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)

  • 2024.09
    -
    Present

    RIKEN, TRIP Headquarters, Visiting Scientist

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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 】

  • Reexamination of the charge-ordered dimer pattern in the spinel compound CuIr<sub>2</sub>S<sub>4</sub> using single-crystal synchrotron x-ray diffraction

    T. Ohashi, N. Katayama, K. Kojima, M. Emi, C. Koyama, T. Hara, K. Hashimoto, S. Kitani, H. Kawaji, H. S. Suzuki, S. Nagata, K. Sugimoto, K. Iida, H. Sawa

    Physical Review B (American Physical Society (APS))  111 ( 22 )  2025.06

    Accepted,  ISSN  2469-9950

  • DC electric field driven discretization of single-particle excitation spectra in a Mott insulator

    Koudai Sugimoto

    Physical Review B (American Physical Society (APS))  111 ( 24 )  2025.06

    Accepted,  ISSN  24699950

    　View Summary

    We theoretically investigate the single-particle excitation spectra of a one-dimensional Hubbard model at half filling using an infinite matrix-product state and elucidate the discretized energy spectra emerging under the influence of a dc electric field. In a weak electric-field regime, we observe two kinds of spectral structures in the density of states. With increasing the electric-field strength, the discretized spectra, the period of which is proportional to the strength, become dominant, and the density of states exhibits the Wannier-Stark ladder in their spectra. In addition, we also simulate time- and angle-resolved photoemission spectroscopy using an ultrashort terahertz pump pulse that approximates a dc electric field. Our results represent a significant step forward in understanding the states in strongly correlated electron systems driven by a static electric field.

  • Ground-State Properties of the <i>t</i>–<i>J</i> Model for the CuO Double-Chain Structure

    Tatsuya Kaneko, Satoshi Ejima, Koudai Sugimoto, Kazuhiko Kuroki

    Journal of the Physical Society of Japan (Physical Society of Japan)  93 ( 8 ) 084703 2024.08

    Accepted,  ISSN  00319015

    　View Summary

    We investigate the ground-state properties of a correlated model for the double-chain structure in cuprates. We consider the t–J model, in which the nearest-neighbor spin interaction J<inf>1</inf> is smaller than the next-nearest neighbor interaction J<inf>2</inf> corresponding to the CuO double-chain structure. We vary J<inf>1</inf> from antiferromagnetic to ferromagnetic values and calculate the correlation functions including the superconducting pair correlation function. Employing the density-matrix renormalization group method, we show that the ground state for antiferromagnetic J<inf>1</inf> exhibits the hallmarks of the Luther–Emery liquid phase, in which the spin-singlet pair and charge-density-wave correlations exhibit power-law decays against distance, and the spin correlation function decays exponentially. Its signatures are gradually dismissed as J<inf>1</inf> approaches the ferromagnetic regime. Our findings suggest that the antiferromagnetic double-chain structure without ferromagnetic bonds is favorable for superconductivity.

  • 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.

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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 】

  • 無機柔粘性結晶の構造的特徴を活用した革新的物質機能の開拓

    2024.04
    -
    2027.03

    日本学術振興会, 科学研究費助成事業, 基盤研究(B), Coinvestigator(s)

  • 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.

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Awards 【 Display / hide 】

  • Young Scientist Award of the Physical Society of Japan

    2026.03, The Physical Society of Japan, Theoretical studies on excitation spectra in strongly correlated electron systems via optical response

  • 教育功労賞

    2024.09, 千葉大学

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Courses Taught 【 Display / hide 】

  • PHYSICS EXERCISE 3

    2025

  • PHYSICS D

    2025

  • PHYSICS C

    2025

  • LITERATURE OF PHYSICS

    2025

  • BACHELOR'S THESIS

    2025

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Social Activities 【 Display / hide 】

  • 第19回高校生理科研究発表会

    千葉大学, 第19回高校生理科研究発表会, 

    , 
    2025.09

  • ２０２５年度塾内高校生のための慶應義塾大学理工学部夏休み研究体験

    慶應義塾大学理工学部, ２０２５年度塾内高校生のための慶應義塾大学理工学部夏休み研究体験, 

    , 
    2025.08

  • 第18回高校生理科研究発表会

    千葉大学, 第18回高校生理科研究発表会, 

    , 
    2024.09

  • 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

  • 研究室公開

    慶應義塾大学理工学部, 慶應義塾湘南藤沢高等部理工学部見学会, 

    , 
    2023.07

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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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