Ohashi, Yoji

写真a

Affiliation

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

Position

Professor

Related Websites

External Links

Career 【 Display / hide

  • 1994.04
    -
    1995.03

    日本学術振興会特別研究員(大阪大学基礎工学部)

  • 1995.04
    -
    1998.04

    筑波大学物理学系助手

  • 1998

    佐賀大学理工学部非常勤講師(併任、集中講義)

  • 1998.05
    -
    2006.03

    筑波大学物理学系講師

  • 1999

    佐賀大学理工学部非常勤講師(併任、集中講義)

display all >>

Academic Background 【 Display / hide

  • 1989.03

    慶應義塾大学, Faculty of Science and Engineering, 物理学科

    University, Graduated

  • 1994.03

    Tokyo Institute of Technology, Graduate School, Division of Science and Engineering, 物理学

    Graduate School, Completed, Doctoral course

Academic Degrees 【 Display / hide

  • 博士(理学), Tokyo Institute of Technology, Coursework, 1994.03

 

Books 【 Display / hide

  • Encyclopedia of Condensed Matter Physics, 2nd edition

    Y. Ohashi, Elsevier, 2023.10,  Page: vol. 3, 10-21

    Scope: BEC-BCS crossover in ultracold atomic gases and neutron stars,  Contact page: Volume 3 , 10-21 , Accepted

  • Foundations of Quantum Mechanics in the Light of New Technology

    S. Tsuchiya, and Y. Ohashi, World Scientific, 2009

    Scope: 37-40

  • Foundations of Quantum Mechanics in the Light of New Technology

    Y. Ohashi, and N. Fukushima, World Scientific, 2009

    Scope: 18-23

  • RECENT PROGRESS IN MANY-BODY THEORIES

    Y. Ohashi, World Scientific, 2006

    Scope: 18-23, 37-40

  • 科学大辞典(第2版)

    OHASHI Yoji, 丸善, 2005

    Scope: 項目執筆

display all >>

Papers 【 Display / hide

  • Stable non-equilibrium Fulde-Ferrell-Larkin-Ovchinnikov state in a spin-imbalanced driven-dissipative Fermi gas loaded on a three-dimensional cubic optical lattice

    Kawamura T., Ohashi Y.

    Physical Review A (Physical Review A)  108 ( 1 ) 013321 2023.07

    Research paper (scientific journal), Last author, Accepted,  ISSN  24699926

     View Summary

    We theoretically investigate a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type superfluid phase transition in a driven-dissipative two-component Fermi gas. The system is assumed to be in the nonequilibrium steady state, which is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. Including pairing fluctuations by extending the strong-coupling theory developed in the thermal-equilibrium state by Nozières and Schmitt-Rink to this nonequilibrium case, we show that a nonequilibrium FFLO (NFFLO) phase transition can be realized without spin imbalance, under the conditions that (1) the two reservoirs imprint a two-edge structure on the momentum distribution of Fermi atoms and (2) the system is loaded on a three-dimensional cubic optical lattice. While the two edges work like two Fermi surfaces with different sizes, the role of the optical lattice is to prevent the NFFLO long-range order from destruction by NFFLO pairing fluctuations. We also draw the nonequilibrium mean-field phase diagram in terms of the chemical potential difference between the two reservoirs, a fictitious magnetic field to tune the spin imbalance of the system, and the environmental temperature of the reservoirs to clarify the relation between the NFFLO state and the ordinary thermal-equilibrium FFLO state discussed in spin-imbalanced Fermi gases.

  • Isothermal compressibility and effects of multi-body molecular interactions in a strongly interacting ultracold Fermi gas

    Daichi Kagamihara, Ryohei Sato, Koki Manabe, Hiroyuki Tajima, Yoji Ohashi

    Physical Review A (American Physical Society APS)  106 ( 3 ) 033308  2022.09

    Research paper (scientific journal), Joint Work, Last author, Accepted,  ISSN  24699926

     View Summary

    We theoretically investigate the isothermal compressibility κT in the normal state of an ultracold Fermi gas. Including pairing fluctuations, as well as preformed-pair formations, within the framework of the self-consistent T-matrix approximation, we evaluate the temperature dependence of this thermodynamic quantity over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. While κT in the weak-coupling BCS regime is dominated by Fermi atoms near the Fermi surface, correlations between tightly bound Cooper-pair molecules are found to play crucial roles in the strong-coupling BEC regime. In the latter region, besides a two-body molecular interaction, a three-body one is shown to sizably affect κT near the superfluid phase-transition temperature. Our results indicate that the strong-coupling BEC regime of an ultracold Fermi gas would provide a unique opportunity to study multibody correlations between Cooper-pair molecules.

  • Feasibility of a Fulde-Ferrell-Larkin-Ovchinnikov superfluid Fermi atomic gas

    Kawamura T., Ohashi Y.

    Physical Review A (Physical Review A)  106 ( 3 )  2022.09

    Research paper (scientific journal), Joint Work, Last author, Accepted,  ISSN  24699926

     View Summary

    We theoretically explore a promising route to achieve the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in a spin-imbalanced ultracold Fermi gas. In the current stage of cold atom physics, search for this exotic Fermi superfluid is facing two serious difficulties: One is the desperate destruction of the FFLO long-range order by FFLO pairing fluctuations, which precludes entering the phase through a second-order transition, even in three dimension. The other is the fierce competition with the phase separation into the BCS (Bardeen-Cooper-Schrieffer) state and the spin-polarized normal state. By including strong FFLO pairing fluctuations within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that the anisotropy of Fermi surface introduced by an optical lattice makes the FFLO state stable against the paring fluctuations. This stabilized FFLO state is also found to be able to overcome the competition with the phase separation under a certain condition. Since the realization of unconventional Fermi superfluids is one of the most exciting challenges in cold atom physics, our results would contribute to the further development of this field.

  • Proposed Fermi-surface reservoir engineering and application to realizing unconventional Fermi superfluids in a driven-dissipative nonequilibrium Fermi gas

    Kawamura Taira, Hanai Ryo, Ohashi Yoji

    PHYSICAL REVIEW A (Physical Review A)  106 ( 1 )  2022.07

    Research paper (scientific journal), Joint Work, Last author,  ISSN  2469-9926

     View Summary

    We develop a theory to describe the dynamics of a driven-dissipative many-body Fermi system to pursue our proposal to realize exotic quantum states based on reservoir engineering. Our idea is to design the shape of a Fermi surface so as to have multiple Fermi edges by properly attaching multiple reservoirs with different chemical potentials to a fermionic system. These emerged edges give rise to additional scattering channels that can destabilize the system into unconventional states, which is exemplified in this work by considering a driven-dissipative attractively interacting Fermi gas. By formulating a quantum kinetic equation using the Nambu-Keldysh Green's function technique, we explore nonequilibrium steady states in this system and assess their stability. We find that, in addition to the Bardeen - Cooper - Schrieffer-type isotropic pairing state, a Fulde-Ferrell-type anisotropic superfluid state being accompanied by Cooper pairs with nonzero center-of-mass momentum exists as a stable solution, even in the absence of a magnetic Zeeman field. Our result implies a great potential of realizing quantum matter beyond the equilibrium paradigm by engineering the shape and topology of Fermi surfaces in both electronic and atomic systems.

  • Beyond Gaussian pair fluctuation theory for strongly interacting Fermi gases II: The broken-symmetry phase

    Brendan C. Mulkerin, Xing-Can Yao, Yoji Ohashi, Xia-Ji Liu, Hui Hu

    arXiv:2201.04798 (arXiv (preprint server))   2022.01

    Research paper (scientific journal), Joint Work

     View Summary

    We theoretically investigate the superfluid properties of a two-band gas of Yb173 Fermi atoms with an orbital Feshbach resonance (OFR). To describe the BCS-BEC crossover region, we include superfluid fluctuations caused by interband and intraband pairing interactions associated with OFR by extending the strong-coupling theory developed by Nozières and Schmitt-Rink to the two-band case below the superfluid phase transition temperature; however, the effects of an experimentally inaccessible deep bound state are removed to model a real Yb173 Fermi gas near OFR. We show that the condensate fraction in the upper closed channel gradually becomes smaller than that in the lower open channel as one moves from the strong- to the weak-coupling regime, because the OFR-pairing mechanism tunes the interaction strengths by adjusting the energy difference between the two bands. However, even when the closed-channel band is much higher in energy than the open-channel band in the weak-coupling regime, the magnitude of the superfluid order parameter in the closed channel is found to be still comparable to that in the open channel. As the reason for this, we point out a pair-tunneling effect by the OFR-induced interband interaction. In addition to these superfluid quantities, we also examine collective modes, such as the Goldstone mode, the Schmid (Higgs) mode, and Leggett mode, to clarify how they appear in the spectral weights of pair-correlation functions in each band. Since the realization of a multiband superfluid Fermi gas is a crucial issue in cold Fermi gas physics, our results would contribute to the basic understanding of this type of Fermi superfluid in the BCS-BEC crossover region.

display all >>

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

Reviews, Commentaries, etc. 【 Display / hide

  • BCS-BEC クロスオーバー

    大橋洋士

    数理科学 (サイエンス社)  58 ( 6 ) 21 - 27 2020.06

    Article, review, commentary, editorial, etc. (scientific journal), Single Work

  • フェルミ原子ガス超流動と中性子星内部状態への応用

    大橋洋士

    日本物理学会誌 (日本物理学会)  73 ( 12 ) 842 - 851 2018.12

    Article, review, commentary, editorial, etc. (scientific journal), Single Work

  • 「Superfluid Fermi atomic gas as a quantum simulator for the study of neutron-star equation of state」(P. van Wyk, H. Tajima, D. Inotani, A. Ohnishi, and Y. Ohashi, Physical Review A 97, 013601(1-13) (2018). )の紹介記事

    大橋洋士

    慶應義塾大学理工学部 (慶應義塾大学理工学部)   2018.04

    Single Work

  • フェルミ原子ガス超流動のBCS-BECクロスオーバー理論と中性子星状態方程式への挑戦

    大橋洋士、Pieter van Wyk, 田島裕之

    原子核研究 61 ( 1 ) 70 - 84 2016.09

    Article, review, commentary, editorial, etc. (scientific journal), Joint Work

  • フェルミ超流動とボース・アインシュタイン凝縮の統一描像

    大橋洋士

    日本物理学会誌 71   450 - 451 2016.07

    Article, review, commentary, editorial, etc. (scientific journal), Single Work

display all >>

Presentations 【 Display / hide

  • Excitation spectrum of odd-frequency Fermi superfluid

    岩崎舜平、大橋洋士

    日本物理学会 (オンライン開催) , 

    2024.03

    Oral presentation (general)

  • Specific heat of spin-orbit coupled ultracold Fermi gas and effects of rashbon bound states

    岡嶋佑典, 竹本功貴, 岩崎舜平, 大橋洋士

    日本物理学会 (オンライン開催) , 

    2024.03

    Oral presentation (general)

  • N-S-N接合系における電圧誘起非平衡FFLO状態

    河村泰良、大橋洋士、Henk Stoof

    日本物理学会 (オンライン開催) , 

    2024.03

    Oral presentation (general)

  • Emergence of inhomogeneous superconducting state in a voltage-driven superconducting structure

    Taira Kawamura, Yoji Ohashi, and Henk Stoof

    NWO Physics 2024 (NH Conference Centre Koningshof, The Netherlands) , 

    2024.01

    Poster presentation, Dutch Research Council (NWO)

  • BCS-BEC crossover of the odd-frequency pairing Fermi gas

    岩崎舜平, 河村泰良,大橋洋士

    新学術領域「量子クラスターで読み解く物質の階層構造」 第9回領域研究会 (理化学研究所 RIBF棟2F 大会議室) , 

    2023.09

    Poster presentation, 新学術領域「量子クラスターで読み解く物質の階層構造」

display all >>

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

  • BCS-BECクロスオーバー領域におけるフェルミ原子ガスの超流動秩序化過程の解明

    2022.04
    -
    2025.03

    Grant-in-Aid for Scientific Research, Research grant, Principal investigator

  • Theoretical study of superfluid ordering process in the BCS-BEC crossover regime of an ultracold Fermi gas

    2022.04
    -
    2025.03

    MEXT,JSPS, Grant-in-Aid for Scientific Research, 基盤研究(C), Principal investigator

  • 強相関量子多体状態の形成過程をフェルミ原子気体の高い操作性を使い解明する研究

    2019.04
    -
    2022.03

    Grant-in-Aid for Scientific Research, Research grant, Principal investigator

  • 物質の階層変化および状態変化に伴う普遍的物理

    2018.06
    -
    2023.03

    Grant-in-Aid for Scientific Research, Research grant, Coinvestigator(s)

  • エクサスケール計算機を想定した量子モデルシミュレーションに対する並列化・高速化

    2018.04
    -
    2021.03

    Grant-in-Aid for Scientific Research, Research grant, Coinvestigator(s)

display all >>

Works 【 Display / hide

  • 大学院集中講義

    OHASHI Yoji

    京都大学, 

    2017.11

    Other, Single

     View Details

    フェルミ原子ガス超流動におけるBCS-BECクロスオーバー

  • 長崎国際テレビ開局25周年記念特別番組「アインシュタイン 美しい国 日本を旅する」監修(物理学)

    大橋洋士

    長崎国際テレビ:2015年10月18日(日)15:00~16:25、BS日テレ:2015年11月1日(日)16:00~17:25, 

    2015.10

    Other, Single

  • 研究分野を変えた時のこと

    大橋洋士

    三田評論 No 1187, 2015年3月号:Researcher's Eye, 

    2015.03
    -
    Present

    Other, Single

  • 大学院集中講義

    OHASHI Yoji

    東京大学, 

    2015.02

    Other, Single

     View Details

    フェルミ原子ガス超流動の物理

  • SFC科学入門講座

    OHASHI Yoji

    慶應義塾湘南藤沢高等学校, 

    2014.03

    Other, Single

     View Details

    量子の世界と超伝導

display all >>

Awards 【 Display / hide

  • 2020年秋季日本物理学会学生優秀発表賞

    河村泰良(研究室研究生)[花井亮、大橋洋士], 2020.09, 日本物理学会, 非平衡定常状態にあるフェルミ超流動の安定性解析 [研究室研究生]

  • KEKオンライン研究会「熱場の量子論とその応用」ポスター賞

    河村泰良(研究室研究生)[大橋洋士], 2020.08, 熱場の量子論とその応用2020, 非平衡フェルミ原子気体の超流動状態の性質 [研究室研究生]

  • 日本物理学会学生優秀発表賞(領域1、2019秋)

    真辺幸喜(研究室学生D1)[真辺幸喜、大橋洋士], 2019.11, 日本物理学会, 調和トラップ中の冷却Bose-Fermi混合気体における光原子分光スペクトル

    Type of Award: Award from Japanese society, conference, symposium, etc.

  • 理研シンポジウム・iTHEMS研究会 「熱場の量子論とその応用」ポスター賞

    真辺幸喜(研究室学生M2)[真辺幸喜、猪谷太輔、大橋洋士], 2018.08, 熱場の量子論とその応用2018, 強結合冷却Bose-Fermi混合気体の1粒子励起における質量インバランス効果[研究室院生]

  • 藤原賞

    田島裕之(研究室学生D3), 2017.03, 慶應義塾大学, フェルミ原子ガス超流動の熱力学的性質の理論研究に対して[研究室院生]

display all >>

 

Courses Taught 【 Display / hide

  • THERMODYNAMICS AND STATISTICAL MECHANICS

    2023

  • STATISTICAL MECHANICS 1

    2023

  • PHYSICS SEMINAR 3

    2023

  • PHYSICS D

    2023

  • PHYSICS C

    2023

display all >>

Courses Previously Taught 【 Display / hide

  • 凝縮系物理学特論

    Keio University

    2014.04
    -
    2015.03

    Autumn Semester, Lecture, Within own faculty

  • 熱統計力学第1

    Keio University

    2014.04
    -
    2015.03

    Spring Semester, Lecture, Within own faculty

 

Media Coverage 【 Display / hide

  • :世紀の大発見!?「室温超伝導」

    テレ東Biz, テレ東Biz 理系通信, 2023.08

     View Summary

    最近、LK-99と呼ばれる物質において「室温超伝導が実現したのではないか」との発表があり大きな話題になっているが、その信ぴょう性や疑問点、室温超伝導の可能性について、配信番組中で専門的観点からコメントを与えた。

Memberships in Academic Societies 【 Display / hide

  • 日本物理学会, 

    1991.09
    -
    Present

Committee Experiences 【 Display / hide

  • 2022

    カルカッタ大学博士審査外部委員(2件), インド カルカッタ大学

  • 2021.12
    -
    2022.03

    研究業績審査委員会専門委員, 日本原子力研究開発機構

  • 2021.06
    -
    2025.05

    AAPPS Bulltin board editor, Association of Asia Pacific Physics Society

  • 2021.03

    主任研究員等研究業績審査委員, 日本原子力研究開発機構

  • 2021.01
    -
    2021.03

    任期付研究員研究業績評価委員, 日本原子力研究開発機構

display all >>