Sugimoto, Koudai

写真a

Affiliation

Faculty of Science and Technology, Department of Physics （Yagami）

Position

Senior Assistant Professor (Non-tenured)/Assistant Professor (Non-tenured)
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Career 【 Display / hide 】

  • 2012.04
    -
    2015.03

    Japan Society for the Promotion of Science, JSPS Research Fellow (DC1)

  • 2015.04
    -
    2019.03

    Chiba University, Center for Frontier Science, Assistant professor

  • 2019.04
    -
    2022.03

    Keio University, Department of Physics, Research Associate (Non-tenured)

  • 2022.04
    -
    Present

    Keio University, Department of Physics, Senior Assistant Professor (Non-tenured)

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Academic Background 【 Display / hide 】

  • 2006.04
    -
    2007.03

    Osaka University, School of Science, Department of Biological Sciences

  • 2007.04
    -
    2010.03

    Osaka University, School of Science, Department of Physics

  • 2010.04
    -
    2015.03

    Kyoto University, Graduate School of Science, Division of Physics and Astronomy

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Research Areas 【 Display / hide 】

  • Natural Science / Semiconductors, optical properties of condensed matter and atomic physics

  • Natural Science / Magnetism, superconductivity and strongly correlated systems

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Papers 【 Display / hide 】

  • Symmetry, topology, duality, chirality, and criticality in a spin-1/2 XXZ ladder with a four-spin interaction

    Matéo Fontaine, Koudai Sugimoto, Shunsuke Furukawa

    Physical Review B （American Physical Society (APS))  109 （ 13 ）  2024.04

    Accepted,  ISSN  24699950

    　View Summary

    We study the ground-state phase diagram of a spin-12 XXZ model with a chirality-chirality interaction (CCI) on a two-leg ladder. This model offers a minimal setup to study an interplay between spin and chirality degrees of freedom. The spin-chirality duality transformation allows us to relate the regimes of weak and strong CCIs. By applying the Abelian bosonization and the duality, we obtain a rich phase diagram that contains distinct gapped featureless and ordered phases. In particular, Néel and vector chiral orders appear for easy-axis anisotropy, while two distinct symmetry-protected topological (SPT) phases appear for easy-plane anisotropy. The two SPT phases can be viewed as twisted variants of the Haldane phase. We also present an effective description in terms of (spinor) hard-core bosons, which reveals critical behavior on the self-dual line in the easy-axis and easy-plane regimes. We perform numerical simulations to confirm the predicted phase structure and critical properties. We further demonstrate that the two SPT phases and a trivial phase are distinguished by topological indices in the presence of certain symmetries. A similar phase structure is expected in a spin-12 XXZ ladder with four-spin ring exchange.

  • Pump-probe spectroscopy of the one-dimensional extended Hubbard model at half filling

    Koudai Sugimoto, Satoshi Ejima

    Physical Review B （American Physical Society (APS))  108 （ 19 ）  2023.11

    Lead author, Corresponding author, Accepted,  ISSN  24699950

    　View Summary

    By utilizing time-dependent tensor-network algorithms in the infinite matrix-product-state representation, we theoretically investigate the pump-probe spectroscopy of the one-dimensional extended Hubbard model at half filling. Our focus lies on nonequilibrium optical conductivity and single-particle excitation spectra. In the spin-density-wave (SDW) phase, we identify an in-gap state in the nonequilibrium optical conductivity due to the formation of excitons (or doublon-holon pairs), generated by the pulse through nonlocal interactions. In the strong-coupling regime, we discern additional multiple in-gap and out-of-gap states. In the charge-density-wave (CDW) phase, we detect not only an in-gap state but also a finite Drude weight, which results from the dissolution of charge order by photoexcitation. Analyzing time-dependent single-particle excitation spectra directly in the thermodynamic limit confirms the origin of these new states in the SDW and CDW phases as the excitation of newly emerged dispersions. We illustrate that the pump-probe spectroscopy simulations in the thermodynamic limit furnish unambiguous spectral structures that allow for direct comparison with experimental results, and the integration of nonequilibrium optical conductivity and time- and angle-resolved photoemission spectroscopy provides comprehensive insights into nonequilibrium states.

  • Zigzag chain order of LiVSe<sub>2</sub> developing away from the vanadium trimer phase transition boundary

    K. Kojima, N. Katayama, K. Sugimoto, N. Hirao, Y. Ohta, H. Sawa

    Physical Review B （American Physical Society (APS))  108 （ 9 ）  2023.09

    Accepted,  ISSN  24699950

    　View Summary

    The phenomenon of self-assembly of constituent elements to form molecules at low temperatures appears ubiquitously in transition metal compounds with orbital degrees of freedom. Recent progress in local structure studies using synchrotron radiation x rays is shifting the interest in structural studies in such molecule-forming systems from the low-temperature ordered phase to the short-range order that appears like a precursor at high temperatures. In this study, we discuss both experimentally and theoretically the relationship between the trimer structure that appears in the layered LiVX2(X = O, S, Se) system with a two-dimensional triangular lattice of vanadium and the zigzag chainlike local structure that appears near the phase transition boundary where molecular formation occurs. The vanadium trimerization that persistently appears in both low-temperature phases of LiVO2 and LiVS2 disappears in LiVSe2, and a regular triangular lattice is thought to be realized in LiVSe2, but this study reveals that the zigzag chain local distortion appears with a finite correlation length. These zigzag chain state local distortions are similar to the motif of local distortions in the high-temperature phase of LiVS2, indicating that the local distortions are persistent away from the trimer phase transition boundary. On the other hand, it is concluded that the zigzag chain order appearing in LiVSe2 is more stable than that in LiVS2 in terms of the temperature variation of atomic displacement and correlation length. The zigzag chain order is considered to be competitive with the trimer order appearing in the LiVX2 system. In this paper, we discuss the similarities and differences between the parameters that stabilize these electronic phases and the local distortions that appear in other molecular formation systems.

  • Wannier-Stark ladders and Stark shifts of excitons in Mott insulators

    Mina Udono, Tatsuya Kaneko, Koudai Sugimoto

    Physical Review B （American Physical Society (APS))  108 （ 8 ）  2023.08

    Last author, Corresponding author, Accepted,  ISSN  24699950

    　View Summary

    External-field driven energy-level discretization, such as Landau quantization or Stark localization, is one of the most intriguing phenomena in quantum systems. We investigate the emergence of the Wannier-Stark ladder coming from the particle-hole continuum and the Stark shifts of the exciton levels in one-dimensional Mott insulators under the dc electric field. The discretized peak structure in the optical-conductivity spectra newly appears by applying the dc electric field, and the positions of these peaks can be reproduced from the energy levels of a simple effective model in the strong-coupling regime. Our results not only suggest that Mott insulators can serve as a viable platform for Stark discretization, but also pave the way for investigations of dynamical properties in correlated many-body systems under a dc electric field.

  • Collective mode excitations and simulated <i>L</i>-edge resonant inelastic x-ray scattering spectra in antiferromagnetic Ca<sub>2</sub>RuO<sub>4</sub>

    Shunsuke Yamamoto, Yukinori Ohta, Koudai Sugimoto

    Physical Review B （American Physical Society (APS))  106 （ 4 ）  2022.07

    Last author, Accepted,  ISSN  24699950

    　View Summary

    Using the three-orbital Hubbard model, we investigate the low-energy excitation spectra in the antiferromagnetic phase of Ca2RuO4. We calculate the dynamical susceptibilities in the low-energy region by the random phase approximation and find that the anisotropic dispersion of the transverse mode is in good agreement with the spectra recently reported by inelastic neutron scattering experiments. By the fast-collision approximation, we simulate the resonant inelastic x-ray scattering (RIXS) spectra of the Ru L3 edge from the dynamical susceptibilities. We show that the dispersion of the transverse mode is clearly observed in the calculated RIXS spectra and that the polarization dependence of the incident x rays enables one to distinguish between the excitations of the in-plane transverse mode and out-of-plane transverse mode.

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Papers, etc., Registered in KOARA 【 Display / hide 】

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Reviews, Commentaries, etc. 【 Display / hide 】

  • d‐p模型によるTa<sub>2</sub>NiSe<sub>5</sub>の励起子絶縁体転移

    杉本高大, 太田幸則

    日本物理学会講演概要集(CD-ROM) 74 ( 1 ) ROMBUNNO.15pS305‐6 2019.03

    ISSN  2189-079X

  • 有限温度変分クラスター法によるハバード模型の解析

    西田尚央, 藤内亮, 杉本高大, 太田幸則

    日本物理学会講演概要集(CD-ROM) 74 ( 1 ) ROMBUNNO.17aG212‐8 2019.03

    ISSN  2189-079X

  • α‐Sr<sub>2</sub>CrO<sub>4</sub>の高圧下電子状態

    高橋諒, 山口伴紀, 杉本高大, 山内徹, 櫻井裕也, 太田幸則

    日本物理学会講演概要集(CD-ROM) 73 ( 2 ) ROMBUNNO.10aPS‐23 2018.09

    ISSN  2189-079X

  • 強結合型励起子絶縁体としてのTa<sub>2</sub>NiSe<sub>5</sub>

    杉本高大, 西本理, 西本理, 金子竜也, 太田幸則

    日本物理学会講演概要集(CD-ROM) 73 ( 2 ) ROMBUNNO.10aPS‐24 2018.09

    ISSN  2189-079X

  • 変分クラスター近似による二軌道ハバード模型の励起子相と超伝導相

    藤内亮, 杉本高大, 太田幸則

    日本物理学会講演概要集(CD-ROM) 73 ( 2 ) ROMBUNNO.10aPS‐93 2018.09

    ISSN  2189-079X

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Presentations 【 Display / hide 】

  • Strong Coupling Nature of the Excitonic Insulator State in Ta2NiSe5

    Koudai Sugimoto, Satoshi Nishimoto, Tatsuya Kaneko, Yukinori Ohta

    The 10th international Conference “New Generation in Strongly Correlated Electron Systems” (NGSCES 2019), 

    2019.09
    , 

    Oral presentation (general)

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Research Projects of Competitive Funds, etc. 【 Display / hide 】

  • Ultrafast spectroscopy simulation of low-dimensional strongly correlated electron systems using tensor network

    2023.04
    -
    2027.03

    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), Principal investigator

  • 電子ドープ型銅酸化物高温超伝導体における擬ギャップ現象の核磁気共鳴法による解明

    2021.04
    -
    2024.03

    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C), Grant-in-Aid for Scientific Research (C), Coinvestigator(s)

    　View Summary

    銅酸化物高温超伝導体はその発見以来、母物質である反強磁性モット絶縁体にキャリアを注入することにより超伝導が発現すると考えられてきた。しかし、近年Nd2CuO4構造いわゆるT’構造をもつ電子ドープ型高温超伝導体において、過剰酸素を取除くことにより、電子をドープしなくても超伝導が発現する可能性が示された。しかし、一方で実験的に示されてきたこのノンドープ超伝導は、実は酸素がCuO2面から取り除かれ電子ドープが起こっており、その超伝導は従来通りのドープされたモット絶縁体としての高温超伝導であるとも、最近の研究では指摘されている。また、これまでにT'型の銅酸化物高温超伝導体においても擬ギャップ現象が本質的に存在することが明らかになってきており、この擬ギャップの起源を探ることが本研究課題の重要な目標である。さらに、擬ギャップ現象と関連して強相関電子系における量子臨界現象と磁性について研究することも重要であり、本年度は周辺物質に関しても研究を行なった。
    銅酸化物に関しては、T’-LE(S,C)COに注目し、系統的に63,65Cu核、139La核のNMR測定を行ない、超伝導対称性および反強磁性ゆらぎの有無・強さを明らかにした。また、重い電子系超伝導体Ce3PtIn11の二段磁気転移におけるq構造をtransferred hyperfine fieldを用いることにより、明らかにした。さらに、単体金属であるalpha-Mnの圧力誘起磁気秩序相における磁気構造についてゼロ磁場NMRの周波数の圧力依存から論じ、フェリ磁性的なモデルが他の実験結果と矛盾しないことを示した。

  • Theory of preformed pair states and nonequilibrium dynamics in materials with strong-coupling excitonic condensation

    2020.04
    -
    2024.03

    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B), Grant-in-Aid for Scientific Research (B), Coinvestigator(s)

    　View Summary

    本申請代表者グループの励起子凝縮に関するこれまでの理論研究を背景に、本研究の目的は次の３点を明らかにすることである。すなわち、① 熱的純粋量子状態に基づく変分クラスター近似を用いた強結合励起子凝縮機構の解明 ② 時間発展ランチョス法・時間依存密度行列繰込み群法による励起子相の非平衡ダイナミクスの解明 ③第一原理計算とその解析に基づく低エネルギー有効電子模型の導出。これらにより、超伝導と並ぶフェルミオン対凝縮系としての励起子凝縮の物理の学理を深化させる。この目的を達成するため、研究分担者と協力者を3チームに分け、計画の4年間で強結合励起子絶縁体の特異性を明らかにし、有限温度量子多体系における秩序と揺らぎという歴史的大問題にひとつの解答を与え、さらにはフェルミオン対凝縮系の非平衡ダイナミクスという未踏の問題に糸口を見出す。
    令和2年度は、励起子凝縮の学理の深化のための出発点を構築した。新型コロナウィルス感染症の予期せぬ拡大により研究活動が様々な側面で大きく抑制されたため、研究期間の1年間の延長を申請し認められた。その結果、次のような研究実績を得ることができた。研究代表者チームは、非平衡グリーン関数による摂動論的解析手法に加え、強相関電子系に対する非摂動論的な量子クラスター法を中心とする数値計算手法を開発した。また、低エネルギー有効模型の導出を行い、物質に即した理論を構築する基礎を得た。研究分担者チームは、関係する種々の強相関電子模型の非平衡ダイナミクスを研究し、光励起による強相関電子系における非平衡ダイナミクスや光誘起相転移に関する研究手法を発展させた。また、強結合励起子系の有限温度における一般理論を構築するため、熱的純粋量子状態に基づく変分クラスター近似の計算手法を開発した。研究成果は幾つかの学術論文として出版し、国内外の学会等で発表した。

  • Excitation dynamics in excitonic condensation phase

    2019.04
    -
    2022.03

    MEXT,JSPS, Grant-in-Aid for Scientific Research, Sugimoto Koudai, Grant-in-Aid for Early-Career Scientists , Principal investigator

    　View Summary

    The phase in which excitons spontaneously form in a semimetal or semiconductor and subsequently undergo Bose condensation is referred to as an excitonic phase. In recent years, some potential materials have been proposed, and the study of excitonic phases has revealed novel perspectives. In this research, I conducted both theoretical and numerical analyses of finite-temperature states and photo-induced nonequilibrium states in strongly correlated electron systems, with particular attention to the physical properties of excitonic phases.
    For the finite-temperature calculations, we expanded the variational cluster approach to include finite temperatures and applied the cluster mean-field approximation to the excitonic phase. In the examination of nonequilibrium states, we proposed light-induced phase transitions within the attractive and repulsive Hubbard models, and elucidated the effect of excitons on higher-harmonic generations in the extended Hubbard model.

  • Development of the microscopic theory on external-field responses of excitonic insulators

    2015.08
    -
    2017.03

    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Sugimoto Koudai, Grant-in-Aid for Research Activity Start-up, Principal investigator

    　View Summary

    An excitonic insulator is a system where electrons and holes spontaneously form pairs (excitons) in a semiconductor or a semimetal and the pairs go into quantum condensation. We have focused on transition-metal chalcogenide Ta2NiSe5 known as a candidate material of an excitonic insulator, and have clarified the behaviors of the microscopic quantum interferences in the ultrasonic attenuation rate and the NMR relaxation rate, the diamagnetic susceptibility, and the optical conductivity from the perspective of the excitonic condensation in it. We have also shown that in cobalt oxide Pr0.5Ca0.5CoO3 the magnetic-multipole order emerges in the excitonic phase, and have studied its low-energy excaitation spectra.

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Courses Taught 【 Display / hide 】

  • PHYSICS EXERCISE 3

    2024

  • PHYSICS D

    2024

  • PHYSICS C

    2024

  • LITERATURE OF PHYSICS

    2024

  • BACHELOR'S THESIS

    2024

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Social Activities 【 Display / hide 】

  • 第17回高校生理科研究発表会

    千葉大学, 第17回高校生理科研究発表会, 

    , 
    2023.09

  • 第16回高校生理科研究発表会

    千葉大学, 第16回高校生理科研究発表会, 

    , 
    2022.09

  • 第15回高校生理科研究発表会

    千葉大学, 第15回高校生理科研究発表会, 

    , 
    2021.09

  • 第14回高校生理科研究発表会

    千葉大学, 第14回高校生理科研究発表会, 

    , 
    2020.12

  • 第13回高校生理科研究発表会

    千葉大学, 第13回高校生理科研究発表会, 

    , 
    2019.09

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Media Coverage 【 Display / hide 】

  • Long-Range Coupling Affects Entanglement Dynamics

    American Physical Society, Physics （https://physics.aps.org/articles/v15/2) , 2022.01

  • 量子もつれなどによる「量子測定誘起相転移」発現のための条件、慶大などが発見

    マイナビ, TECH+ （https://news.mynavi.jp/techplus/article/20220108-2245465/) , 2022.01

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Committee Experiences 【 Display / hide 】

  • 2022.04
    -
    2023.03

    領域運営委員, 一般社団法人日本物理学会

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