Updated on 2026/08/11

写真a

 
OKUBO Tsuyoshi
 
Organization
Academic Assembly Institute of Science and Technology Faculty of Fundamental Sciences Associate Professor
Graduate School of Science and Technology Fundamental Sciences Associate Professor
Faculty of Science Department of Science Associate Professor
Title
Associate Professor
Homepage
https://tsuyoshi-okubo.com
External link

Degree

  • PhD in Physics ( 2008.3   Kyushu University )

  • M.Sc. in Physics ( 2005.3   Kyushu University )

  • B.Sc. in Physics ( 2003.3   Kyushu University )

Research Interests

  • フラストレート磁性体

  • 量子計算

  • 統計物理

  • 計算物理

  • テンソルネットワーク

Research Areas

  • Natural Science / Mathematical physics and fundamental theory of condensed matter physics

  • Natural Science / Magnetism, superconductivity and strongly correlated systems

Research History (researchmap)

  • The University of Tokyo   Graduate School of Science

    2026.4

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  • Niigata University   Associate Professor

    2026.4

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  • The University of Tokyo   Project associate professor

    2021.7 - 2026.3

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  • 科学技術振興機構   戦略的創造研究推進事業   さきがけ研究者（兼任）

    2019.10 - 2022.3

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  • The University of Tokyo   Department of Physics, Graduate School of Science   Project Lecturer

    2017.4 - 2021.6

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  • The University of Tokyo   The Institute for Solid State Physics   Project Researcher

    2016.4 - 2017.3

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  • The University of Tokyo   The Institute for Solid State Physics   Project Researcher

    2012.4 - 2016.3

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  • Osaka University   Department of Earth and Space Science, Graduate School of Science   Project Researcher

    2008.4 - 2012.3

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  • Japan Society for the Promotion of Science   Research Fellowship for Young Scientist (DC1)

    2005.4 - 2008.3

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Research History

  • Niigata University   Fundamental Sciences, Institute of Science and Technology, Academic Assembly   Associate Professor

    2026.4

  • Niigata University   Department of Science, Faculty of Science   Associate Professor

    2026.4

  • Niigata University   Fundamental Sciences, Graduate School of Science and Technology   Associate Professor

    2026.4

Education

  • Kyushu University   Graduate School of Science   Department of Condensed Matter Science

    2003.4 - 2008.3

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  • Kyushu University   School of Science   Department of Physics

    1999.4 - 2003.3

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Professional Memberships

Committee Memberships

  • 日本物理学会   計算物理領域 運営委員  

    2025.4   

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    Committee type：Academic society

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  • 日本物理学会   領域３運営委員  

    2023.4 - 2024.3   

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    Committee type：Academic society

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  • 日本物理学会   代議員  

    2019.4 - 2021.3   

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    Committee type：Academic society

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  • 日本物理学会   代議員  

    2017.4 - 2019.3   

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    Committee type：Academic society

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Papers

  • Efficient Tomography of Microwave Photonic Cluster States Reviewed

    Yoshiki Sunada, Shingo Kono, Jesper Ilves, Takanori Sugiyama, Yasunari Suzuki, Tsuyoshi Okubo, Shuhei Tamate, Yutaka Tabuchi, Yasunobu Nakamura

    PRX Quantum   7 ( 1 )   010323   2026.2

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    Entanglement among a large number of qubits is a crucial resource for many quantum algorithms. Such many-body states have been efficiently generated by entangling a chain of itinerant photonic qubits in the optical or microwave domain. However, it has remained challenging to fully characterize the generated many-body states by experimentally reconstructing their exponentially large density matrices. Here, we develop an efficient tomography method based on the matrix-product-operator formalism and demonstrate it on a cluster state of up to 35 microwave photonic qubits by reconstructing its 2 35 × 2 35 density matrix. The full characterization enables us to detect the performance degradation of our photon source which occurs only when generating a large cluster state. This tomography method is generally applicable to various physical realizations of entangled qubits and provides an efficient benchmarking method for guiding the development of high-fidelity sources of entangled photons.

    DOI： 10.1103/3vnw-yjd5

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/3vnw-yjd5/fulltext

  • Plastic tensor networks for interpretable generative modeling Reviewed

    Katsuya O Akamatsu, Kenji Harada, Tsuyoshi Okubo, Naoki Kawashima

    Machine Learning: Science and Technology   7 ( 1 )   015014 - 015014   2026.1

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

    Abstract

    A structural optimization scheme for a single-layer nonnegative adaptive tensor tree (NATT) that models a target probability distribution is proposed as an alternative paradigm for generative modeling. The NATT scheme, by construction, automatically searches for a tree structure that best fits a given discrete dataset whose features serve as inputs, and has the advantage that it is interpretable as a probabilistic graphical model. We consider the NATT scheme and a recently proposed Born machine ATT optimization scheme and demonstrate their effectiveness on a variety of generative modeling tasks where the objective is to infer the hidden structure of a provided dataset. Our results show that in terms of minimizing the negative log-likelihood, the single-layer scheme has model performance comparable to the Born machine scheme, though not better. The tasks include deducing the structure of binary bitwise operations, learning the internal structure of random Bayesian networks given only visible sites, and a real-world example related to hierarchical clustering where a cladogram is constructed from mitochondrial DNA sequences. In doing so, we also show the importance of the choice of network topology and the versatility of a least-mutual information criterion in selecting a candidate structure for a tensor tree, as well as discuss aspects of these tensor tree generative models including their information content and interpretability.

    DOI： 10.1088/2632-2153/ae3048

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    Other Link： https://iopscience.iop.org/article/10.1088/2632-2153/ae3048/pdf

  • TeNeS-v2: Enhancement for real-time and finite temperature simulations of quantum many-body systems Reviewed

    Yuichi Motoyama, Tsuyoshi Okubo, Kazuyoshi Yoshimi, Satoshi Morita, Tatsumi Aoyama, Takeo Kato, Naoki Kawashima

    Computer Physics Communications   315   109692   2025.10

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

    DOI： 10.1016/j.cpc.2025.109692

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  • Universal scaling laws of absorbing phase transitions in artificial deep neural networks Reviewed

    Keiichi Tamai, Tsuyoshi Okubo, Truong Vinh, Truong Duy, Naotake Natori, Synge Todo

    Physical Review Research   7   033072   2025.6

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/jp61-6sp2

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/jp61-6sp2/fulltext

  • Tensor tree learns hidden relational structures in data to construct generative models Reviewed

    Kenji Harada, Tsuyoshi Okubo, Naoki Kawashima

    Machine Learning: Science and Technology   6 ( 2 )   025002 - 025002   2025.4

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

    Abstract

    Based on the tensor tree network with the Born machine framework, we propose a general method for constructing a generative model by expressing the target distribution function as the amplitude of the quantum wave function represented by a tensor tree. The key idea is dynamically optimizing the tree structure that minimizes the bond mutual information. The proposed method offers enhanced performance and uncovers hidden relational structures in the target data. We illustrate potential practical applications with four examples: (i) random patterns, (ii) QMNIST handwritten digits, (iii) Bayesian networks, and (iv) the pattern of stock price fluctuation pattern in S&P500. In (i) and (ii), the strongly correlated variables were concentrated near the center of the network; in (iii), the causality pattern was identified; and in (iv), a structure corresponding to the eleven sectors emerged.

    DOI： 10.1088/2632-2153/adc2c7

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    Other Link： https://iopscience.iop.org/article/10.1088/2632-2153/adc2c7/pdf

  • Spin Seebeck Effect as a Probe for Majorana Fermions in Kitaev Spin Liquids Reviewed

    Yasuyuki Kato, Joji Nasu, Masahiro Sato, Tsuyoshi Okubo, Takahiro Misawa, Yukitoshi Motome

    Physical Review X   15 ( 1 )   011050   2025.3

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    Quantum entanglement in strongly correlated electron systems often leads to exotic elementary excitations. Quantum spin liquids provide a paradigmatic example, where the elementary excitations are described by fractional quasiparticles such as spinons. However, such fractional quasiparticles behave differently from electrons, making their experimental identification challenging. Here, we theoretically investigate the spin Seebeck effect, which is a thermoelectric response via a spin current, as an efficient probe of the fractional quasiparticles in quantum spin liquids, focusing on the Kitaev honeycomb model. By comprehensive studies using real-time dynamics, perturbation theory, and linear spin-wave theory based on the tunnel spin-current theory, we find that the spin current is induced by thermal gradient in the Kitaev spin liquid via the low-energy fractional Majorana excitations. This identification underscores the ability of Majorana fermions to carry spin current, despite lacking spin angular momentum. Furthermore, we find that the induced spin current changes its sign depending on the sign of the Kitaev interaction, indicating that the Majorana fermions contribute to the spin current with (up-) down-spin-like nature when the exchange coupling is (anti)ferromagnetic. Thus, in contrast to the negative spin current already found in a one-dimensional quantum spin liquid, our calculation reveals that the spin Seebeck effect can exhibit either positive or negative signals, contingent upon the nature of fractional excitations in the quantum spin liquids. We also clarify contrasting field-angle dependence between the Kitaev spin liquid in the low-field limit and the high-field ferromagnetic state, which is useful for the experimental identification. Our finding suggests that the spin Seebeck effect could be used not only to detect fractional quasiparticles emerging in quantum spin liquids but also to generate and control them.

    Published by the American Physical Society 2025

    DOI： 10.1103/physrevx.15.011050

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevX.15.011050/fulltext

  • Nuclear norm regularized loop optimization for tensor network Reviewed

    Kenji Homma, Tsuyoshi Okubo, Naoki Kawashima

    Physical Review Research   6 ( 4 )   043102   2024.11

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    We propose a loop optimization algorithm based on nuclear norm regularization for the tensor network. The key ingredient of this scheme is to introduce a rank penalty term proposed in the context of data processing. Compared to the standard variational periodic matrix product states method, this algorithm can circumvent the local minima related to short-ranged correlation in a simpler fashion. We demonstrate its performance when used as a part of the tensor network renormalization algorithms [S. Yang, Z.-C. Gu, and X.-G. Wen, ] for the critical two-dimensional Ising model. The scale invariance of the renormalized tensors is attained with higher accuracy while the higher parts of the scaling dimension spectrum are obtained in a more stable fashion.

    Published by the American Physical Society 2024

    DOI： 10.1103/physrevresearch.6.043102

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.6.043102/fulltext

  • Quantum phase transition between spin liquid and spin nematics in spin-1 Kitaev honeycomb model Reviewed

    Tohru Mashiko, Tsuyoshi Okubo

    Physical Review Research   6 ( 3 )   033110   2024.7

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    Besides the exactly solvable spin-1/2 Kitaev model, higher spin-S ones, not exactly solvable, are promising playgrounds for researches on the quantum spin liquid as well. One of the main interests in higher spin-S cases is the interplay between the Kitaev spin liquid (KSL) and spin nematics. We probe this interplay in a spin-1 model on the honeycomb lattice with competing bilinear-biquadratic and Kitaev interactions. Utilizing the 2D infinite projected entangled-pair state (iPEPS), we map out the phase diagram for the ferro-biquadratic interaction. In the phase diagram, we discover the direct phase transitions between the KSL phases and ferro-quadrupolar ordered spin nematic phase, which we call the FQ phase. It has been revealed that the ferro KSL exhibits robustness against perturbations from ferro-quadrupolar interactions. Also, the FQ phase is extended to the parameter region near the antiferro-Kitaev limit.

    Published by the American Physical Society 2024

    DOI： 10.1103/physrevresearch.6.033110

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.6.033110/fulltext

  • Hunting for quantum-classical crossover in condensed matter problems Reviewed

    Nobuyuki Yoshioka, Tsuyoshi Okubo, Yasunari Suzuki, Yuki Koizumi, Wataru Mizukami

    npj Quantum Information   10 ( 1 )   2024.4

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    Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

    Abstract

    The intensive pursuit for quantum advantage in terms of computational complexity has further led to a modernized crucial question of when and how will quantum computers outperform classical computers. The next milestone is undoubtedly the realization of quantum acceleration in practical problems. Here we provide a clear evidence and arguments that the primary target is likely to be condensed matter physics. Our primary contributions are summarized as follows: 1) Proposal of systematic error/runtime analysis on state-of-the-art classical algorithm based on tensor networks; 2) Dedicated and high-resolution analysis on quantum resource performed at the level of executable logical instructions; 3) Clarification of quantum-classical crosspoint for ground-state simulation to be within runtime of hours using only a few hundreds of thousand physical qubits for 2d Heisenberg and 2d Fermi-Hubbard models, assuming that logical qubits are encoded via the surface code with the physical error rate of p = 10<sup>−3</sup>. To our knowledge, we argue that condensed matter problems offer the earliest platform for demonstration of practical quantum advantage that is order-of-magnitude more feasible than ever known candidates, in terms of both qubit counts and total runtime.

    DOI： 10.1038/s41534-024-00839-4

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    Other Link： https://www.nature.com/articles/s41534-024-00839-4

  • Quantum spin state stabilized by coupling with classical spins Reviewed

    H. Yamaguchi, T. Okubo, A. Matsuo, T. Kawakami, Y. Iwasaki, T. Takahashi, Y. Hosokoshi, K. Kindo

    Physical Review B   109 ( 10 )   2024.3

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    Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/physrevb.109.l100404

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.109.L100404/fulltext

  • Possibility of a Topological Phase Transition in Two-dimensional <i>RP</i><sup>3</sup> Model Reviewed

    Tsuyoshi Okubo, Naoki Kawashima

    Journal of the Physical Society of Japan   92 ( 11 )   114701-1 - 114701-7   2023.11

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    Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

    DOI： 10.7566/jpsj.92.114701

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  • TeNeS: Tensor network solver for quantum lattice systems Reviewed

    Yuichi Motoyama, Tsuyoshi Okubo, Kazuyoshi Yoshimi, Satoshi Morita, Takeo Kato, Naoki Kawashima

    Computer Physics Communications   279   108437 - 108437   2022.10

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1016/j.cpc.2022.108437

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  • Optimized implementation for calculation and fast-update of Pfaffians installed to the open-source fermionic variational solver mVMC Reviewed

    RuQing G. Xu, Tsuyoshi Okubo, Synge Todo, Masatoshi Imada

    Computer Physics Communications   277   108375 - 108375   2022.8

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1016/j.cpc.2022.108375

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  • Bond-weighted tensor renormalization group Reviewed

    Daiki Adachi, Tsuyoshi Okubo, Synge Todo

    Physical Review B   105 ( 6 )   L060402-1 - L060402-6   2022.2

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/physrevb.105.l060402

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.105.L060402/fulltext

  • Phase shift in skyrmion crystals Reviewed

    Satoru Hayami, Tsuyoshi Okubo, Yukitoshi Motome

    Nature Communications   12 ( 1 )   6927   2021.12

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    Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

    DOI： 10.1038/s41467-021-27083-0

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    Other Link： https://www.nature.com/articles/s41467-021-27083-0

  • Gapped ground state in a spin-12 frustrated square lattice Reviewed

    H. Yamaguchi, N. Uemoto, T. Okubo, Y. Kono, S. Kittaka, T. Sakakibara, T. Yajima, S. Shimono, Y. Iwasaki, Y. Hosokoshi

    Physical Review B   104 ( 6 )   L060411   2021.8

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    Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/physrevb.104.l060411

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.104.L060411/fulltext

  • Quantum critical phenomena in a spin-12 frustrated square lattice with spatial anisotropy Reviewed

    H. Yamaguchi, Y. Iwasaki, Y. Kono, T. Okubo, S. Miyamoto, Y. Hosokoshi, A. Matsuo, T. Sakakibara, T. Kida, M. Hagiwara

    Physical Review B   103 ( 22 )   L220407   2021.6

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    Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/physrevb.103.l220407

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.103.L220407/fulltext

  • Anisotropic tensor renormalization group Reviewed

    Daiki Adachi, Tsuyoshi Okubo, Synge Todo

    Physical Review B   102 ( 5 )   054432   2020.8

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    Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/physrevb.102.054432

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.102.054432/fulltext

  • Zero-energy excitation in the classical kagome antiferromagnet NaBa2Mn3F11 Reviewed

    Shohei Hayashida, Hajime Ishikawa, Yoshihiko Okamoto, Tsuyoshi Okubo, Zenji Hiroi, Gøran J. Nilsen, Hannu Mutka, Takatsugu Masuda

    Physical Review B   101 ( 21 )   1 - 6   2020.6

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/physrevb.101.214409

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.101.214409/fulltext

  • Magnetic field induced quantum phases in a tensor network study of Kitaev magnets Reviewed International coauthorship

    Hyun-Yong Lee, Ryui Kaneko, Li Ern Chern, Tsuyoshi Okubo, Youhei Yamaji, Naoki Kawashima, Yong Baek Kim

    Nature Communications   11 ( 1 )   1639   2020.4

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

    DOI： 10.1038/s41467-020-15320-x

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    Other Link： http://www.nature.com/articles/s41467-020-15320-x

  • Abelian and non-Abelian chiral spin liquids in a compact tensor network representation Reviewed

    Hyun-Yong Lee, Ryui Kaneko, Tsuyoshi Okubo, Naoki Kawashima

    Physical Review B   101 ( 3 )   035140-1   2020.1

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/physrevb.101.035140

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.101.035140/fulltext

  • Multiple- q states of the J1−J2 classical honeycomb-lattice Heisenberg antiferromagnet under a magnetic field Reviewed

    Tokuro Shimokawa, Tsuyoshi Okubo, Hikaru Kawamura

    Physical Review B   100 ( 22 )   224404   2019.12

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

    DOI： 10.1103/physrevb.100.224404

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    Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.100.224404/fulltext

  • Gapless Kitaev Spin Liquid to Classical String Gas through Tensor Networks Reviewed

    Hyun-Yong Lee, Ryui Kaneko, Tsuyoshi Okubo, Naoki Kawashima

    Physical Review Letters   123 ( 8 )   087203   2019.8

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/PhysRevLett.123.087203

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  • A series of magnon crystals appearing under ultrahigh magnetic fields in a kagomé antiferromagnet Reviewed

    R. Okuma, D. Nakamura, T. Okubo, A. Miyake, A. Matsuo, K. Kindo, M. Tokunaga, N. Kawashima, S. Takeyama, Z. Hiroi

    Nature Communications   10 ( 1 )   1229   2019.3

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1038/s41467-019-09063-7

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  • Field-enhanced quantum fluctuation in an S=1/2 frustrated square lattice Reviewed

    H. Yamaguchi, Y. Sasaki, T. Okubo, M. Yoshida, T. Kida, M. Hagiwara, Y. Kono, S. Kittaka, T. Sakakibara, M. Takigawa, Y. Iwasaki, Y. Hosokoshi

    Physical Review B   98   094402   2018.9

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/PhysRevB.98.094402

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  • Magnetic state selected by magnetic dipole interaction in the kagome antiferromagnet NaBa2 Mn3 F11 Reviewed

    Shohei Hayashida, Hajime Ishikawa, Yoshihiko Okamoto, Tsuyoshi Okubo, Zenji Hiroi, Maxim Avdeev, Pascal Manuel, Masato Hagihala, Minoru Soda, Takatsugu Masuda

    Physical Review B   97 ( 5 )   054411   2018.2

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    Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

    DOI： 10.1103/PhysRevB.97.054411

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  • Quantum phase transitions driven by rhombic-type single-ion anisotropy in the S=1 Haldane chain Reviewed

    Yu-Chin Tzeng, Hiroaki Onishi, Tsuyoshi Okubo, Ying-Jer Kao

    PHYSICAL REVIEW B   96 ( 6 )   060404(R)   2017.8

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/PhysRevB.96.060404

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  • Weak ferromagnetic order breaking the threefold rotational symmetry of the underlying kagome lattice in CdCu3(OH)(6)(NO3)(2) center dot H2O Reviewed

    Ryutaro Okuma, Takeshi Yajima, Daisuke Nishio-Hamane, Tsuyoshi Okubo, Zenji Hiroi

    PHYSICAL REVIEW B   95 ( 9 )   095427   2017.3

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/PhysRevB.95.094427

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  • Massively parallel Monte Carlo simulation of a possible topological phase transition in two-dimensional frustrated spin systems Invited

    Tsuyoshi Okubo

    ISSP Supercomputer Activity Report 2016   2017

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  • Ground state properties of $\mathrm{Na_2IrO_3}$ determined from $\textit{ab initio}$ Hamiltonian and its extensions containing Kitaev and extended Heisenberg interactions Reviewed

    Tsuyoshi Okubo, Kazuya Shinjo, Youhei Yamaji, Naoki Kawashima, Shigetoshi Sota, Takami Tohyama, Masatoshi Imada

    Physical Review B   96   054434   2016.11

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    Language：English   Publishing type：Research paper (scientific journal)  

    We investigate the ground state properties of $\mathrm{Na_2IrO_3}$ based on
    numerical calculations of the recently proposed {\it ab initio} Hamiltonian
    represented by Kitaev and extended Heisenberg interactions. To overcome the
    limitation posed by small tractable system sizes in the exact diagonalization
    study employed in a previous study (Yamaji $\textit{et al.}$, Phys. Rev. Lett.
    $\textbf{113}$, 107201 (2014)), we apply two-dimensional density matrix
    renomalization group, and infinite-size tensor-network method. By calculating
    at much larger system sizes, we critically test the validity of the exact
    diagonalization results. The results consistently indicate that the ground
    state of $\mathrm{Na_2IrO_3}$ is a magnetically ordered state with zigzag
    configuration in agreement with experimental observations and the previous
    diagonalization study. Applications of the two independent methods in addition
    to the exact diagonalization study further uncover a consistent and rich phase
    diagram near the zigzag phase beyond the accessibility of the exact
    diagonalization. For example, in the parameter space away from the $\textit{ab
    initio}$ value of $\mathrm{Na_2IrO_3}$ controlled by the trigonal distortion,
    we find three phases: (i) an ordered phase with the magnetic moment aligned
    mutually in $120$ degrees orientation on every third hexagon, (ii) a
    magnetically ordered phase with a $16$-site unit-cell, and (iii) an ordered
    phase with presumably incommensurate periodicity of the moment. It suggests
    that potentially rich magnetic structures may appear in
    $A_\mathrm{2}\mathrm{IrO_3}$ compounds for $A$ other than Na. The present
    results also serve to establish the accuracy of the first-principles approach
    in reproducing the available experimental results thereby further contribute to
    find a route to realize the Kitaev spin liquid.

    DOI： 10.1103/PhysRevB.96.054434

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  • Experimental Realization of a Quantum Pentagonal Lattice Reviewed

    Hironori Yamaguchi, Tsuyoshi Okubo, Shunichiro Kittaka, Toshiro Sakakibara, Koji Araki, Kenji Iwase, Naoki Amaya, Toshio Ono, Yuko Hosokoshi

    SCIENTIFIC REPORTS   5   15327   2015.10

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1038/srep15327

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  • SU(N) Heisenberg model with multicolumn representations Reviewed

    Tsuyoshi Okubo, Kenji Harada, Jie Lou, Naoki Kawashima

    PHYSICAL REVIEW B   92 ( 13 )   134404   2015.10

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/PhysRevB.92.134404

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  • Scaling relation for dangerously irrelevant symmetry-breaking fields Reviewed

    Tsuyoshi Okubo, Kosei Oshikawa, Hiroshi Watanabe, Naoki Kawashima

    PHYSICAL REVIEW B   91 ( 17 )   174417   2015.5

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/PhysRevB.91.174417

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  • Snapshot Spectrum and Critical Phenomenon for Two-Dimensional Classical Spin Systems Reviewed

    Yukinari Imura, Tsuyoshi Okubo, Satoshi Morita, Kouichi Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   83 ( 11 )   114002   2014.11

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    Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.7566/JPSJ.83.114002

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  • Field-induced incommensurate phase in the strong-rung spin ladder with ferromagnetic legs Reviewed

    H. Yamaguchi, H. Miyagai, M. Yoshida, M. Takigawa, K. Iwase, T. Ono, N. Kase, K. Araki, S. Kittaka, T. Sakakibara, T. Shimokawa, T. Okubo, K. Okunishi, A. Matsuo, Y. Hosokoshi

    PHYSICAL REVIEW B   89 ( 22 )   220402   2014.6

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    DOI： 10.1103/PhysRevB.89.220402

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  • Kagome-Triangular Lattice Antiferromagnet NaBa2Mn3F11 Reviewed

    Hajime Ishikawa, Tsuyoshi Okubo, Yoshihiko Okamoto, Zenji Hiroi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   83 ( 4 )   043703   2014.4

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    DOI： 10.7566/JPSJ.83.043703

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  • Possibility of deconfined criticality in SU(N) Heisenberg models at small N Reviewed

    Kenji Harada, Takafumi Suzuki, Tsuyoshi Okubo, Haruhiko Matsuo, Jie Lou, Hiroshi Watanabe, Synge Todo, Naoki Kawashima

    PHYSICAL REVIEW B   88 ( 22 )   220408(R)   2013.12

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    DOI： 10.1103/PhysRevB.88.220408

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  • Various regimes of quantum behavior in an S=1/2 Heisenberg antiferromagnetic chain with fourfold periodicity Reviewed

    H. Yamaguchi, T. Okubo, K. Iwase, T. Ono, Y. Kono, S. Kittaka, T. Sakakibara, A. Matsuo, K. Kindo, Y. Hosokoshi

    PHYSICAL REVIEW B   88 ( 17 )   174410   2013.11

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    DOI： 10.1103/PhysRevB.88.174410

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  • Multiple-q states and skyrmion lattice in frustrated triangular-lattice Heisenberg antiferromagnet Invited

    Tsuyoshi Okubo, Hikaru Kawamura

    ISSP Supercomputer Activity Report 2012   2013

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  • Finite-Temperature Transition of the Antiferromagnetic Heisenberg Model on a Distorted Kagome Lattice Reviewed

    Hiroshi Masuda, Tsuyoshi Okubo, Hikaru Kawamura

    PHYSICAL REVIEW LETTERS   109 ( 5 )   057201   2012.8

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    DOI： 10.1103/PhysRevLett.109.057201

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  • Multiple-q States and the Skyrmion Lattice of the Triangular-Lattice Heisenberg Antiferromagnet under Magnetic Fields Reviewed

    Tsuyoshi Okubo, Sungki Chung, Hikaru Kawamura

    PHYSICAL REVIEW LETTERS   108 ( 1 )   017206   2012.1

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    DOI： 10.1103/PhysRevLett.108.017206

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  • Cubic and noncubic multiple-q states in the Heisenberg antiferromagnet on the pyrochlore lattice Reviewed

    Tsuyoshi Okubo, Trung Hai Nguyen, Hikaru Kawamura

    PHYSICAL REVIEW B   84 ( 14 )   144432   2011.10

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    DOI： 10.1103/PhysRevB.84.144432

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  • Novel Spin-Liquid States in the Frustrated Heisenberg Antiferromagnet on the Honeycomb Lattice Reviewed

    Soichiro Okumura, Hikaru Kawamura, Tsuyoshi Okubo, Yukitoshi Motome

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   79 ( 11 )   114705 - 114705   2010.11

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    DOI： 10.1143/JPSJ.79.114705

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  • Signature of a Z(2) Vortex in the Dynamical Correlations of the Triangular-Lattice Heisenberg Antiferromagnet Reviewed

    Tsuyoshi Okubo, Hikaru Kawamura

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   79 ( 8 )   084706   2010.8

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    DOI： 10.1143/JPSJ.79.084706

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  • Phase transition of the three-dimensional chiral Ginzburg-Landau model: Search for the chiral phase Reviewed

    Tsuyoshi Okubo, Hikaru Kawamura

    PHYSICAL REVIEW B   82 ( 1 )   014404   2010.7

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    DOI： 10.1103/PhysRevB.82.014404

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  • Z(2)-Vortex Ordering of the Triangular-Lattice Heisenberg Antiferromagnet Reviewed

    Hikaru Kawamura, Atsushi Yamamoto, Tsuyoshi Okubo

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   79 ( 2 )   023701   2010.2

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    DOI： 10.1143/JPSJ.79.023701

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  • Mean-field analysis of phase transitions in the emergence of hierarchical society Reviewed

    Tsuyoshi Okubo, Takashi Odagaki

    PHYSICAL REVIEW E   76 ( 3 )   036105   2007.9

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    DOI： 10.1103/PhysRevE.76.036105

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  • Dissipative process under a boundary perturbation Reviewed

    Tsuyoshi Okubo, Takashi Odagaki

    Physical Review E   73 ( 2 )   026128   2006

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    Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

    DOI： 10.1103/PhysRevE.73.026128

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  • Packing and Percolation of poly-disperse discs and spheres

    Takashi Odagaki, Tsuyoshi Okubo, Ryusei Ogata, Keiji Okazaki

    The Proceedings of Second COE Workshop on Sphere Packings   187 - 197   2005

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  • Random packing of binary hard discs Reviewed

    T Okubo, T Odagaki

    JOURNAL OF PHYSICS-CONDENSED MATTER   16 ( 37 )   6651 - 6659   2004.9

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    DOI： 10.1088/0953-8984/16/37/002

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Books

  • Advanced Mathematical Science for Mobility Society

    （ Role： Contributor）

    Springer-Nature New York Inc  2024.4  （ ISBN:9819997747 ）

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    Total pages：215  

    ASIN

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  • 現代の物性物理学 (物理学叢書111)

    小田垣 孝, 吉留 崇, 大久保 毅（ Role： Joint translator）

    吉岡書店  2021.9  （ ISBN:4842703733 ）

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    Total pages：612  

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MISC

  • 物性物理における「量子加速」の探索 Invited

    吉岡 信行, 大久保 毅, 鈴木 泰成, 水上 渉

    応用物理   93 ( 3 )   165 - 168   2024.3

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    DOI： 10.11470/oubutsu.93.3_165

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  • 拡張キタエフ模型における熱ホール伝導度のテンソルネットワーク法による研究

    大久保毅

    固体物理   57 ( 11 )   633 - 642   2022.11

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  • 量子多体系とテンソルネットワーク

    大久保毅

    数理科学2022年12月号   58 - 64   2022.11

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  • テンソルネットワークの将来

    大久保毅

    数理科学2022年2月号   65 - 72   2022.2

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  • テンソルネットワーク形式の進展と応用

    西野友年, 大久保毅

    日本物理学会誌   72 ( 10 )   2017.10

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  • フラストレート磁性体における多重Q秩序とスカーミオン格子状態

    大久保毅, 川村光

    日本物理学会誌   67 ( 7 )   2012.7

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Presentations

  • Tensor network approaches for condensed matter physics Invited

    Tsuyoshi Okubo

    16th Symposium on Discovery, Fusion, Creation of New Knowledge by Multidisciplinary Computational Sciences  2024.10 

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    Event date： 2024.10

    Language：English   Presentation type：Oral presentation (invited, special)  

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  • テンソルネットネットワーク法による二次元量子格子模型の解析 Invited

    大久保毅

    物性研究所スパコン共同利用・CCMS合同研究会「計算物質科学の現在と未来」  2024.4 

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    Event date： 2024.4

    Language：Japanese   Presentation type：Oral presentation (invited, special)  

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  • Tensor-Network-Based Approach to Kitaev Spin Liquid Invited

    Tsuyoshi Okubo

    Annual Meeting of the Physical Society of Taiwan 2024  2024.1 

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    Event date： 2024.1

    Language：English   Presentation type：Oral presentation (invited, special)  

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  • Quantum-classical entangled approach using tensor networks for investigating quantum spin liquid Invited

    Tsuyoshi Okubo

    Mini-workshop: Tensor Network algorithms and applications 2024  2024.1 

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    Event date： 2024.1

    Language：English   Presentation type：Oral presentation (invited, special)  

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  • テンソルネットワーク状態による二次元フラストレート磁性体の解析と有限温度への拡張 Invited

    大久保毅

    Tensor Network 2023  2023.11 

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    Event date： 2023.11

    Language：Japanese   Presentation type：Oral presentation (invited, special)  

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  • テンソルネットワークと量子計算 Invited

    大久保毅

    場の理論の新しい計算方法2023  2023.10 

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    Event date： 2023.10

    Language：Japanese   Presentation type：Oral presentation (invited, special)  

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  • テンソルネットワークによる量子古典エンタングルアルゴリズムの提案 Invited

    大久保毅

    物性研究のための量子アルゴリズム最前線  2023.3 

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    Event date： 2023.3

    Language：Japanese   Presentation type：Oral presentation (invited, special)  

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  • Tensor network representation of Kitaev spin liquid and its application to quantum circuit-based variation calculations Invited

    Tsuyoshi Okubo

    Novel Quantum States in Condensed Matter 2022 (NQS2022)  2022.11 

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    Event date： 2022.10 - 2022.12

    Language：English   Presentation type：Oral presentation (invited, special)  

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  • Tensor network approach to Kitaev spin liquid Invited

    Tsuyoshi Okubo

    The 15th Asia Pacific Physics Conference  2022.8 

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    Event date： 2022.8

    Language：English   Presentation type：Oral presentation (invited, special)  

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  • テンソルネットワーク法の量子多体問題への適用と量子計算機への展開 Invited

    大久保毅

    量子多体計算のフロンティア  2022.3 

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    Event date： 2022.3

    Language：Japanese   Presentation type：Oral presentation (invited, special)  

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  • 量子物理とテンソルネットワーク Invited

    大久保毅

    第1回量子ソフトウェアワークショップ：テンソルネットワークと量子計算の展望  2021.12 

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    Event date： 2021.12

    Language：Japanese   Presentation type：Oral presentation (invited, special)  

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  • 拡張キタエフ模型における熱ホール伝導度の数値的研究 Invited

    大久保毅

    キタエフ量子スピン液体研究の新展開  2021.8 

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    Event date： 2021.8

    Presentation type：Oral presentation (invited, special)  

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  • Large spin fluctuation in the magnetization process of frustrated square lattice Heisenberg magnets Invited

    Tsuyoshi Okubo

    The International Workshop on Quantum Magnets in Extreme Conditions  2021.3 

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    Event date： 2021.3

    Language：English   Presentation type：Oral presentation (invited, special)  

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  • Tensor Network study of honeycomb lattice Kitaev model Invited

    Tsuyoshi Okubo

    Recent progress in theoretical physics based on quantum information theory  2021.3 

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    Event date： 2021.3

    Language：English   Presentation type：Oral presentation (invited, special)  

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  • Finite-temperature tensor-network methods for quantum spin systems and their application to extended Kitaev model Invited

    Tsuyoshi Okubo

    APS Global Physics Summit 2026  2026.3 

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  • Applications of Tensor Networks to Finite-Temperature Quantum States and Machine Learning Invited

    Tsuyoshi Okubo

    Current and Future Computational Approaches to Quantum Many-Body Systems 2026 (CompQMB2026)  2026.3 

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  • Quantum computing and tensor networks: Applications to quantum many-body problems and machine learning Invited

    Tsuyoshi Okubo

    Bridging Realities: Quantum Computing Meets Artificial Intelligence  2025.12 

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  • Thermal Hall transport in Kitaev and Kagome-lattice spin systems Invited

    Tsuyoshi Okubo

    Tensor Network States: Algorithms and Applications (TNSAA) 2025  2025.12 

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  • 今後の計算物質科学におけるソフトウェア開発のあり方について Invited

    大久保毅

    物性研究所ソフトウェア開発・高度化プロジェクト研究会 〜計算物質科学の発展を支えるオープンソースソフトウェアの開発と普及  2025.10 

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    Language：Japanese   Presentation type：Symposium, workshop panel (nominated)  

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  • テンソルネットワーク法による量子スピン模型解析ソフトウェアの開発 Invited

    大久保毅

    物性研究所ソフトウェア開発・高度化プロジェクト研究会 〜計算物質科学の発展を支えるオープンソースソフトウェアの開発と普及  2025.10 

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  • テンソルネットワーク法を用いた磁性体の研究 Invited

    大久保毅

    磁気記録・情報ストレージ研究会（MRIS）  2025.10 

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  • Quantum-classical hybrid approach via tensor networks Invited

    Tsuyoshi Okubo

    日本物理学会第80回年次大会シンポジウム「Computational materials science based on quantum-classical hybrid algorithms」  2025.9 

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  • Generative Models Using Tensor Networks and Their Embedding into Quantum Circuits Invited

    Tsuyoshi Okubo

    Q2B25 Toky  2025.5 

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  • テンソルネットワーク法による二次元量子スピン系の有限温度物性の解析 Invited

    大久保毅

    物性研究所スパコン共同利用・CCMS合同研究会「機械学習と計算物性科学の未来」  2025.4 

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  • 量子技術の応用:量子多体問題と機械学習への展開 Invited

    大久保毅

    第三回 Agora 講演会 ―現在の量子技術とその先―  2025.3 

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  • Tensor Network approaches to quantum spin liquid Invited

    Tsuyoshi Okubo

    The 38th Annual CSP Workshop  2025.2 

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  • Application of Tensor Network States and Their Embeddings to Quantum Circuit Invited

    Tsuyoshi Okubo

    Korea-US-Japan Quantum International Collaborative Research Workshop 2025  2025.2 

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  • Tensor Network Methods for Quantum Many-Body Problems Invited

    Tsuyoshi Okubo

    International Workshop on Massively Parallel Programming for Quantum Chemistry and Physics (MPQCP 2025)  2025.1 

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  • Recent progress in tensor network approach for condensed matter physics Invited

    Tsuyoshi Okubo

    KEK-THEORY Workshop 2024  2024.12 

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  • Tensor network Approaches to S=1/2 and S=1 Kitaev spin liquids Invited

    Tsuyoshi Okubo

    Correlated Quantum Materials + beyond  2024.11 

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  • Tensor Network and Quantum Computing Invited

    Tsuyoshi Okubo

    Q2B24 Tokyo  2024.7 

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  • Tensor network approach to two-dimensional frustrated spin system Invited

    Tsuyoshi Okubo

    APCTP-IACS-SNBNCBS Workshop on Computational Methods for Emergent Quantum Matter: From Theoretical Concepts to Experimental Realization,  2022.11 

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    Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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  • テンソルネットワーク法による量子スピン系の基底状態計算 Invited

    大久保毅

    物性研究所スパコン共同利用・CCMS 合同研究会「計算物質科学の新展開 2020」  2020.12 

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  • Tensor network study on extended Kitaev models Invited International conference

    Tsuyoshi Okubo

    The 33rd Annual CSP Workshop  2020.2 

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  • Anisotropic tensor renormalization group and BTRG Invited International conference

    Tsuyoshi Okubo

    Workshop on Tensor Network States: Algorithms and Applications  2019.12 

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  • Tensor network approach to frustrated spin systems Invited International conference

    Tsuyoshi Okubo

    Computational Approaches to Quantum Many-body Problems  2019.8 

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  • Large spin fluctuation in the magnetization process of frustrated square lattice Heisenberg model Invited International conference

    Tsuyoshi Okubo

    Computational Approaches to Quantum Many-body Problems  2019.8 

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  • Tensor network study on Kitaev materials: Search for Kitaev spin liquid Invited International conference

    Tsuyoshi Okubo

    The 2nd Asia Pacific Workshop on Quantum Magnetism (APFM2018)  2018.12 

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  • フラストレートした正方格子ハイゼンベルグ模型における大きなスピン揺らぎと1/2磁化プラトー Invited

    大久保毅

    スピン系物理の最前線  2018.11 

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  • Tensor network quantum states and their application to condensed matter physics Invited International conference

    Tsuyoshi Okubo

    Interdisciplinary Workshop on Tensor Network  2018.10 

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  • Tensor network study on Kitaev material Invited International conference

    Tsuyoshi Okubo

    International Conference on Magnetism (ICM) 2018  2018.7 

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  • 並列モンテカルロ法によるフラストレート磁性体におけるトポロジカル転移の探求 Invited

    大久保毅, 川島直輝

    物性研究所スパコン共同利用・CCMS合同研究会「計算物質科学の今と未来」  2018.4 

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  • Tensor network study on Kitaev materials Invited International conference

    Tsuyoshi Okubo

    Novel Quantum States in Condensed Matter 2017  2017.11 

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  • テンソルネットワーク法によるS=1/2カゴメ格子模型の磁化過程の研究 Invited

    大久保毅

    Conference on Unusual Physics in Kagome Lattice  2017.3 

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  • Magnetization process of kagome lattice Heisenberg antiferromagnets: 1/3 plateau state and effects of Dzyaloshinskii-Moriya interaction Invited International conference

    Tsuyoshi Okubo, Naoki Kawashima

    International Workshop on Tensor Networks and Quantum Many-Body Problems (TNQMP2016)  2016.7 

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  • Tensor network study on magnetization process of the kagome lattice Heisenberg antiferromagnets Invited International conference

    Tsuyoshi Okubo, Naoki Kawashima

    Trends in Theory of Correlated Materials 2016 (TTCM2016)  2016.5 

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  • Ground state properties of frustrated honeycomb and kagome lattice antiferromagnets Invited International conference

    Tsuyoshi Okubo, Naoki Kawashima

    The 75th Okazaki Conference Tensor Network States: Algorithms and Applications 2016  2016.1 

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  • フラストレーションが生み出す磁性体の新奇秩序 Invited

    大久保毅

    ナノ構造・物性-ナノ機能・応用部会合同シンポジウム「若い力が切り拓くナノサイエンス・テクノロジー」  2014.12 

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  • PESPテンソルネットワークを用いた量子スピン模型の基底状態計算 Invited

    大久保毅

    量子多体系の新しい潮流ーテンソルネットワーク・繰り込み群・エンタングルメントー  2014.12 

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  • Ground state calculation of the generalized Kitaev-Heisenberg model using PEPS tensor network method Invited International conference

    Tsuyoshi Okubo, Naoki Kawashima

    CMSI International Workshop 2014: Tensor Network Algorithms in Materials Science  2014.10 

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  • Tensor network renormalization study of the distorted Kitaev-Heisenberg model Invited International conference

    Tsuyoshi Okubo, Naoki Kawashima

    CMSI Kobe International Satellite Meeting 2013:Recent Progress in Tensor Network Algorithms  2013.10 

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  • Multiple-Q states and the skyrmion lattice of the triangular-lattice Heisenberg antiferromagnet under magnetic fields Invited International conference

    Tsuyoshi Okubbo, Chung Songki, Hikaru Kawamura

    Trends in Theory of Correlated Materials 2013  2013.10 

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  • Ordering and Dynamics of Topological Excitation in Frustrated Magnets Invited International conference

    Tsuyoshi Okubo, Sungki Chung, Hikaru Kawamura

    26th Annual CSP Workshop  2013.2 

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Awards

  • 第８回日本物理学会若手奨励賞

    2014.3   フラストレート磁性体におけるトポロジカル相転移と多重Q秩序の研究

    大久保毅

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Research Projects

  • Computational materials science based on quantum-classical hybrid algorithms

    Grant number：23H03816

    2023.4 - 2026.3

    System name：Grants-in-Aid for Scientific Research

    Research category：Grant-in-Aid for Transformative Research Areas (B)

    Awarding organization：Japan Society for the Promotion of Science

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    Authorship：Coinvestigator(s) 

    Grant amount：\32240000 （ Direct Cost: \24800000 、 Indirect Cost：\7440000 ）

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  • 波動関数理論と量子アルゴリズムの融合

    Grant number：23H03818

    2023.4 - 2026.3

    System name：科学研究費助成事業 学術変革領域研究(B)

    Research category：学術変革領域研究(B)

    Awarding organization：日本学術振興会

    大久保 毅, 三澤 貴宏, 山地 洋平, 森田 悟史, 井戸 康太

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    Authorship：Principal investigator 

    Grant amount：\36790000 （ Direct Cost: \28300000 、 Indirect Cost：\8490000 ）

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  • テンソルネットワーク形式に基づく古典・量子エンタングルアルゴリズムの開発

    Grant number：22K18682

    2022.6 - 2025.3

    System name：科学研究費助成事業 挑戦的研究(萌芽)

    Research category：挑戦的研究(萌芽)

    Awarding organization：日本学術振興会

    大久保 毅

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    Authorship：Principal investigator 

    Grant amount：\6370000 （ Direct Cost: \4900000 、 Indirect Cost：\1470000 ）

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  • テンソルネットワーク法の展開によるフラストレート磁性体の新奇物性探究

    Grant number：23K22450

    2022.4 - 2025.3

    System name：科学研究費助成事業

    Research category：基盤研究(B)

    Awarding organization：日本学術振興会

    大久保 毅

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    Grant amount：\17030000 （ Direct Cost: \13100000 、 Indirect Cost：\3930000 ）

    本年度の研究では、テンソルネットワーク法に基づくフラストレート磁性体の解析手法として、特に、有限温度状態、励起状態の計算手法の確立を目指し、(i)有限温度計算の高精度化と(ii)励起状態計算法の開発を行った。
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    (i)有限温度計算の高精度化においては、有限温度状態を記述する密度演算子をテンソルネットワークで表現するアプローチを検討した。密度演算子のテンソルネットワーク表現では、密度演算子を純粋状態とみなすpurificationに基づいたテンソルネットワーク表現と、密度行列を演算子として取り扱うテンソルネットワーク表現の双方を比較し、現状の最適化手法では、後者の方が低温状態をよく表現できることを確認した。また、後者の表現において、無限に大きな系での最適化を検討し、直接結合した最近接の二つのテンソルだけを考慮した最適化ではなく、複数テンソルのクラスターを用いた最適化により低温領域の計算精度が上がることも再確認できた。加えて、対象とするフラストレート磁性体に合わせて、密度行列を近似するテンソルネットワークの構造を変えることでも、虚時間発展の近似精度が向上することも明らかとなった。
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    (ii)励起状態計算法の開発では、基底状態に不純物テンソルを挿入する励起状態計算方法の指針を検討したが、与えられたテンソルネットワーク表現を最適化する手法について、複数の可能性を比較している段階に留まっており、さらなる検討と実問題での検証が必要である。

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  • テンソルネットワーク法の展開によるフラストレート磁性体の新奇物性探究

    Grant number：22H01179

    2022.4 - 2025.3

    System name：科学研究費助成事業 基盤研究(B)

    Research category：基盤研究(B)

    Awarding organization：日本学術振興会

    大久保 毅

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    Authorship：Principal investigator 

    Grant amount：\17030000 （ Direct Cost: \13100000 、 Indirect Cost：\3930000 ）

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  • 量子スピン液体におけるマヨラナ粒子の創出と制御

    Grant number：20H00122

    2020.4 - 2025.3

    System name：科学研究費助成事業 基盤研究(A)

    Research category：基盤研究(A)

    Awarding organization：日本学術振興会

    求 幸年, 大久保 毅, 那須 譲治, 加藤 康之, 三澤 貴宏

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    Authorship：Coinvestigator(s) 

    Grant amount：\44590000 （ Direct Cost: \34300000 、 Indirect Cost：\10290000 ）

    研究計画に沿って研究を推進し、以下の研究成果を得た。(0) 計算機環境の構築：本研究計画をスムーズに遂行するために、新しいPCクラスタシステム一式及び計算サーバーを導入し、代表者と分担者で共有して使用できる計算機環境を構築した。(1) キタエフ量子スピン液体が実現する新たな舞台を探索する目的で、高スピン系や冷却原子系を念頭に置いた数値計算を行い、キタエフ量子スピン液体が実現するパラメタ領域を明らかにした。(2) 乱れのあるキタエフ模型における熱力学量や熱ホール伝導度の計算を行い、サイト欠陥とボンド乱れに対して対照的な振る舞いが現れるを明らかにした。(3) キタエフ量子スピン液体においてスピンの分数化によって現れるフラックスを自在に生成し制御する手法として、局所的な格子歪みの効果を調べ、実際に生成・移動・交換等の操作が可能なことを示した。(4) キタエフ量子スピン液体に対する交代磁場の効果を調べる目的で、マヨラナのバンド構造や熱伝導係数を調べた。(5) キタエフ量子スピン液体へのスピン流注入に関する研究に着手し、計算のフレームワークの整備と予備的な数値計算を行った。(6) 磁場中で現れる半整数量子化熱ホール効果に関して、非キタエフ相互作用、磁場、温度の効果を網羅的に調べるため、計算のフレームワークの整備と予備的な数値計算を行った。(7) キタエフ候補物質α-RuCl3が磁場中で示すトポロジカル相転移に関して、実験グループとの共同研究をおこなった。

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  • テンソルネットワーク状態を活用した量子多体系基底状態計算手法の開発

    2019.10 - 2023.3

    System name：さきがけ

    Awarding organization：科学技術振興機構

    大久保毅

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    Authorship：Principal investigator  Grant type：Competitive

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  • テンソルネットワーク変分法による有限温度フラストレート磁性体の物性解明

    Grant number：19K03740

    2019.4 - 2022.3

    System name：科学研究費助成事業 基盤研究(C)

    Research category：基盤研究(C)

    Awarding organization：日本学術振興会

    大久保 毅

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    Authorship：Principal investigator 

    Grant amount：\4290000 （ Direct Cost: \3300000 、 Indirect Cost：\990000 ）

    昨年度の研究により、密度行列のテンソルネットワーク表示としては、purificaiton を用いない直接表現の方が優れていることが明らかとなった。一方で、キタエフスピン液体などの興味深い基底状態を持つ系では、低温での計算精度が不十分であることも徐々に明らかとなっていた。この状況を踏まえ、本年度は、計算精度の向上を目指した二つの試みを行った。
    まず、無限に大きな二次元量子スピン系において、従来法で計算精度が低くなる原因を検証したところ、Suzuki-Trotter分解した虚時間発展演算子をかけた後のテンソル最適化が不十分であることが一つの問題であることが判明した。この点を解決するため、従来の２サイトだけに注目した最適化範囲を拡張し、複数サイトで最適化を行うクラスター最適化法を考案した。このクラスター最適化法を基底状態が量子スピン液体になっているキタエフ模型に適用したところ、低温比熱の計算精度が大幅に向上することが明らかとなった。クラスター最適化法は従来法に比べて計算量の増加もそれほど多くないため、今後、より大きなテンソルを用いた計算により、無限に大きな二次元量子スピン系の有限温度計算が高精度に行える目処がたった。
    第二に、最適化の問題を大幅に改善するために、無限に広がった二次元ではなく、有限サイズの二次元系を考え、その密度行列を一次元のテンソル積状態（行列積状態）で近似する手法を検討した。行列積状態は、カノニカル形式と呼ばれる、二次元以上のテンソル積状態にはない、規格化された表現が存在しており、高精度のテンソル最適化が可能であることが知られている。この点に着目し、磁場中のキタエフ模型で種々の物性について精度検証を行った結果、課題だった比熱を含め、実験的に注目されている熱伝導度などの複雑な物理量も高精度で計算可能であることが明らかになった。

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  • Study on frustrated quantum spin systems by tensor network method

    Grant number：15K17701

    2015.4 - 2019.3

    System name：Grants-in-Aid for Scientific Research Grant-in-Aid for Young Scientists (B)

    Research category：Grant-in-Aid for Young Scientists (B)

    Awarding organization：Japan Society for the Promotion of Science

    Tsuyoshi Okubo

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    Authorship：Principal investigator 

    Grant amount：\3900000 （ Direct Cost: \3000000 、 Indirect Cost：\900000 ）

    In this study, we have investigated several frustrated quantum spin systems by using the tensor network method. First, for S=1/2 kagome lattice Heisenberg model, we have shown that several magnetization plateaus were stabilized under a magnetic field. Especially, at the 1/3 plateau, we clalyfied that the six spins on hexagons formed a resonated state. Second, for an ab initio Hamiltonian of Na2IrO3, which has strong Kitaev interaction, we found ground state phase diagram consistent with experimental observations. Finally, for frustrated square lattice Heisenberg model with ferromagnetic and antiferromagnetic interactions, we found that quantum spin fluctuation was enhanced under a magnetic field, which was consistent with experimental observations on corresponding an organic compound.

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  • 初到達時間に基づく境界摂動の理論構築

    Grant number：05J06188

    2005.4 - 2008.3

    System name：科学研究費助成事業 特別研究員奨励費

    Research category：特別研究員奨励費

    Awarding organization：日本学術振興会

    大久保 毅

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    Authorship：Principal investigator 

    Grant amount：\2700000 （ Direct Cost: \2700000 ）

    今年度は、前年度までの研究結果を整理し直し、種々の境界摂動実験と比較することにより、境界摂動に対する応答に共通してみられる、摂動の詳細に依存しない性質を明らかにした。
    第一に、半導体微粒子を混合した系での光の照射に対する電流の応答、触媒上での化学反応における容器の圧力変化に対する反応速度の応答、溶液の境界温度変化に対する熱流の応答、といった種々の境界摂動実験を比較し共通の性質を調べた。その結果、これらの境界摂動実験は、粒子の流入、移動、流出という三つの本質的な過程を共通して含んでおり、これまで我々が用いてきた粒子の流入・流出に基づくモデルが境界摂動実験の一般的な性質を調べる道具として適当であることを確認した。
    第二に、粒子の移動が量子力学に従う場合に加えて、運動が古典力学や確率分布の時間発展で記述される場合についても解析を進めることにより、境界摂動に対する応答と到達時間分布の関係が、種々の系において普遍的に成り立つことを明らかにした。過去に川崎らによって解析された粒子の移動がランダムウォークに従うモデルでは、境界摂動、及び、吸着壁がない状態での初到達時間分布が境界摂動に対する応答関数であるという結果が得られていた。一般的なモデルに基づいてこの結果を再検討することにより、川崎らの結果は特殊な状況でのみ成立し、一般の設定では、これまでの量子系の研究で明らかにしてきた関係式と同様に、吸着壁の性質に依存するように定義した到達時間分布が、境界摂動に対する応答関数になることを示した。

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Teaching Experience (researchmap)

  • 統計力学IIIA, B

    2026.4

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  • Data compression in computational science and quantum computing

    2022.10

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  • 物理学ゼミナール

    2018.10
    -
    2019.3

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  • 計算科学における情報圧縮

    2017.10
    -
    2022.3
    Institution name：東京大学大学院

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  • 多体問題の計算科学

    2017.4
    Institution name：東京大学大学院

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  • 計算科学概論

    2017.4
    -
    2022.3

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Teaching Experience

  • 統計力学IIIB

    2026
    Institution name：新潟大学

  • 統計力学IIIA

    2026
    Institution name：新潟大学

  • 現代物理学セミナーB

    2026
    Institution name：新潟大学

  • 現代物理学セミナーA

    2026
    Institution name：新潟大学

  • 物理学基礎A II

    2026
    Institution name：新潟大学

  • 計算物性学

    2026
    Institution name：新潟大学

  • 物性物理学

    2026
    Institution name：新潟大学

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