Kazutaka Nishiguchi, Chul-Ho Lee, Kazuhiko Kuroki

  Physical Society of Japan, Feb. 2025, Journal of the Physical Society of Japan, 94(2) (2)

  [Refereed]

  Scientific journal


• Characterization of Planar Defect in Layered Perovskite Photocatalyst Y2Ti2O5S2 by Electron Microscopy and First-Principles Calculations

  Mamiko Nakabayashi, Kazutaka Nishiguchi, Xizhuang Liang, Takashi Hisatomi, Tsuyoshi Takata, Takashi Tsuchimochi, Naoya Shibata, Kazunari Domen, Seiichiro L. Ten-no

  Layered perovskite Y2Ti2O5S2 is a promising semiconductor photocatalyst with an electronic structure suitable for overall water splitting under visible light. However, similar to other photocatalysts, structural defects during synthesis should be controlled. Transmission electron microscopy (TEM) revealed extremely large planar defects composed of S-Mg-S layers in Y2Ti2O5S2 synthesized using the flux method. We determined the planar defect structure and electronic structure using first-principles calculations based on the density functional theory. The evaluation of the formation energy suggests that S-and Mg-poor conditions may prevent defect formation. Furthermore, we discuss the impurity levels caused by the planar defects and their effects on the electronic state and catalytic performance.

  Lead, AMER CHEMICAL SOC, Apr. 2023, JOURNAL OF PHYSICAL CHEMISTRY C, 127(16) (16), 7887 - 7893, English

  [Refereed]

  Scientific journal

  Link to Kobe Univ. Repository (Kernel)


• Possibility of N-type Doping in CaAl₂Si₂-type Zintl Phase Compound CaZn₂X₂ (X = As, P)

  Kazutaka Nishiguchi, Masayuki Ochi, Chul-Ho Lee, Kazuhiko Kuroki

  Lead, Physical Society of Japan, Jun. 2022, Journal of the Physical Society of Japan, 91(6) (6)

  [Refereed]

  Scientific journal


• First-Principles Investigation on the Heterostructure Photocatalyst Comprising BiVO4 and Few-Layer Black Phosphorus

  Takashi Tsuchimochi, Kaname Takaoki, Kazutaka Nishiguchi, Seiichiro L. Ten-no

  American Chemical Society (ACS), Oct. 2021, The Journal of Physical Chemistry C, 125(40) (40), 21840 - 21850

  [Refereed]

  Scientific journal


• Effect of On-site Coulomb Repulsion on Ferromagnetic Fluctuations in Heavily Overdoped Cuprates

  Shingo Teranishi, Kazutaka Nishiguchi, Seiji Yunoki, Koichi Kusakabe

  Physical Society of Japan, Sep. 2021, Journal of the Physical Society of Japan, 90(9) (9), 094707 - 094707, English

  [Refereed]

  Scientific journal


• Material Optimization of Potential High-Tc Superconducting Single-layer Cuprates

  Shingo Teranishi, Kazutaka Nishiguchi, Koichi Kusakabe

  Apr. 2021, J. Phys. Soc. Jpn. 90, 054705 (2021)

  [Refereed]


• Possible superconductivity induced by a large spin-orbit coupling in carrier doped iridium oxide insulators: A weak coupling approach

  Kazutaka Nishiguchi, Tomonori Shirakawa, Hiroshi Watanabe, Ryotaro Arita, Seiji Yunoki

  We study possible superconductivity in a carrier-doped iridium oxide
  insulator Sr$_{2}$IrO$_{4}$ based on an effective $t_{2g}$ three-orbital
  Hubbard model on the square lattice with a large spin-orbit coupling (SOC).
  Numerically solving the linearized Eliashberg equation for the superconducting
  (SC) gap function with the random phase approximation, we systematically
  examine both singlet and triplet SC gap functions with possible pairing
  symmetries and the parameter dependence of the superconductivity. For the
  realistic SOC $\lambda$ and Hund's coupling $J/U$ relevant to
  Sr$_{2}$IrO$_{4}$, namely, for a large $\lambda$ and small $J/U$ region, we
  find that the intra-band antiferromagnetic (AF) pseudospin $\bm{j}_{\text{eff } }
  = -\bm{l}+\bm{s}$ fluctuations favor a $d_{x^{2}-y^{2 } }$-wave pseudospin
  $j_{\text{eff } }=1/2$ singlet pairing in the electron-doping. We also find that
  the $d_{x^{2}-y^{2 } }$-wave pairing is more stabilized with increasing the SOC
  and decreasing the Hund's coupling. Furthermore, we show for a small $\lambda$
  and large $J/U$ region that an $s_{\pm}$-wave singlet pairing is favored in the
  hole-doped region. The origin of the $s_{\pm}$-wave pairing is due to the
  inter-band pair scattering arising from the intra-orbital AF spin $\bm s$
  fluctuations. Although the possibility of a pseudospin triplet pairing is
  considered, we find it always unfavorable for all parameters studied here. The
  experimental consequences for other strongly correlated materials with a large
  SOC are also discussed.

  Lead, Physical Society of Japan, Apr. 2019, J. Phys. Soc. Jpn. 88 (2019), 094701, 88(9) (9), 094701 - 094701, English

  [Refereed]

  Scientific journal


• Tunable induced magnetic moment and in-plane conductance of graphene in Ni/graphene/Ni nano-spin-valve-like structure: a first principles study

  Yusuf Wicaksono, Shingo Teranishi, Kazutaka Nishiguchi, Koichi Kusakabe

  This study theoretically investigated the magnetic properties and electronic
  structure of a graphene-based nano-spin-valve-like structure. Magnetic nickel
  layers on both sides of the graphene were considered. A spin-polarized
  generalized-gradient approximation determined the electronic states. In an
  energetically stable stacking arrangement of graphene and two nickel layers,
  the anti-parallel spin configuration of the underlayer and overlayer magnetic
  moments had the lowest energy, which is in agreement with previous experimental
  studies. The spin density mapping and obtained band-structure results show that
  when the upper and lower Ni(111) slabs have an anti-parallel (parallel)
  magnetic-moment configuration, the carbon atoms of sublattices A and B will
  have an antiferromagnetic (ferromagnetic) spin configuration. A band gap at the
  Dirac cone was open when the alignment had an anti-parallel configuration and
  closed when the alignment had a parallel configuration. Therefore, the in-plane
  conductance of the graphene layer depends on the magnetic alignment of the two
  nickel slabs when the Fermi level is adjusted at the Dirac point. Both the
  magnetic properties and electronic structures of the Ni/graphene/Ni
  nanostructure cause the system to be a new prospective spintronic device
  showing controllable in-plane magnetoresistance.

  Nov. 2018, Carbon 143 (2019) 828-836

  [Refereed]

  Scientific journal


• Edge States Caused by Shift of Dirac Points at the Armchair Edge of Distorted Nanographene

  Koichi Kusakabe, Kazutaka Nishiguchi, Shingo Teranishi, Yusuf Wicaksono

  We present a theoretical study of non-bonding edge modes at the mono-hydrogenated armchair edge (A1 edge) of a graphene ribbon by modifying its bulk electronic structure. Structural deformation of graphene bulk is known to lead Dirac cones to change their location in the Brillouin zone. We adopt the Ryu–Hatsugai criterion on the edge states to identify a non-trivial topological phase in a parametrized Hamiltonian space for graphene ribbons. For the armchair ribbon, the Dirac-cone shift happens along the 1st Brillouin zone and a non-trivial winding number appears except for a special case where the shifted Dirac points are mapped always to the Γ point irrespective of the deformation. We also discuss the electronic band structure of the ribbon by performing DFT-LDA calculation to show the indication of edge state occurrence. By utilizing the tight-binding model to examine a large-size limit of the deformed ribbon structure, an explicit function form of the edge state at A1 is derived. Our results from the Ryu–Hatsugai criterion and the band structure calculations show a physical proof of the existence of the edge states at the armchair graphene ribbon edge when the bulk graphene structure is properly deformed. Observation of this phenomenon is expected to be induced by coherent phonons in a µm graphene sample having a long armchair edge.

  Jul. 2018, J. Phys. Soc. Jpn. 87, 084706 (2018)

  [Refereed]


• Superconductivity arising from layer-differentiation in multi-layer cuprates

  Kazutaka Nishiguchi, Shingo Teranishi, Koichi Kusakabe, Hideo Aoki

  In order to theoretically identify the factors governing superconductivity in
  multi-layer cuprates, a three-layer Hubbard model is studied with the
  two-particle self-consistent (TPSC) approach so as to incorporate electron
  correlations. The linearized Eliashberg equation is then solved for the gap
  function in a matrix form to resolve the role of outer CuO$_2$ planes (OPs) and
  inner plane (IP). We show that OPs dominate IP in the $d_{x^{2}-y^{2 } }$-wave
  superconductivity, while IP dominates in the antiferromagnetism. This comes
  from an electron correlation effect in that the correlation makes the doping
  rates different between OPs and IP (i.e., a self-doping effect), which occurs
  in intermediate and strong correlation regimes. Namely, the antiferromagnetic
  fluctuations in IP are stronger due to a stronger electron correlation, which
  simultaneously reduces the quasiparticle density of states in IP with a
  suppressed $d_{x^{2}-y^{2 } }$-wave superconductivity. Intriguingly, while the
  off-diagonal (inter-layer) elements in the gap function matrix are tiny, {\it
  inter-layer pair scattering} processes are in fact at work in enhancing the
  superconducting transition temperature $T_{\text{c } }$ through the inter-layer
  Green's functions. This actually causes the trilayer system to have higher
  $T_{\text{c } }$ than the single-layer in a weak- and intermediate-coupling
  regimes. This picture holds for a range of the on-site Hubbard repulsion $U$
  that contains those estimated for the cuprates. The present result is
  qualitatively consistent with nuclear magnetic resonance experiments in
  multi-layer cuprates superconductors.

  Lead, Jun. 2018, Phys. Rev. B 98, 174508 (2018)

  [Refereed]

  Scientific journal


• Material-dependent screening of Coulomb interaction in single-layer cuprates

  Shingo Teranishi, Kazutaka Nishiguchi, Koichi Kusakabe

  To explore material dependence of cuprate superconductors, we evaluate
  effective Coulomb interactions for Hg1201 and Tl1201, where Tl1201 having a
  nearly half value of T_c of Hg1201 even at the optimal oxygen concentration.
  Although structures are similar for these superconductors, there is an apparent
  difference in the occupied levels below EF. The characteristic difference in
  the band structure is correlated with oxygen contents in the buffer layer. By
  using constrained Random Phase Approximation, effective screened Coulomb
  interactions are estimated for HgBa_2CuO_4 and TlBa_2CuO_5. The results shows
  that the value of screened on-site Coulomb interaction in Hg1201 is nearly
  twice bigger than that in Tl1201. In addition, The eigenvalues of the
  linearized Eliashberg equation of single-band Hubbard model within FLEX can
  show apparent difference in T_c. When we assume that the twice big screened
  on-site Coulomb for Hg, the material dependent T_c might be explained.

  May 2018, J. Phys. Soc. Jpn. 87, 114701 (2018)

  [Refereed]

  Scientific journal


• 強相関電子系へのAdvance/PHASEの適用—Application of Advance/PHASE to Strongly Correlated Electron Systems—特集 アドバンスソフトにおける研究開発 ; アドバンスソフトにおける外部機関との連携

  西口 和孝, 草部 浩一

  Lead, 東京 : アドバンスソフト出版事業部, Jan. 2018, アドバンスシミュレーション = Advance simulation / アドバンスソフト株式会社編集委員会 編, 25, 43 - 54, Japanese

  [Refereed][Invited]


• Self-Doping Effect Arising from Electron Correlations in Multilayer Cuprates

  Kazutaka Nishiguchi, Shingo Teranishi, Koichi Kusakabe

  A self-doping effect between outer and inner CuO2 planes (OPs and IPs) in multilayer cuprate superconductors is studied. When one considers a three-layer tight-binding model of the Hg-based three-layer cuprate derived from first-principles calculations, the electron concentration becomes larger in the OPs than in the IP. This is inconsistent with the experimental finding that more hole carriers tend to be introduced into the OPs than into the IP. We investigate a three-layer Hubbard model with the two-particle self-consistent approach for multilayer systems to incorporate electron correlations. We observe that the double occupancy (antiferromagnetic instability) in the IP decreases (increases) more than that in the OPs, and also reveal that more electrons tend to be introduced into the IP than into the OPs to obtain an energy gain from the on-site Hubbard interaction. These results are consistent with the experimental findings, and this electron distribution between the OPs and IP can be interpreted as a self-doping effect arising from strong electron correlations.

  Lead, PHYSICAL SOC JAPAN, Aug. 2017, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 86(8) (8), English

  [Refereed]

  Scientific journal


• Random phase approximation for superconducting states in multi-orbital Hubbard models with spin-orbit coupling

  Kazutaka Nishiguchi, Tomonori Shirakawa, Hiroshi Watanabe, Seiji Yunoki

  To better understand electronic and magnetic properties of superconducting (SC) states in 5d transition metal oxides, we derive an analytical formula of the dynamical correlation function for multi-orbital systems with the spin-orbit coupling (SOC), which is accessible to the experimental responses. Starting with a multi-orbital BCS Hamiltonian including the SC gap function, we introduce the dynamical correlation function in the non-interacting case. Based on the diagrammatic techniques, we derive the dynamical correlation function for the interacting case within the random phase approximation. With the help of a numerical calculation, we also show the temperature dependence of the magnetic susceptibility for two different SC states in the presence of the SOC.

  Lead, Institute of Physics Publishing, Apr. 2017, Journal of Physics: Conference Series, 807(5) (5), English

  [Refereed]

  International conference proceedings


• An enhancement mechanism of two-particle exchange interactions in single- and multi-layer cuprate superconductors

  Shingo Teranishi, Satoaki Miyao, Kazutaka Nishiguchi, Koichi Kusakabe

  To explore material dependence of layered cuprate superconductors, we examine
  effective two-particle interactions for Hg1201 and Tl1201, where Tl1201 having
  a nearly half value of Tc of Hg1201 even at the optimal oxygen concentration.
  Although the 3dx_2-y_2 band, the Fermi surface, and its Wannier-orbitals are
  similar for these superconductors, there is an apparent difference in the
  unoccupied levels above EF. Based on a multi-reference
  density-functional-theory formulation, effective two-particle exchange
  interactions are estimated to derive enhancement in intra-layer exchange
  interactions for HgBa2CuO4, while it is weakened in TlBaLaCuO5 and furthermore
  it is weak in TlBa2CuO5. The characteristic difference in the band structure is
  correlated with oxygen contents in the buffer layer. We also comment on the
  similar feature in triple-layered compounds. Our spin-fluctuation enhancement
  mechanism in an electron-correlation regime is consistent with the experimental
  fact.

  Apr. 2017

  Scientific journal


• 強相関電子系へのAdvance/PHASEの適用

  西口和孝, 草部浩一

  Lead, 2017, アドバンスシミュレーション, Vol. 25. (2017)

  [Refereed][Invited]


• Magnetism in Sr2IrO4: a Weak Coupling Study

  Kazutaka Nishiguchi, Hiroshi Watanabe, Seiji Yunoki

  To reveal a possible electronic instability toward various symmetry broken ordered states of Sr2IrO4, we investigate generalized susceptibilities of a three-orbital Hubbard model with spin–orbit coupling in the random phase approximation (RPA). By calculating the magnetic susceptibilities, we find that the in-plane antiferromagnetic instability of an effective total angular momentum Jeff = 1/2 is the strongest for a realistic set of parameters for Sr2IrO4. This is in good agreement with the strong coupling theoretical study and the experimental observation. We also discuss the possibility of superconductivity in doped Sr2IrO4.

  Lead, Sep. 2013, JPS Conf. Proc. 3, 015037 (2014)

  [Refereed]


• Superconductivity assisted by interlayer pair hopping in multilayered cuprates

  Kazutaka Nishiguchi, Kazuhiko Kuroki, Ryotaro Arita, Takashi Oka, Hideo Aoki

  In order to explore why the multilayered cuprates have such high T c values, we have examined various interlayer processes. Since the interlayer one-electron hopping has little effect on the band structure, we turn to the interlayer pair hopping. The superconductivity in a double-layer Hubbard model with and without the interlayer pair hopping, as studied by solving the Eliashberg equation with the fluctuation exchange approximation, reveals that the interlayer pair hopping acts to increase the pairing interaction and the self-energy simultaneously, but that the former effect supersedes the latter and enhances the superconductivity, along with how the sign of the interlayer off-site pair hopping determines the relative configuration of d waves between the adjacent layers. Study of the triple-layer case with the interlayer pair hopping further reveals that the superconductivity is further enhanced but tends to be saturated toward the triple-layer case. © 2013 American Physical Society.

  Jul. 2013, Physical Review B - Condensed Matter and Materials Physics, 88(1) (1), English

  [Refereed]

  Scientific journal


• Theory of high Tc superconductivity in multi-layered cuprates

  Kazutaka Nishiguchi

  Lead, Mar. 2013, The University of Tokyo

  [Refereed]

  Doctoral thesis

• 人工光合成による水素生成：ペロブスカイト半導体光触媒の第一原理解析

  西口和孝

  近畿化学協会コンピュータ化学部会（第120回例会）公開講演会, Oct. 2024

  [Invited]

  Public discourse


• First-principles analysis of photocatalytic activities in perovskite oxynitride ATaO2N (A = Ba, Sr, Ca)

  Kazutaka Nishiguchi, Takuya Fujimoto, Seiichiro L. Ten-no

  The 18th Annual Meeting of Japan Society for Molecular Science, Sep. 2024

  Poster presentation


• Machine learning-based prediction of crystal phases in perovskite compounds

  Ryota Yamamoto, Kazutaka Nishiguchi, Seiichiro L. Ten-no

  The 18th Annual Meeting of Japan Society for Molecular Science, Sep. 2024

  Poster presentation


• Material design of high-performance perovskite semiconductor photocatalysts for hydrogen evolution through artificial photosynthesis

  Kazutaka Nishiguchi

  The 3rd Research Workshop of ”Program for Promoting Research on the Supercomputer Fugaku", Mar. 2024

  Poster presentation


• Ferromagnetic fluctuations in thermoelectric Heusler alloy Fe2VAl: A weak-coupling approach

  Kazutaka Nishiguchi

  2024 APS March Meeting, Mar. 2024, English

  Oral presentation


• Ferromagnetic fluctuations in Heusler thermoelectric material Fe₂VAl: A weak-coupling approach

  西口和孝

  日本物理学会講演概要集(CD-ROM), 2024


• First-principles analysis of planar defect in layered perovskite Y2Ti2O5S2

  Kazutaka Nishiguchi

  第２回「富岳」成果創出加速プログラム 研究交流会, Mar. 2023

  Poster presentation


• First-principles analysis of defects in layered perovskite photocatalyst Y2Ti2O5S2

  Kazutaka Nishiguchi

  PCoMS Symposium & Annual meeting of Supercomputing Consortium for Computational Materials Science 2022, Feb. 2023

  Oral presentation


• First-Principles Analysis of Doped Layered Perovskite Semiconductor Photocatalyst Y₂Ti₂O₅S₂

  尼岸倫長, 西口和孝, 天能精一郎

  分子科学討論会講演プログラム&要旨(Web), 2023


• First-principles analysis of doped perovskite semiconductor photocatalysts: toward promoting photocatalytic performance

  西口和孝, 尼岸倫長, 天能精一郎

  分子科学討論会講演プログラム&要旨(Web), 2023


• Theoretical study of spin fluctuations in Heusler compound Fe₂VAl

  西口和孝

  日本物理学会講演概要集(CD-ROM), 2023


• Theoretical Study of Planar Defect in Layered Perovskite Photocatalyst Y₂Ti₂O₅S₂

  西口和孝, 中林麻美子, LIANG Xizhuang, 久富隆史, 土持崇嗣, 土持崇嗣, 柴田直哉, 柴田直哉, 堂免一成, 堂免一成, 天能精一郎

  分子科学討論会講演プログラム&要旨(Web), 2022


• Theoretical study of spin fluctuations in Heusler compound Fe₂VAl

  西口和孝

  日本物理学会講演概要集(CD-ROM), 2022


• Possibility of n-type doping in CaAl2Si2-type Zintl phase compound CaZn2X2 (X = As, P)

  Kazutaka Nishiguchi, Masayuki Ochi, Chul-Ho Lee, Kazuhiko Kuroki

  1st Japan-France Virtual Workshop on Thermoelectrics, Sep. 2021, English

  Poster presentation


• Theoretical study of n-type doping and its formation energy for thermoelectric material 122-Zintl phase compound with the first-principles calculations

  西口和孝, 越智正之, LEE Chul-Ho, 黒木和彦

  日本物理学会講演概要集(CD-ROM), 2021


• Effects of electron correlations on ferromagnetic fluctuations in heavily over-doped cuprates

  寺西慎伍, 西口和孝, 柚木清司, 柚木清司, 柚木清司, 草部浩一

  日本物理学会講演概要集(CD-ROM), 2020


• Theoretical study of impurity doping and its formation energy for thermoelectric material 122-Zintl phase compound with the first-principles calculations

  西口和孝, 越智正之, LEE Chul-Ho, 黒木和彦

  日本物理学会講演概要集(CD-ROM), 2020


• Magnetic Field Dependence of Opening and Closing Dirac Cone in Ni/Graphene/Ni nano-spin-valve-like structure

  Yusuf Wicaksono, Shingo Teranishi, Kazutaka Nishiguchi, Koichi Kusakabe

  1 & 2DM Conference, English

  Oral presentation


• Nd₂CuO₄における3d/4f局在電子軌道の第一原理有効ハミルトニアンの理論的評価

  玉垣侑也, 草部浩一, 寺西慎伍, 西口和孝

  応用物理学会春季学術講演会講演予稿集(CD-ROM), 2019


• Nd₂CuO₄における3d/4f局在電子軌道の超交換相互作用の理論的評価

  玉垣侑也, 草部浩一, 寺西慎伍, 西口和孝

  応用物理学会秋季学術講演会講演予稿集(CD-ROM), 2019


• 電子相関の層分化から生じる多層系銅酸化物の高温超伝導

  西口和孝, 寺西慎伍, 草部浩一, 青木秀夫, 青木秀夫

  日本物理学会講演概要集(CD-ROM), 2018


• 銅酸化物におけるバッファ層構造の違いに起因する遮蔽効果の物質依存性

  寺西慎伍, 西口和孝, 草部浩一

  日本物理学会講演概要集(CD-ROM), 2018


• コヒーレントフォノン導入によるディラック点の移動とアームチェア・エッジ状態の理論

  草部浩一, 西口和孝, 寺西慎伍, WICAKSONO Yusuf

  日本物理学会講演概要集(CD-ROM), 2018


• An effective 2-band ���� model of sulfur hydride H3S for high-��c superconductivity

  Kazutaka Nishiguchi, Shingo Teranishi, Satoaki Miyao, Goh Matsushita, Koichi Kusakabe

  APS March Meeting 2017, Mar. 2017, English

  [Invited]

  Oral presentation


• 強相関電子系の量子時間発展演算子表現におけるアルゴリズム開発

  草部浩一, 西口和孝, 松下剛

  日本物理学会講演概要集(CD-ROM), 2017


• Photon Pass processで記述した第一原理的電子正孔系量子ダイナミクス解析法

  松下剛, 草部浩一, 西口和孝

  日本物理学会講演概要集(CD-ROM), 2017


• 多層系銅酸化物超伝導体における反強磁性不安定性とフィリングの層依存性の多軌道二体自己無撞着法による解析

  西口和孝, 草部浩一

  日本物理学会講演概要集(CD-ROM), 2017


• 銅酸化物超伝導体における二電子交換相互作用増加効果の物質依存性と超伝導性解析

  寺西慎伍, 西口和孝, 草部浩一

  日本物理学会講演概要集(CD-ROM), 2017


• ルテニウム酸化物Sr₂RuO₄における超伝導の一軸性圧力効果の弱結合理論による研究

  西口和孝, 越智正之, 草部浩一, 黒木和彦

  日本物理学会講演概要集(CD-ROM), 2017


• Knight shift of Sr2IrO4 in Pseudospin Singlet Superconductivity Induced by Large Spin-Orbit Coupling

  Kazutaka Nishiguchi, Tomonori Shirakawa, Hiroshi Watanabe, Ryotaro Arita, Seiji Yunoki

  The International Conference on STRONGLY CORRELATED ELECTRON SYSTEMS, May 2016, English

  Poster presentation


• イリジウム酸化物の超伝導状態におけるナイトシフトの乱雑位相近似による理論的解析

  西口和孝, 白川知功, 渡部洋, 有田亮太郎, 柚木清司, 白川知功, 渡部洋, 有田亮太郎, 柚木清司, 柚木清司

  日本物理学会講演概要集(CD-ROM), 2016


• Possible Superconductivity Induced by Strong Spin-Orbit Coupling in Carrier Doped Iridium Oxides Insulators

  Kazutaka Nishiguchi, Tomonori Shirakawa, Hiroshi Watanabe, Ryotaro Arita, Seiji Yunoki

  APS March Meeting 2014, Mar. 2014, English

  Oral presentation


• Theoretical study for the superconductivity in Sr2IrO4

  Kazutaka Nishiguchi, Hiroshi Watanabe, Seiji Yunoki

  The International Conference on STRONGLY CORRELATED ELECTRON SYSTEMS, Aug. 2013, English

  Poster presentation


• Effect of inter-layer single-electron hopping and pair-hopping on superconductivity in multi-layered cuprates

  Kazutaka Nishiguchi, Kazuhiko Kuroki, Ryotaro Arita, Takashi Oka, Hideo Aoki

  APS March Meeting 2012, Feb. 2012, English

  Poster presentation


• Effect of inter-layer pair hopping and single electron-hopping on superconductivity in multi-layered high-Tc cuprates

  Kazutaka Nishiguchi, Kazuhiko Kuroki, Ryotaro Arita, Takashi Oka, Hideo Aoki

  Materials & Mechanisms of Superconductivity Conference (M2S), 2012, English

  Poster presentation


• Effect of inter-layer Pair hopping and One-electron hopping on Superconductivity in Multi-layered high-Tc Cuprates

  Kazutaka Nishiguchi, Kazuhiko Kuroki, Ryotaro Arita, Takashi Oka, Hideo Aoki

  GCOE International Symposium on Physical Sciences Frontier, 2012, English

  Poster presentation


• Theoretical Study of Superconductivity in Multi-layered Cuprates

  Kazutaka Nishiguchi, Kazuhiko Kuroki, Ryotaro Arita, Hideo Aoki

  XVI Training Course in the Physics of Strong Correlated Systems, 2011, English

  Oral presentation

• THE PHYSICAL SOCIETY OF JAPAN

• Theoretical study and material design of high performance thermoelectric materials arising from ferromagnetic spin fluctuations

  西口 和孝

  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), Kobe University, Apr. 2021 - Mar. 2024

  強磁性揺らぎによる熱電性能の増強のメカニズムを理解するために、代表物質であるホイスラー化合物Fe$_2$VAlに対して密度汎関数理論に基づく第一原理計算を行い、基本的な電子状態構造（状態密度やバンド構造）を得た。特に、フェルミレベル付近を構成する軌道成分としては、Feのd軌道やVのd軌道だけでなく、Alのs/p軌道も強く混成していることが見出された。更に、ドーパントの効果を調べるために、ドープされたFe$_2$VAl（Fe$_2$V$_{0.9}$Cr$_{0.1}$Al$_{0.9}$Si$_{0.1}$やFe$_{2.2}$V$_{0.8}$Al$_{1-y}$Si$_y$）の電子状態についてスーパーセルを用いて数値解析を行った。スーパーセル計算では、CrのV置換ドープやVのFe置換ドープの場合には、キャリアドープとともにスピン分極し磁気モーメントが誘起され、SiのAl置換ドープの場合には、単純にキャリアがドープされることを見出した。また、強相関ソルバーによる強磁性揺らぎを考慮するために、最局在ワニエ関数を用いてFe$_2$VAlの第一原理有効模型を構築した。そこでは、フェルミレベル付近の軌道成分を取り込んだ低エネルギー模型として、Feのd軌道、Vのd軌道、そしてAlのs/p軌道を考慮した19（=5*2+5+1+3）軌道模型を見出した。これらの軌道成分は上述の通り強くエンタングルしており、これら全てを考慮することでフェルミレベル付近のバンド構造を再現することが出来た。