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Yamamoto Shoji

Faculty of Science　Physics　Condensed Matter PhysicsProfessor

Researcher basic information

■ Degree
  • Dr. of Science, Kyoto University
■ URL
researchmap URLホームページURL■ Various IDs
J-Global ID■ Research Keywords and Fields
Research Keyword
  • quasicrystal
  • itinerant electron magnetism
  • Kitaev model
  • quantum spin liquid
  • modified spin wave theory
  • photoinduced phase transition
  • frustrated magnetism
  • Statistical Physics
  • Condensed Matter Theory
Research Field
  • Natural Science, Magnetism, superconductivity and strongly correlated systems
  • Natural Science, Semiconductors, optical properties of condensed matter and atomic physics
■ Educational Organization

Career

■ Career
Career
  • Jul. 2002 - Present
    Hokkaido University
  • Oct. 1996 - Jun. 2002
    Okayama University
  • 2002
    Professor of Hokkaido University
  • Mar. 1996 - Oct. 1996
    Hannover University, Invited Researcher
  • Jul. 1993 - Oct. 1996
    Osaka University
  • 1996
    Associate Professor of Okayama University
  • 1993
    Research Associate of Osaka University
Educational Background
  • 1993, Kyoto University, 理学研究科, 物理学第一専攻, Japan
  • 1993, Kyoto University, Graduate School, Division of Natural Science, Division of Physics I
  • 1988, Kyoto University, Faculty of Science, 物理学科, Japan
  • 1988, Kyoto University, Faculty of Science, Department of Physics

Research activity information

■ Papers
  • Raman Characterization of Two-Dimensional Quasiperiodic Antiferromagnets on Various Lattices: Spin–Orbit Mechanism
    Takashi Inoue; Shoji Yamamoto
    Journal of the Physical Society of Japan, 95, 6, Physical Society of Japan, 15 Jun. 2026, [Peer-reviewed], [Corresponding author]
    English, Scientific journal

    Preprint
  • Superconfined Antiferromagnons on the Two-Dimensional Penrose Lattice
    Takashi Inoue; Shoji Yamamoto
    Crystals, 16, 6, 370, 370, MDPI AG, 01 Jun. 2026, [Peer-reviewed], [Corresponding author]
    Scientific journal, We find novel confined states in the spin-S nearest-neighbor antiferromagnetic Heisenberg model on the two-dimensional Penrose lattice. Linear spin waves have massively degenerate eigenstates strictly confined to tricoordinated sites. They contrast with the well-known itinerant analogs in the tight-binding model, where electrons are confined but extended to both tricoordinated and pentacoordinated sites. It is the site potentials in the spin-wave Hamiltonian, originating from Coulomb interactions between electrons, that confine spin waves to minimally coordinated sites only. Confined states in the tight-binding Hamiltonian consist of six types of building blocks, whereas those in the spin-wave Hamiltonian consist of only four of them. Confined spin waves are robust against 1/S corrections. Emergent O(S0) interactions further confine—superconfine—spin waves into two separate groups within tricoordinated sites.

    open-access paper
  • Breakdown of the conventional spin-wave dynamics and its double-constraint modification in the spin-1/2 triangular-prism Heisenberg antiferromagnet
    Shoji Yamamoto; Jun Ohara
    Europhysics Letters, 151, 3, 36003-1, 36003-7, IOP Publishing, 09 Jul. 2025, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal, Abstract

    Spontaneous magnon decays in an $S=\frac{1},{2}$ Heisenberg antiferromagnet on the equilateral triangular prism are investigated in terms of modified magnon Green's functions. In one dimension, the so-called infrared divergence prevents us from calculating any---whether static or dynamic---structure factor within the conventional spin-wave theory even at zero temperature. The well-known modified spin-wave theory initiated by Takahashi completely fails to treat anharmonicities to cause transverse-to-longitudinal coupling which are quite characteristic of noncollinear antiferromagnets. We propose imposing a double-constraint condition on spin waves to solve all these difficulties and get a full view of the nonlinear spin-wave dynamics in one-dimensional frustrated noncollinear antiferromagnets. We reveal a novel instability of the single-particle spectrum in the absence of any well-defined magnetically ordered ground state., 36902156

    Preprint
  • Photoproduction of Loop Currents in Coronene Isomers Without Any Applied Magnetic Field
    Jun Ohara; Shoji Yamamoto
    Solids, 5, 4, 640, 650, MDPI AG, 06 Dec. 2024, [Peer-reviewed], [Last author]
    English, Scientific journal, Applying an extended Peierls–Hubbard model to π electrons in a coronene isomer, we investigate their ground-state properties and photoinduced dynamics with particular interest in possible loop current states. Once we switch on a static magnetic field perpendicular to the coronene disk, diamagnetic (diatropic) and paramagnetic (paratropic) loop currents appear on the rim circuit and inner hub, respectively. Besides this well-known homocentric two-loop current state, heterocentric multiloop current states can be stabilized by virtue of possible electron–lattice coupling. These multiloop current states generally have a larger diamagnetic moment than the conventional two-loop one, and hence it follows that coronene, or possibly polycyclic conjugated hydrocarbons in general, may become more aromatic than otherwise with their π electrons being coupled to phonons. When we photoirradiate a ground-state coronene isomer without applying a static magnetic field, loop currents are induced in keeping with the incident light polarization. Linearly and circularly polarized lights induce heterocentric two-loop and multiloop currents, respectively, without and together with two homocentric loop currents of the conventional type, respectively. The heterocentric two-loop currents occur in a mirror-symmetric manner, which reads as the emergence of a pair of antiparallel magnetic moments, whereas the heterocentric multiloop ones appear at random in both space and time, which reads as the emergence of disordered local magnetic moments.
  • Magnon Confinement on the Two-Dimensional Penrose Lattice: Perpendicular-Space Analysis of the Dynamic Structure Factor
    Shoji Yamamoto; Takashi Inoue
    Crystals, 14, 8, 702-1, 702-26, MDPI AG, 01 Aug. 2024, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal, Employing the spin-wave formalism within and beyond the harmonic-oscillator approx-imation, we study the dynamic structure factors of spin-12 nearest-neighbor quantum Heisenberg antiferromagnets on two-dimensional quasiperiodic lattices with particular emphasis on a mag-netic analog to the well-known confined states of a hopping Hamiltonian for independent electrons on a two-dimensional Penrose lattice. We present comprehensive calculations on the C5v Penrose tiling in comparison with the C8v Ammann–Beenker tiling, revealing their decagonal and octagonal antiferromagnetic microstructures. Their dynamic spin structure factors both exhibit linear soft modes emergent at magnetic Bragg wavevectors and have nearly or fairly flat scattering bands, signifying magnetic excitations localized in some way, at several different energies in a self-similar manner. In particular, the lowest-lying highly flat mode is distinctive of the Penrose lattice, which is mediated by its unique antiferromagnons confined within tricoordinated sites only, unlike their itinerant electron counterparts involving pentacoordinated, as well as tricoordinated, sites. Bringing harmonic antiferromagnons into higher-order quantum interaction splits, the lowest-lying nearly flat scattering band in two, each mediated by further confined antiferromagnons, which is fully demonstrated and throughly visualized in the perpendicular as well as real spaces. We disclose superconfined antiferromagnons on the two-dimensional Penrose lattice., 36902156

    open access
  • Thermal features of Heisenberg antiferromagnets on edge- versus corner-sharing triangular-based lattices: a message from spin waves
    Shoji Yamamoto; Jun Ohara
    Journal of Physics Communications, 7, 6, 065004-1, 065004-34, IOP Publishing, 01 Jun. 2023, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal, Abstract

    We propose a new scheme of modifying spin waves so as to describe the thermodynamic properties of various noncollinear antiferromagnets with particular interest in a comparison between edge- versus corner-sharing triangular-based lattices. The well-known modified spin-wave theory for collinear antiferromagnets diagonalizes a bosonic Hamiltonian subject to the constraint that the total staggered magnetization be zero. Applying this scheme to frustrated noncollinear antiferromagnets ends in a poor thermodynamics, missing the optimal ground state and breaking the local U(1) rotational symmetry. We find such a plausible double-constraint condition for spin spirals as to spontaneously go back to the traditional single-constraint condition at the onset of a collinear Néel-ordered classical ground state. We first diagonalize only the bilinear terms in Holstein-Primakoff boson operators on the order of spin magnitude S and then bring these linear spin waves into interaction in a perturbative rather than variational manner. We demonstrate specific-heat calculations in terms of thus-modified interacting spin waves on various triangular-based lattices. In zero dimension, modified-spin-wave findings in comparison with finite-temperature Lanczos calculations turn out so successful as to reproduce the monomodal and bimodal specific-heat temperature profiles of the triangular-based edge-sharing Platonic and corner-sharing Archimedean polyhedral-lattice antiferromagnets, respectively. In two dimensions, high-temperature series expansions and tensor-network-based renormalization-group calculations are still controversial especially at low temperatures, and under such circumstances, modified spin waves interestingly predict that the specific heat of the kagome-lattice antiferromagnet in the corner-sharing geometry remains having both mid-temperature broad maximum and low-temperature narrow peak in the thermodynamic limit, while the specific heat of the triangular-lattice antiferromagnet in the edge-sharing geometry retains a low-temperature sharp peak followed by a mid-temperature weak anormaly in the thermodynamic limit. By further calculating one-magnon spectral functions in terms of our newly developed double-constraint modified spin-wave theory, we reveal that not only the elaborate modification scheme but also quantum corrections, especially those caused by the O(S 0) primary self-energies, are key ingredients in the successful description of triangular-based-lattice noncollinear antiferromagnets over the whole temperature range of absolute zero to infinity.

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  • Topological Characterization of Kitaev Spin Nanoribbons with Ordered Flux Configurations
    Ryuto Tadokoro; Shoji Yamamoto
    JPS Conference Proceedings, 38, 011160-1, 011160-6, Journal of the Physical Society of Japan, 22 May 2023, [Peer-reviewed], [Last author, Corresponding author]
    English, International conference proceedings

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  • Multimagnon-mediated Raman scattering in two-dimensional quasiperiodic Heisenberg antiferromagnets
    Shoji Yamamoto; Jun Ohara
    Journal of Physics: Conference Series, 2461, 1, 012012-1, 012012-8, IOP Publishing, 01 Mar. 2023, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal, Abstract

    We demonstrate configuration-interaction and Green’s function approaches to the Shastry-Shraiman fourth-order magnetic Raman response of Heisenberg antiferromagnets on the two-dimensional C5v Penrose and C8v Ammann-Beenker lattices. Beyond the Loudon-Fleury second-order perturbation scheme, two more symmetry species A1 and A2 become Raman-active to break the depolarized Loudon-Fleury Raman response of E2 symmetry. The A1 and A2 modes are activated by dynamic ring exchange and chiral spin fluctuations, respectively, the former of which owes much to multimagnon scatterings in a complicated manner, while the latter of which owes much to aperiodic arrangements of different rhombuses., 36902156

    Preprint
  • Polarized Raman Response of Two-Dimensional Quasiperiodic Antiferromagnets: Configuration-Interaction versus Green’s Function Approaches
    Takashi Inoue; Shoji Yamamoto
    Journal of the Physical Society of Japan, 91, 5, 053701-1, 053701-5, Physical Society of Japan, 15 May 2022, [Peer-reviewed], [Last author, Corresponding author]
    English, Scientific journal, 36902156

    Preprint
  • Photoinduced Bidirectional Magnetism against Monodirectional Electronics
    Shoji Yamamoto
    Solid State Physics, 56, 8, 395, 408, Aug. 2021, [Peer-reviewed], [Invited], [Lead author, Corresponding author]
    Japanese, Scientific journal

    Preprint
  • Photoinduced bidirectional magnetism against monodirectional electronics in square-antiprismatic octacyanometalates
    Jun Ohara; Shoji Yamamoto
    Journal of the Physical Society of Japan, 89, 9, 093706-1, 093706-5, Physical Society of Japan, Sep. 2020, [Peer-reviewed], [Last author, Corresponding author]
    English, Scientific journal

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  • Optical Observation of Quasiperiodic Heisenberg Antiferromagnets in Two Dimensions
    Takashi Inoue; Shoji Yamamoto
    physica status solidi (b), 257, 11, 2000118-1, 2000118-8, Wiley, 06 Jul. 2020, [Peer-reviewed], [Last author, Corresponding author]
    English, Scientific journal

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  • Projective symmetry group analysis of inelastic light scattering in Kitaev spin balls
    Taku Kimura; Shoji Yamamoto
    Physical Review B, 101, 21, 214411-1, 214411-18, Jul. 2020, [Peer-reviewed], [Last author, Corresponding author]
    English, Scientific journal

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  • Raman scattering polarization and single spinon identification in two-dimensional
    Shoji Yamamoto; Taku Kimura
    Taku Kimura;, 89, 6, 063701-1, 063701-5, Jun. 2020, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal

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  • Raman spectra of ring-exchange antiferromagnets on the Penrose lattice
    Inoue Takashi; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 75.1, 1500, 1500, The Physical Society of Japan, 2020
    Japanese
  • Raman scattering mediated by Majorana fermions in Kitaev nanoribbons
    Shoji Yamamoto; Kosuke Suzuki
    Journal of Physcis Conference Series, 1220, 1, 012046-1, 012046-4, May 2019, [Peer-reviewed]
    English, International conference proceedings

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  • Optical observation of fermionic partons in Kitaev spin balls
    Shoji Yamamoto; Taku Kimura
    Journal of Physics Conference Series, 1220, 1, 012003-1, 012003-4, May 2019, [Peer-reviewed]
    English, International conference proceedings

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  • Spin-wave thermodynamics of square-lattice antiferromagnets revisited
    Shoji Yamamoto; Yusaku Noriki
    Physical Review B, 99, 09, 094412-1, 094412-17, Mar. 2019, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal

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  • Magnetic Raman scattering beyond the Loudon-Fleury mechanism in the Heisenberg antiferromagnet on the Penrose lattice
    Inoue Takashi; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 74.1, 1696, 1696, The Physical Society of Japan, 2019
    Japanese
  • One- and two-magnon-mediated Raman scattering in Penrose-lattice antiferromagnets
    Inoue Takashi; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 74.2, 1342, 1342, The Physical Society of Japan, 2019
    Japanese
  • Photocontrollable Mixed-Valent States in Platinum-Halide Ladder Compounds
    Jun Ohara; Shoji Yamamoto
    Applied Sciences, 8, 11, 2126-1, 2126-15, Nov. 2018, [Peer-reviewed]
    English, Scientific journal
  • Magnetic Raman scattering in two-dimensional quasiperiodic Heisenberg antiferromagnets
    Inoue Takashi; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 73.2, 1149, 1149, The Physical Society of Japan, 2018
    Japanese
  • Modified Spin-Wave Theory on Low-Dimensional Heisenberg Ferrimagnets: A New Robust Formulation
    Yusaku Noriki; Shoji Yamamoto
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 86, 3, 034714-1, 034714-19, Mar. 2017, [Peer-reviewed], [Last author, Corresponding author]
    English, Scientific journal

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  • Photoinduced itinerant ferromagnetism in copper octacyanomolybdates
    Jun Ohara; Shoji Yamamoto
    Journal of Physics Conference Series, 868, 1, 012013-1, 012013-8, 2017, [Peer-reviewed]

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  • 21aBD-13 Theory of magnetic Raman scattering in antiferromagnetic spin tubes
    INOUE Takashi; YAMAMOTO Shoji
    Meeting Abstracts of the Physical Society of Japan, 71, 1231, 1231, The Physical Society of Japan (JPS), 2016
    Japanese
  • 21aBD-14 Nuclear magnetic relaxation theory on low-dimensional ferrimagnets
    Noriki Yusaku; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 71, 1232, 1232, The Physical Society of Japan (JPS), 2016
    Japanese
  • 21pBT-11 Modified spin-wave theory on fully frustrated antiferromanets : How to modify the conventional spin waves
    Yamamoto Shoji; Ohara Jun
    Meeting Abstracts of the Physical Society of Japan, 71, 3053, 3053, The Physical Society of Japan (JPS), 2016
    Japanese
  • 22aBC-7 Theory of magnetic Raman scattering in triangular lattice antiferromagnets
    Ohara Jun; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 71, 1294, 1294, The Physical Society of Japan (JPS), 2016
    Japanese
  • Nuclear magnetic relaxation theory on the one-dimensional ferrimagnets Ca3Cu3-xNix(PO4)4 (x=0,1,2)
    Noriki Yusaku; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 71, 962, 962, The Physical Society of Japan, 2016
    Japanese,

    Cu(II) (S=1/2)と Ni(II) (S=1)の3量体が鎖を形成する1次元フェリ磁性体の核スピン-格子緩和率 1/T_1 の計算及びその実験との比較。修正スピン波理論，Raman 過程，3マグノン過程，2次摂動。

  • The magnetic specific heat and susceptibility of triangular-lattice Heisenberg antiferromagnets
    Ohara Jun; Yamamoto Shoji; Schmidt Burkhard
    Meeting Abstracts of the Physical Society of Japan, 71, 839, 839, The Physical Society of Japan, 2016
    Japanese,

    表題模型及び関連物質の磁気比熱、帯磁率の理論計算。有限サイズ・クラスタ、熱力学極限、その振る舞いの詳細な検討。修正スピン波理論、Lanczos対角化法。

  • Nuclear spin-lattice relaxation at field-induced level crossings in a Cr8F8 pivalate single crystal
    Shoji Yamamoto
    PHYSICA B-CONDENSED MATTER, 481, 224, 231, Jan. 2016, [Peer-reviewed]
    English, Scientific journal

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  • Antiferromagnetic order in single crystals of the S=2 quasi-one-dimensional chain MnCl3(bpy)
    Shin-ichi Shinozaki; Akira Okutani; Daichi Yoshizawa; Takanori Kida; Tetsuya Takeuchi; Shoji Yamamoto; Olivia N. Risset; Daniel R. Talham; Mark W. Meisel; Masayuki Hagiwara
    PHYSICAL REVIEW B, 93, 1, 014407-1, 014407-6, Jan. 2016, [Peer-reviewed]
    English, Scientific journal

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  • 16pCG-11 Dimensional crossover in bimetallic ferrimagnets
    Noriki Yusaku; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 70, 811, 811, The Physical Society of Japan (JPS), 2015
    Japanese
  • 16pCG-12 Modified spin-wave theory on quadrangular-prism antiferromagnets
    Kimura Taku; Noriki Yusaku; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 70, 812, 812, The Physical Society of Japan (JPS), 2015
    Japanese
  • 16aCQ-8 Bosonic representations of the thermodynamic properties of the triangular Heisenberg antiferromagnet
    Ohara Jun; Yamamoto Shoji
    Meeting Abstracts of the Physical Society of Japan, 70, 2534, 2534, The Physical Society of Japan (JPS), 2015
    Japanese
  • 16aCQ-7 Schwinger-boson representation of anisotropic quantum spin models
    INOUE Takashi; YAMAMOTO Shoji
    Meeting Abstracts of the Physical Society of Japan, 70, 2533, 2533, The Physical Society of Japan (JPS), 2015
    Japanese
  • Photomagnetism in octacyanomolybdates of the square antiprism configuration
    Jun Ohara; Shoji Yamamoto
    Molybdenum and its Compounds: Applications, Electrochemical Properties and Geological Implications, 123, 152, Nova Science Publishers, Inc., 01 Jul. 2014
    English, In book
  • Photoinduced Directional and Bidirectional Phase Transitions in Bistable Linear Polycyclic Aromatic Compounds
    Longlong Zhang; Shoji Yamamoto
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 83, 6, 064708-1, 064708-14, Jun. 2014, [Peer-reviewed]
    English, Scientific journal

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  • Phototunable Magnetism in Copper Octacyanomolybdate
    Jun Ohara; Shoji Yamamoto
    SCIENTIFIC WORLD JOURNAL, 2014, 762765-1, 762765-13, May 2014, [Peer-reviewed]
    English, Scientific journal

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  • Symmetry argument of cyano-bridged copper-molybdenum complexes
    Jun Ohara; Shoji Yamamoto
    JOURNAL OF THE KOREAN PHYSICAL SOCIETY, 62, 12, 1797, 1801, Jul. 2013, [Peer-reviewed]
    English, Scientific journal
  • Platinum-halide square-prism compounds: Novel ground states and their optical features
    Shoji Yamamoto; Jun Ohara
    Platinum: Compounds, Production and Applications, 123, 145, Nova Science Publishers, Inc., Jan. 2013
    English, In book
  • Magnetic Properties of theS=2 Heisenberg Antiferromagnetic Chain Compound MnCl3(bpy)
    M Hagiwara; Y Idutsu; Z Honda; S Yamamoto
    Journal of Physics: Conference Series, 400, 3, 032014-1, 032014-4, IOP Publishing, 17 Dec. 2012, [Peer-reviewed]
    Scientific journal
  • Phase competition and photomagnetism in supramolecular assemblies based on octacyanomolybdates of square antiprism configuration
    Shoji Yamamoto; Jun Ohara
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 12, 9, 12, 2565, 2569, 2012, [Peer-reviewed]
    English, International conference proceedings
  • Bottom-up realization of a porous metal-organic nanotubular assembly
    Kazuya Otsubo; Yusuke Wakabayashi; Jun Ohara; Shoji Yamamoto; Hiroyuki Matsuzaki; Hiroshi Okamoto; Kiyofumi Nitta; Tomoya Uruga; Hiroshi Kitagawa
    NATURE MATERIALS, 10, 4, 291, 295, Apr. 2011, [Peer-reviewed]
    English, Scientific journal
  • Optical distinction and photoinduced phase transition between degenerate ground states of polyacene
    Shoji Yamamoto; Jun Ohara
    PHYSICA B-CONDENSED MATTER, 405, 11, S365, S368, Jun. 2010, [Peer-reviewed]
    English, Scientific journal
  • Ground-State Properties of a Peierls-Hubbard Triangular Prism
    Shoji Yamamoto; Jun Ohara; Masa-aki Ozaki
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 79, 4, 044709-1, 044709-8, Apr. 2010, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal

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  • Competing ground states of a Peierls-Hubbard nanotube
    Jun Ohara; Shoji Yamamoto
    Europhysics Letters, 87, 1, 17006-1, 17006-6, Jul. 2009, [Peer-reviewed], [Last author, Corresponding author]
    English, Scientific journal

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  • Optical observations of new halogen-bridged platinum complexes assembled within a ladder lattice
    Shoji Yamamoto; Jun Ohara
    JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 20, S367, S370, Feb. 2009, [Peer-reviewed]
    English, Scientific journal

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  • Multichanneled relaxation of halogen-bridged diplatinum chain complexes photoexcited
    Shoji Yamamoto; Jun Ohara
    Physica Status Solidi (C) Current Topics in Solid State Physics, 6, 1, 144, 148, 2009, [Peer-reviewed]
    English, International conference proceedings
  • Multichanneled relaxation of halogen-bridged diplatinum chain complexes photoexcited
    Shoji Yamamoto; Jun Ohara
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1, 6, 1, 144, 148, 2009, [Peer-reviewed]
    English, International conference proceedings
  • Photoproduction of spin and charge carriers in halogen-bridged binuclear platinum chain complexes
    Shoji Yamamoto; Jun Ohara
    JOURNAL OF PHYSICS-CONDENSED MATTER, 20, 41, 415215-1, 415215-9, Sep. 2008, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal

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  • Optical characterization of platinum-halide ladder compounds
    Shoji Yamamoto; Jun Ohara
    PHYSICAL REVIEW B, 76, 23, 235116-1, 235116-8, Dec. 2007, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal

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  • Low-energy structure of the homometallic intertwining double-chain ferrimagnets A(3)Cu(3)(PO4)(4) (A=Ca,Sr,Pb)
    Shoji Yamamoto; Jun Ohara
    PHYSICAL REVIEW B, 76, 7, 014409-1, 014409-8, Jul. 2007, [Peer-reviewed], [Lead author, Corresponding author]
    English, Scientific journal

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  • Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds
    Jun Ohara; Shoji Yamamoto
    Physical Review B - Condensed Matter and Materials Physics, 73, 4, 2006, [Peer-reviewed]
    English, Scientific journal
  • Transport properties of organic spin-gapped antiferromagnets
    T Sakai; S Yamamoto
    SYNTHETIC METALS, 152, 1-3, 477, 480, Sep. 2005
    English, Scientific journal
  • Photoinduced infrared absorption of quasi-one-dimensional halogen-bridged binuclear transition-metal complexes
    Jun Ohara; Shoji Yamamoto
    Journal of Physics and Chemistry of Solids, 66, 8-9, 1571, 1574, Aug. 2005, [Peer-reviewed]
    English, International conference proceedings
  • Transport in Gapped Quantum Antiferromagnets
    Sakai Tôru; Yamamoto Shoji
    Journal of the Physical Society of Japan, 74, 191, 195, The Physical Society of Japan, 2005
    English, The transport properties of quantum systems are one of the most interesting current topics in statistical physics. Heat transport can directly be observed through thermal conductivity measurements, while spin transport can be extracted from nuclear spin–lattice relaxation-time measurements. Numerous experiments have indeed been performed in an attempt to settle the argument about whether finite-temperature energy transport should be diffusive or ballistic. From the theoretical point of view, there is an argument that integrable Hamiltonians exhibit ballistic transport, whereas nonintegrable ones are diffusive or insulators. Since nonintegrable systems have much less been studied in this context so far, we investigate spin-½ ferromagnetic-antiferromagnetic bond-alternating chains, which exhibit common features of Haldane-gap antiferromagnets. Employing the Jordan–Wigner spinless-fermion representation, their level statistics is investigated by the exact-diagonalization method. We argue a possibility of the diffusive transport occurring in the system.
  • Modified Spin-Wave Theory of Nuclear Magnetic Relaxation in One-Dimensional Quantum Ferrimagnets: Three-Magnon versus Raman Processes
    Yamamoto Shoji; Hori Hiromitsu
    Journal of the Physical Society of Japan, 74, 75, 79, The Physical Society of Japan, 2005
    English, Nuclear spin–lattice relaxation in one-dimensional Heisenberg ferrimagnets is studied by means of a modified spin-wave theory. Calculating beyond the first-order mechanism, where a nuclear spin directly interacts with spin waves through the hyperfine coupling, we demonstrate that the exchange-scattering-enhanced three-magnon nuclear relaxation may generally predominate over the Raman one with increasing temperature and decreasing field. Recent proton spin–lattice relaxation-time (T1) measurements on the ferrimagnetic chain compound NiCu(C7H6N2O6)(H2O)3·2H2O suggest that the major contribution to 1/T1be made by the three-magnon scattering.
  • Low-temperature thermodynamics of one-dimensional alternating-spin Heisenberg ferromagnets
    Yamamoto Shoji; Hori Hiromitsu
    PHYSICAL REVIEW B, 72, 54423, 54423, American Physical Society, 2005
    English, Motivated by a novel bimetallic chain compound in which alternating magnetic centers are ferromagneticallycoupled, we investigate thermodynamic properties of one-dimensional spin-(S, s) Heisenberg ferromagnetsboth numerically and analytically. On the one hand, quantum Monte Carlo calculations illuminate theoverall thermal behavior. The specific heat may exhibit a double-peaked structure at intermediate temperaturesfor S3s in general. On the other hand, a modified spin-wave theory precisely describes the low-temperatureproperties. Expanding the specific heat and the magnetic susceptibility, we reveal an analogy and a contrastbetween mixed-spin ferromagnets and ferrimagnets.
  • Photoinduced absorption spectra of halogen-bridged binuclear metal complexes: Possible contrast between R4[Pt2(P 2O5H2)4X]·nH2O and Pt2(CH3CS2)4I
    Jun Ohara; Shoji Yamamoto
    Journal of the Physical Society of Japan, 74, 1, 250, 253, Jan. 2005, [Peer-reviewed], [Last author, Corresponding author]
    English, Scientific journal

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  • Soliton-induced optical absorption of halogen-bridged mixed-valence binuclear metal complexes
    Jun Ohara; Shoji Yamamoto
    Physical Review B - Condensed Matter and Materials Physics, 70, 11, 1, 115112, Sep. 2004, [Peer-reviewed]
    English, Scientific journal
  • Nuclear Spin–Lattice Relaxation in One-Dimensional Heisenberg Ferrimagnets: Three-Magnon vs Raman Processes
    Hori Hiromitsu; Yamamoto Shoji
    J Phys Soc Jpn, 73, 6, 1453, 1456, The Physical Society of Japan (JPS), 15 Jun. 2004
    English, Nuclear spin–lattice relaxation in one-dimensional Heisenberg ferrimagnets is studied in terms of a modified spin-wave theory. We consider the second-order process, where a nuclear spin flip induces virtual spin waves which are then scattered thermally via the four-magnon exchange interaction, as well as the first-order process, where a nuclear spin directly interacts with spin waves via the hyperfine interaction. We point out a possibility of the three-magnon relaxation process predominating over the Raman one and suggest model experiments.
  • Nuclear Magnetic Relaxation in the Haldane-Gap Antiferromagnet Ni(C2H8N2)2NO2(ClO4)
    Yamamoto Shoji; Hori Hiromitsu
    J Phys Soc Jpn, 73, 4, 822, 825, The Physical Society of Japan (JPS), 15 Apr. 2004
    English, A new theory is proposed to interpret nuclear spin–lattice relaxation-time ($T_{1}$) measurements on the spin-1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C2H8N2)2NO2(ClO4) (NENP). While Sagi and Affleck [Phys. Rev. B 53 (1996) 9188] pioneeringly discussed this subject in terms of field-theoretical languages, there is no theoretical attempt yet to explicitly simulate the novel observations of $T_1^{-1}$ reported by Fujiwara et al. [Phys. Rev. B 47 (1993) 11860]. By means of modified spin waves, we interpret \textit{the minimum of $T_1^{-1}$ as a function of an applied field}, pending for the past decade.
  • Static and Dynamic Properties of Antiferromagnetic Heisenberg Ladders: Fermionic versus Bosonic Approaches
    Hori Hiromitsu; Yamamoto Shoji
    J Phys Soc Jpn, 73, 3, 549, 552, The Physical Society of Japan (JPS), 15 Mar. 2004
    English, In terms of spinless fermions via the Jordan–Wigner transformation along a snakelike path and spin waves modified so as to restore the sublattice symmetry, we investigate the static and dynamic properties of two-leg antiferromagnetic Heisenberg ladders. The specific heat is finely reproduced by the spinless fermions, whereas the magnetic susceptibility is well described by the modified spin waves. The nuclear spin–lattice relaxation rate is discussed in detail with particular emphasis on its novel field dependence.
  • Spin–Wave Description of Haldane-Gap Antiferromagnets
    Yamamoto Shoji; Hori Hiromitsu
    J Phys Soc Jpn, 72, 4, 769, 772, The Physical Society of Japan (JPS), 15 Apr. 2003
    English, Modifying the conventional antiferromagnetic spin–wave theory which is plagued by the difficulty of the zero-field sublattice magnetizations diverging in one dimension, we describe magnetic properties of Haldane-gap antiferromagnets. The modified spin waves, constituting a grand canonical bosonic ensemble so as to recover the sublattice symmetry, not only depict well the ground-state correlations but also give useful information on the finite-temperature properties.
  • Quantum phase competition in antiferromagnetic spin-1 ladders
    Shoji Yamamoto; Tôru Sakai; Akihisa Koga
    Progress of Theoretical Physics, 176, 181, 01 Dec. 2002
  • Spin-Wave Description of Nuclear Spin-Lattice Relaxation in Mn12O12 Acetate
    Yamamoto Shoji; Nakanishi Takashi
    PHYSICAL REVIEW LETTERS, 89, 15, 157603, 157603, Oct. 2002
    English, In response to recent nuclear-magnetic-resonance (NMR) measurements on the molecular cluster Mn12O12 acetate, we study the nuclear spin-lattice relaxation rate 1/T1, developing a modified spin-wave theory. Our microscopic new approach, which is distinct from previous macroscopic treatments of the cluster as a rigid spin of S=10, not only excellently interprets the observed temperature and applied-field dependences of 1/T1 for 55Mn nuclei but also strongly supports the 13C NMR evidence for spin delocalization over the entire molecule.
  • Fermionic Description of Spin-Gap States of Antiferromagnetic Heisenberg Ladders in a Magnetic Field
    Hori Hiromitsu; Yamamoto Shoji
    J Phys Soc Jpn, 71, 6, 1607, 1608, The Physical Society of Japan (JPS), 15 Jun. 2002
    English
  • Re-Entrant Quantum Phase Transitions in Antiferromagnetic Spin-1 Ladders
    Yamamoto Shoji; Sakai Tôru; Koga Akihisa
    J Phys Soc Jpn, 71, 5, 1250, 1253, The Physical Society of Japan (JPS), 15 May 2002
    English, In response to recent attempts to construct higher-spin ladder materials from organic polyradicals, we study the ground-state properties of a wide class of antiferromagnetic spin-1 ladders. Employing various numerical tools, we reveal the rich phase diagram and correct a previous nonlinear-sigma-model prediction. A variational analysis interprets the phase competition well with particular emphasis on the re-entrant phase boundary as a function of the rung interaction.
  • Quantum Phase Competition in Antiferromagnetic Spin-1 Ladders :
    YAMAMOTO Shoji; SAKAI Toru; KOGA Akihisa
    Progress of theoretical physics. Supplement, 145, 176, 181, Progress of Theoretical Physics, 2002
    English, Motivated by recent chemical explorations into organic-radical-based higher-spin ladder systems, we study the ground-state properties of a wide class of antiferromagnetic spin-1 ladders. Numerical analysis featuring the level-spectroscopy technique reveals the rich phase diagram, correcting a preceding nonlinear-sigma-model prediction. A variational analysis well interprets the phase competition with particular emphasis on the re-entrant phase boundary on the way from single to coupled chains.
  • Quantum magnetization plateaux in ferrimagnetic spin chains
    Tôru Sakai; Shoji Yamamoto
    Journal of Magnetism and Magnetic Materials, 226-230, I, 645, 646, Elsevier B.V., 2001
    English, Scientific journal
  • Coexistent quantum and classical aspects of magnetization plateaux in alternating-spin chains
    T Sakai; S Yamamoto
    JOURNAL OF PHYSICS-CONDENSED MATTER, 12, 47, 9787, 9797, Nov. 2000
    English, Scientific journal
  • Multiplateau magnetization curves of one-dimensional Heisenberg ferrimagnets
    S Yamamoto; T Sakai
    PHYSICAL REVIEW B, 62, 6, 3795, 3800, Aug. 2000, [Peer-reviewed]
    English, Scientific journal
  • Characterization of ferrimagnetic Heisenberg chains according to the constituent spins
    S Yamamoto; T Fukui; T Sakai
    EUROPEAN PHYSICAL JOURNAL B, 15, 2, 211, 219, May 2000
    English, Scientific journal
  • Multiplateau magnetization curves of one-dimensional Heisenberg ferrimagnets
    Shoji Yamamoto; Tôru Sakai
    Physical Review B - Condensed Matter and Materials Physics, 62, 6, 3795, 3800, 2000
    English, Scientific journal
  • Magnetization Plateau in One-Dimensional Quantum Ferrimagnets :
    SAKAI Toru; YAMAMOTO Shoji
    Progress of theoretical physics. Supplement, 138, 513, 514, Progress of Theoretical Physics, 2000
    English, Quantum critical behavior in the magnetization process of alternating-spin chains is studied by the exact diagonalization and the size scaling technique based on the conformal field theory. The plateau at the spontaneous magnetization is revealed to vanish due to the anisotropy at the Kosterlitz-Thouless transition point in the ferromagnetic region. It is also found that the bond alternation creates a novel plateau as an essential quantum effect.
  • Critical behavior of anisotropic Heisenberg mixed-spin chains in a field
    T Sakai; S Yamamoto
    PHYSICAL REVIEW B, 60, 6, 4053, 4056, Aug. 1999, [Peer-reviewed]
    English, Scientific journal
  • Breakdown of a magnetization plateau due to anisotropy in Heisenberg mixed-spin chains
    S Yamamoto; T Sakai
    JOURNAL OF PHYSICS-CONDENSED MATTER, 11, 26, 5175, 5186, Jul. 1999
    English, Scientific journal
  • Critical behavior of anisotropic Heisenberg mixed-spin chains in a field
    Tôru Sakai; Shoji Yamamoto
    Physical Review B - Condensed Matter and Materials Physics, 60, 6, 4053, 4056, 1999
    English, Scientific journal
  • Low-Energy Structure of Heisenberg Ferrimagnetic Spin Chains
    Yamamoto Shoji; Sakai Tôru
    J Phys Soc Jpn, 67, 11, 3711, 3714, The Physical Society of Japan (JPS), 15 Nov. 1998
    English, Static and dynamic structure factors of Heisenberg ferrimagneticspin chains are numerically investigated.There exist two distinct branches of elementary excitations, whichexhibit ferromagnetic and antiferromagnetic aspects.The ferromagnetic feature is smeared out with the increase of temperature, whereas the antiferromagnetic one persists up to higher temperatures.The scattering intensity is remarkably large at lower boundariesof the ferromagnetic and antiferromagnetic spectra.All these observations are consistent with theferromagnetic-to-antiferromagnetic crossover in the thermalbehavior which has recently been reported.
  • MAGNETIZATION OF S=1 ANTIFERROMAGNETIC HEISENBERG-CHAINS
    S YAMAMOTO; S MIYASHITA
    PHYSICAL REVIEW B, 51, 6, 3649, 3654, Feb. 1995
    English, Scientific journal
  • Ground-State Properties of S=1 Antiferromagnetic Heisenberg Chains with Bond Alternation
    Yamamoto Shoji
    J Phys Soc Jpn, 63, 12, 4327, 4330, The Physical Society of Japan (JPS), 15 Dec. 1994
    English, Ground-state properties of S=1 antiferromagnetic Heisenberg chains with bond alternation, H=J ∑i {1-(-1)iδ} Si· Si+1, are studied by a quantum Monte Carlo method. Spin correlation between chain ends on the open chain appears for δ<0.25, while it vanishes for δ≥0.25. At $\delta \simeq 0.25$, the spin correlation length has a peak and the S=1/2 edge states for the open chain disappear. The fourfold degeneracy of the ground state of the open chain holds for δ<0.25, while it is removed for δ≥0.25. All the results support the continuous phase transition at $\delta \simeq 0.25$ demonstrated in the previous works.
  • Anisotropy effects on the magnetic properties of an S=1 antiferromagnetic Heisenberg chain
    Shoji Yamamoto; Seiji Miyashita
    Physical Review B, 50, 9, 6277, 6288, 1994
    English, Scientific journal
  • THERMODYNAMIC PROPERTIES OF S = 1 ANTIFERROMAGNETIC HEISENBERG-CHAINS AS HALDANE SYSTEMS
    S YAMAMOTO; S MIYASHITA
    PHYSICAL REVIEW B, 48, 13, 9528, 9538, Oct. 1993
    English, Scientific journal
  • THERMODYNAMIC PROPERTIES OF S = 1 ANTIFERROMAGNETIC HEISENBERG-CHAINS
    S MIYASHITA; S YAMAMOTO
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 62, 5, 1459, 1462, May 1993
    English, Scientific journal
  • Effects of edges in S=1 Heisenberg antiferromagnetic chains
    Seiji Miyashita; Shoji Yamamoto
    Physical Review B, 48, 2, 913, 919, 1993
    English, Scientific journal
■ Other Activities and Achievements
■ Syllabus
  • 電子物性物理学, 2024年, 修士課程, 理学院
  • 物理学外国語文献講読Ⅱ, 2024年, 学士課程, 理学部
  • 統計力学Ⅱ, 2024年, 学士課程, 理学部
  • 量子力学Ⅱ, 2024年, 学士課程, 理学部
■ Affiliated academic society
  • 電子スピンサイエンス学会
  • 日本物理学会
  • Physical Society of Japan
■ Works
  • Spin Dynamics in Magnetic Clusters
    2002
  • Mixed Magnetism in Ferrimagnets
    1996 - 1998
■ Research Themes
  • Magnons in quasiperiodic magnets: Exploration of novel Nambu-Goldstone modes
    Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
    Apr. 2022 - Mar. 2026
    山本 昌司
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Hokkaido University, Principal investigator, 22K03502
  • Photomanipulation of varying magnetism in transition-metal complexes of the square antiprisum type
    Grants-in-Aid for Scientific Research
    01 Apr. 2012 - 31 Mar. 2016
    Yamamoto Shoji
    We investigate the photoinduced bidirectional magnetism in Cu2[Mo(CN)8] consisting of square-antiprism-type Mo complexes. If we irradiate this material with blue laser light, it turns from paramagnetic to ferromagnetic. When we further irradiate this magnetized state with orrange laser light, the material is again demagnetized strongly.


    First we elucidate the ground-state properties through a group-theoretical analysis. There appear a few different types of antiferromagnetism as well as ferromagnetism of our particular interest. With a realistic set of parameters, the material lies in the paramagnetic down to low temperatures. Then we numerically integrate the time-dependent Schroedinger equation and track the magnetization. The magnetization is indeed enhanced by blue laser light irradiation and again reduced by orrange laser light irradiation. Surprisingly, the final steady demagnetized state is qualitatively different from the initial static paramagnetic state.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Hokkaido University, Principal investigator, Competitive research funding, 24540342
  • 芳香族有機化合物を用いた超高速多機能光スイッチング・デバイスの探求
    研究助成
    2011 - 2012
    山本 昌司
    寿原記念財団, Principal investigator, Competitive research funding
  • Itinerant electron nanomagnetism -Photoinduced magnetism accompanied by itinerant electrons-
    Grants-in-Aid for Scientific Research
    2008 - 2011
    YAMAMOTO Shoji
    We have theoretically characterized nanotubular halogen-bridged platinum complexes in cooperation with experimental research groups. Half-metallic charge-density-wave states as well as novel mixed-valent ground states were revealed. We have found out photoinduced phase transitions of one-way aspect in polyacene, which consists of linearly fused benzene rings. Strucural isomers of the so-called cis and trans types are energetically degenerate but optically distinguishable and therefore the present findings may open a way to a brandnew photochromic device. We have further constructed a nuclear-magnetic-relaxation theory for the field-induced level crossings in single-molecular ring antiferromagnets. Novel NMR observations in octanuclear chromic wheels were theoretically solved.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Hokkaido University, 20540327
  • リング交換・フラストレーションをもつ磁場中量子スピン系におけるカイラリティ秩序
    科学研究費補助金
    2006 - 2007
    引原 俊哉
    昨年度に引き続き、リング交換・フラストレーションをもつ磁場中量子スピン系における、新奇量子相の探求を行った。まず、リング交換を含んだ磁場中二本鎖梯子ハイゼンベルグ量子スピン系の解析を行い、この系の磁気相図を決定した。その結果、この系では、2つのマグノンが作るマグノン束縛対がBose凝縮したスピンネマティック状態や、ベクトルカイラリティが支配的な朝永・ラッティンジャー流体などの新奇量子相が実現されることを発見し、それら新奇量子相の出現条件と、その特性を明らかにした。
    また、リング交換を含んだジグザグ梯子ハイゼンベルグ量子スピン系に関する研究を行い、これまでに知られていなかった部分強磁性相や新奇非磁性相を含んだ、基底状態相図を決定した。さらに、実在の量子細線の電子密度や閉じ込めポテンシャル強度と、スピンモデルの相互作用パラメータを関連付けることで、今回発見した新奇量子相が、実際の量子細線において実現可能と期待されるパラメータ領域に存在することを示した。
    さらに、最近接強磁性相互作用と次近接反強磁性相互作用が競合した、磁場中ジグザグ梯子ハイゼンベルグ量子スピン系の研究も行い、この系の磁気相図を決定した。その結果、この系では、低磁場領域でベクトルカイラリティ長距離秩序を伴うベクトルカイラリティ相が、また、高磁場領域で複数のマグノンが束縛状態を形成するマグノン束縛相が実現されることを明らかにした。この結果は、LiCuVO_4など、実在の擬一次元磁性体の性質を理解する上で重要な結果である。
    また、ダイヤモンド型量子スピン鎖におけるリング交換相互作用の効果についても調べ、リング交換相互作用による新奇な非磁性状態の出現を予言する結果を得た。
    文部科学省, 特定領域研究, 北海道大学, Competitive research funding, 18043003
  • Construction of the nuclear-magnetic-relaxation theory on low-dimensional magnets
    Grants-in-Aid for Scientific Research
    2004 - 2007
    YAMAMOTO Shoji
    A modified spin-wave theory is developed and applied to low-dimensional quantum magnets. Double-peaked specific heat for one-dimensional ferrimagnets, nuclear spin-lattice relaxation in ferrimagnetic chains and clusters, and thermal behavior of Haldane-gap antiferromagnets are described within the scheme. Mentioning other bosonic and fermionic representations as well, we demonstrate that spin waves are still effective in low dimensions.
    Mesoscopic magnetism is one of the hot topics in materials science, where we can observe a quantum-to-classical crossover on the way from molecular to bulk magnets. Metal-ion magnetic clusters are thus interesting and among others is a series of molecular rings, which allows us to systematically investigate zero-dimensional spin dynamics as a function of the molecule size and the spin magnitude. The nuclear spin-lattice relaxation time Ti works as a powerful probe to the low-frequency spin dynamics and has indeed been measured for various metallic wheels containing copper, chromium, and iron ions. Thus motivated, we microscopically calculate 1/ Ti for both ferromagnetic and antiferromagnetic ring clusters, taking account of crystal-field anisotropies and anisotropic spin-spin interactions as well as the Heisenberg interaction. The following figure compares our calculations with experiments] on the ferromagnetic ring cluster Cu6, where we stress the lifetime broadening of descrete energy levels in proportion to Ti and the nonnegligible anisotropy effect as the key factors to interpret the characteristic peaks of 1/ Ti as a function of T. We calculate the antiferromagnetic ring cluster Cu8 as well and point out possible significance of the Dzyaloshinsky-Moriya interaction.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Hokkaido University, Principal investigator, Competitive research funding, 16540300
  • ナノスケール分子磁性体の量子緩和機構の解明
    研究助成
    2004 - 2005
    山本 昌司
    池谷科学技術振興財団, Principal investigator, Competitive research funding
  • 分子スピン制御による機能性磁性材料の構築
    学術研究助成
    2003 - 2004
    山本 昌司
    日産科学振興財団, Principal investigator, Competitive research funding
  • 低次元金属錯体の多彩な電子相と光物性
    基礎科学研究助成
    2001 - 2003
    山本 昌司
    住友財団, Principal investigator, Competitive research funding
  • ハロゲン架橋金属錯体に観る一次元電子・格子物性 ―金属多核化による新展開―
    科学研究費助成事業
    2001 - 2002
    山本 昌司
    ハロゲン架橋金属錯体は,強い吸収2色性,高次にわたるRaman散乱,大きなStokesシフトを伴う発光スペクトルなど,興味深い光学活性物質である.近年この物質系において金属サイトの複核化が成功し,いわゆるMMX錯体の物性測定が盛んになってきている.本研究課題では,これを理論的にリードすることを目指している.
    研究初年度においては,群論による解析的対称性の議論に基づき,可能な基底状態を系統的に導出し,その物理的性質を明示した.数値計算も行って,実際にどのような条件下でいかなる基底状態が存在し得るかを議論した.計算は絶対零度だけでなく有限温度にも拡張し,量子相転移から熱転移,さらには圧力誘起相転移など,さまざまな内的・外的条件下での相競合の実態を予言・解釈した.特に圧力誘起相転移に関しては実験研究との緊密な連携を図り,東大工学部・物理工学専攻・岡本研究室による観測に,明快な理論解釈を与え,その後の実験研究にも方向性を与えた.
    研究2年度目においては,上記初年度の成果をもとに,非線形光学励起へと計算を進めた.この物質系の原形であるMX錯体では,ソリトン,ポーラロンなど低次元特有の素励起が観測され,その高速ダイナミクスが多くの興味を集めてきた.MMX錯体でも同様な観測が期待されるが,そこではより有効質量が小さく易動度が高いキャリアが励起され得ることを示した.特に,ポリアセチレンで脚光を浴びた解析連続体模型が本物質系に転用できることを数学的に示し,その類似性に基づき,多くの得意な低次元物性を予言した.
    日本学術振興会, 若手研究(B), 13740188
  • フェリ磁性混合スピン系の量子物性---機能性分子磁石を求めて---
    第36回学術研究助成
    1999 - 2000
    山本 昌司
    山陽放送学術文化財団, 岡山大学
  • フェリ磁性混合スピン系の量子物性-機能性分子磁石の可能性を探る-
    科学研究費助成事業
    1999 - 2000
    山本 昌司
    フェリ磁性体には,massless,massive双方の励起モードが存在し,これらはそれぞれ強磁性的・反強磁性的性質を帯びている.低温域ではmassless励起が支配的であるため強磁性的性質を示し,中温域ではmassive励起モードが効きはじめて反強磁性的挙動を示すようになり,高温域では古典的常磁性に移行する.フェリ磁性の融合磁性という新しい捉え方,これこそが本研究の動機であり,この考え方を理論・実験のフィードバックを繰り返すことにより定量化し,強磁性的側面を有効的に抽出する有機分子フェリ磁石へとつなげてゆくことが,本研究の究極目標であった.
    研究2年目にあっては,この最終目標に向かって新たなステップが取られた.研究初年度において一次元フェリ磁性すなわち鎖内物性について徹底的に解明したが,これに基づき,鎖間相互作用を考慮した場合の高次元フェリ磁性へ歩を進めた.分子化学分野では,複核金属ないしポリラジカルを利用してフェリ磁性鎖を構成し,これを束ねてバルク磁石を造るという試みが進行しているが,これに理論的指標を与えるべく計算を進めた.スピン波理論を駆使して,高次元化する際の磁化の定性的・半定量的変化を明らかにし,さらに量子モンテカルロ法や厳密対角化法を併用して,物質パラメータを意識した定量性の高い計算へと発展させた.
    そうした中,分子科学研究所で梯子形状をもつ有機フェリ磁性体が合成された.これを受け,いち早くフェリ磁性梯子の計算に着手し,最も強磁性的側面が強くなるような鎖間相互作用の大きさを明示した.この論文は5月に掲載予定で,今後の物質研究と理論研究の架橋関係を加速するものと期待される.
    日本学術振興会, 奨励研究(A), 岡山大学, 11740206
  • 一次元磁性体における量子協力現象の解明
    第9回学術研究助成
    1997 - 1998
    山本 昌司
    岡山工学振興会, 岡山大学
  • 低次元磁性体における新しいタイプの量子揺動の探究-数値・解析両面からのアプローチ-
    科学研究費助成事業
    1997 - 1998
    山本 昌司
    低次元磁性体の低温量子物性を量子揺動の探究という微視的立場から解明する一方,量子物性研究における方法論的見地からの発展も意図し,数値的・解析的新たな理論体系の開拓にも力を注いだ.低次元特有の量子揺動を縦糸として,スピン・ギャップ系から,フェリ磁性体や金属錯体にも議論を広げた.
    方法論的観点からは,まず量子モンテカルロ・プログラムを,さまざまな空間構造や相互作用に対応できるよう整備した.そして,モンテカルロ・スナップショットにより量子揺動の視覚化に務める一方,行列のテンソル積により波動関数を表現する変分法の運用に力を注いだ.これにより,量子揺動の発現機構の包括的理解が可能となった.さらに,スピン波理論にグランド・カノニカル集団の考え方を加味することにより,その熱力学記述能力の従来常識を覆し,幅広い温度領域に渡って半定量的な議論が可能であることを示した.またこれと並行して,密度行列繰り込み群によるd+1次元転送行列計算を発展させ,低温熱力学の研究に新たな道を開いた.一方現象面では,さまざまな低次元電子・スピン系を議論し,低次元特有の量子揺動や格子との結合に起因する,新奇な量子物性を次々と開拓した.ハルデイン磁性体では,隠れた秩序に生ずるソリトン的揺らぎの役割を解明し,ハルデイン相の量子力学的崩壊機構の理解に貢献した.一方新しい低次元量子磁性開拓を目指して議論し始めたフェリ磁性体では強磁性・反強磁性の交錯という,融合磁性とも呼べる興味深い現象を明らかにした.さらに,電子相関に加えて電子・格子相互作用がユニークかつ重要な役割を果たす,ハロゲン架橋金属錯体に注目した.ハロゲン原子や配位子を調節することにより,ハロゲン原子が磁性を担う,新奇なスピン密度波状態が存在可能であることを指摘した. 『新しいタイプの量子揺動の探究』は,十二分に達成されたものと確信する.今後,実験研究分野からのフィードバックが期待されるところである.
    日本学術振興会, 奨励研究(A), 岡山大学, 09740286
  • 低次元磁性体におけるスピン・格子協力現象
    科学研究費助成事業
    1996 - 1996
    山本 昌司
    全般に一次元反強磁性体の低温量子物性を、量子モンテカルロ法、転送行列法などの統計力学的手法を用いて研究した。当初の研究計画を遂行する中で議論は多様性を帯び、現象、方法論の両面において、新たな方向性を打ち出すこともできたものと認識している。
    スピンと格子の協力現象を系統的に研究する立場から、まず断熱近似の範囲内で、量子揺動や臨界現象を議論した。そこでは1/2から2までの広範なスピン量子数を取扱い、基底状態の性質や低励起構造が、量子性の強い場合から古典的描像に近づいてゆく中で、どのように変化してゆくかが明らかにされた。格子に誘起される交換相互作用の空間的変調を考慮し、その関数としての基底状態の性質を議論した。こうした系ではスピン量子数に依存する一定の規則に基づく相転移が観測されることが検証された。この一連の理論は、現在少しづつ実験研究にもフィードバックされ、結合交替系での相転移を陰に陽に観測しようという試みが始まっている。一方でS=1スピン鎖の基底状態量子揺動を議論した。これは量子揺動を、変分的解析手法で計算する一方、量子モンテカルロ・スナップショットを用いて視覚化、検証するというユニークなもので、現象としての興味もさることながら、今後の低次元量子物性研究における方法論的新たな可能性も提起するものである。
    さらに、断熱近似を越えて格子揺動を議論すべく、経路積分に基づきスピン、格子の自由度を同時に量子化する、量子モンテカルロ・プログラムをスピン量子数が1/2の場合について作成した。当初想定したよりも、プログラム・アルゴリズムは複雑化したため計算時間は長期化し、現在も東京大学物性研究所の大型計算機を使用して計算を続けている。そうした中で、低温において自発的に格子が歪む様子が既に観測されており、スピン Peierls 転移のより直接的議論に道が開かれつつある。
    日本学術振興会, 奨励研究(A), 岡山大学, 08740279
  • 量子モンテカルロ法による有機伝導体の電子・格子状態の研究
    平成6年度研究助成
    1995 - 1996
    山本 昌司
    小笠原科学技術振興財団, 大阪大学
  • ハルデイン磁性体を対象としたS=1反強磁性ハイゼンベルグ模型の数値的研究
    科学研究費助成事業
    1994 - 1994
    山本 昌司
    全般に、S=1反強磁性量子スピン鎖の物性を、量子モンテカルロ法を用いて研究した。当初の研究計画を達成するに留まらず、新たな方向性を開拓する事にも成功したものと考えている。
    まず、現実のハルデイン磁性体を説明する理論的モデルとして、異方性、外場等まで含めたS=1反強磁性ハイゼンベルグ鎖を設定し、これの量子モンテカルロプログラムを作成した。これを用いて、エネルギーギャップ、鎖端磁化、帯磁率などへの異方性の効果、また外場に対する磁化曲線などの計算を行った。これらの研究成果は、従来の実験結果をよく説明するばかりではなく、今後の実験的研究に対する示唆的な役割も十分に果たした。実際既に我々の研究成果は、東京大学物性研究所、理化学研究所、九州大学工学部等の実験研究グループによって比較検討され、論文においても参照されている。
    一方、虚時間相関数を低温で計算することにより、エネルギーの分散関係を精度良く導出することに成功した。この方法は従来のアイデアを基礎としてはいるものの、その応用において斬新なものであり、今後の量子多体系の素励起の研究に一石を投ずるものである。得られた結果は、中性子散乱の実験結果をよく説明する。
    さらに上記の研究とは別に、結合交替量子スピン鎖の研究に着手した。これは、結合交替まで含めた量子スピン鎖は、スピン量子数の整数、半奇整数を問わず、ある一定の条件のもとで臨界性を示す(ギャップレスとなる)という、場の理論による提言に刺激を受けてのものであり、ハルデイン問題を、量子スピン系の横断的、普遍的問題として問直したいという動機による。既にS=1スピン鎖において、臨界現象を視覚化することに成功し、その臨界性がセントラルチャージ1の共形場理論で記述されることを明らかにした。
    日本学術振興会, 奨励研究(A), 大阪大学, 06740289
  • ナノスケールの遍歴電子磁性
    科学研究費補助金
    山本 昌司
    文部科学省, Principal investigator, Competitive research funding
  • Spin Dynamics in Low-dimensional Quantum Magnets
    Competitive research funding
  • photoinduced cooperative phenomena in transition metal complexes
    Competitive research funding