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)

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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

  • 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

  • Encyclopedia of Physics

    H. Matsumoto and Y. Ohashi, Wiley-VCH, 2005

    Scope: Bose-Einstein condensation(218-221)

Papers 【 Display / hide

  • Fermi-surface-engineered unconventional superfluid states in a driven-dissipative non-equilibrium Fermi gas

    Taira Kawamura, Ryo Hanai, Yoji Ohashi

    submitted Physical Review A, ArXiv:2111,02594 (American Physical Society APS)   2021.11

    Research paper (scientific journal), Joint Work, Accepted

  • Proposed Fermi-surface reservoir-engineering and application to realizing unconventional Fermi superfluids

    Taira Kawamura, Ryo Hanai, Yoji Ohashi

    submitted Physical Review, ArXiv:2111,01385 (American Physical Society APS)   2021.11

    Research paper (scientific journal), Joint Work, Accepted

  • Thermodynamic stability, compressibility matrices, and effects of mediated interactions in a strongly interacting Bose-Fermi mixture

    Koki Manabe, and Yoji Ohashi

    Physical Review A (American Physical Society APS)  103 ( 6 ) 063317(1-12) 2021.06

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

     View Summary

    We theoretically investigate the thermodynamic stability of a normal-state Bose-Fermi mixture, with a tunable Bose-Fermi pairing interaction -UBF<0 associated with a heteronuclear Feshbach resonance, as well as a weak repulsive Bose-Bose interaction UBB≥0. Including strong heteropairing fluctuations associated with the former interaction within the self-consistent T-matrix approximation, as well as the latter within the mean-field level, we calculate the compressibility matrix, to assess the stability of this system against density fluctuations. In the weak- and the intermediate-coupling regimes with respect to -UBF, we show that an effective attractive interaction between bosons mediated by density fluctuations in the Fermi component makes the system unstable below a certain temperature Tclp (leading to density collapse). When UBB=0, Tclp is always higher than the Bose-Einstein condensation (BEC) temperature Tc. When UBB>0, the density collapse is suppressed, and the BEC transition becomes possible. It is also suppressed by the formation of tightly bound Bose-Fermi molecules when the heteropairing interaction -UBF is strong; however, since the system may be viewed as a molecular Fermi gas in this case, the BEC transition does not also occur. Since quantum gases involving Bose atoms are known to be sensitive to interparticle correlations, our results would be useful for the study of many-body properties of a Bose-Fermi mixture in a stable manner, without facing the unwanted density collapse.

  • Superfluid properties of an ultracold Fermi gas with an orbital Feshbach resonance in the BCS-BEC crossover region

    Taro Kamihori, Daichi Kagamihara, and Yoji Ohashi

    Physical Review A (American Physical Society APS)  103 ( 5 ) 053319(1-14) 2021.05

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

     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.

  • Strong-coupling effects on quantum transport in an ultracold Fermi gas

    K. Furutani, D. Kagamihara, and Y. Ohashi

    J. Low Temp. Phys. (Journal of Low Temperature Physics)  201 ( 1-2 ) 49 - 57 2020.06

    Research paper (scientific journal), Joint Work, Accepted,  ISSN  00222291

     View Summary

    We theoretically investigate quantum conductance across a point contact between two strongly-interacting ultracold Fermi gases. We evaluate the tunneling current, taking into account pairing fluctuations in the Fermi gases within the framework of a combined strong-coupling T-matrix approximation with the Schwinger-Keldysh non-equilibrium formalism. We show that quasi-particle conductance is gradually suppressed, as one passes through the BCS–BEC crossover region from the weak-coupling side. On the other hand, with increasing the strength of a pairing interaction, the fluctuation-pair conductance is found to remarkably increase in the crossover region, which is followed by the suppression in the strong-coupling regime.

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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

    Introduction and explanation (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

    Introduction and explanation (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

    Introduction and explanation (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

    Introduction and explanation (scientific journal), Single Work

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Presentations 【 Display / hide

  • Shear viscosity and Kovtun-Son-Starinets conjecture in an ultracold Fermi gas

    大橋洋士、鏡原大地,佐藤龍平、真辺幸喜、田島裕之

    第6回分野横断インフォーマルミーティング (オンライン開催) , 2021.10, Oral Presentation(guest/special), 新学術領域「量子クラスターで読み解く物質階層構造」

  • 強結合フェルミ気体における奇周波数クーパーペア

    岩崎舜平、真辺幸喜、大橋洋士

    日本物理学会 (オンライン開催20200920-23) , 2021.09, Oral Presentation(general), 日本物理学会

  • FFLO状態の対形成揺らぎに対する安定性

    河村泰良、大橋洋士

    日本物理学会 (オンライン開催20200920-23) , 2021.09, Oral Presentation(general), 日本物理学会

  • 超流動フェルミ原子気体のずり粘性率II

    鏡原大地、土屋俊二、大橋洋士

    日本物理学会 (オンライン開催20200920-23) , 2021.09, Oral Presentation(general), 日本物理学会

  • 強結合Bose-Fermi混合原子気体における分子間有効相互作用効果II

    真辺幸喜、大橋洋士

    日本物理学会 (オンライン開催20200920-23) , 2021.09, Oral Presentation(general), 日本物理学会

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

  • 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, Co-investigator

  • 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, Co-investigator

  • 非平衡状態のフェルミ原子ガス超流動を用いて中性子星の冷却機構を解明する理論研究

    2016.04
    -
    2019.03

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

  • 量子多体問題に対するアクセラレータを用いた高速化・並列化手法の研究開発

    2015.10
    -
    2018.03

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

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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

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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: Awards of National Conference, Council and Symposium

  • 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]

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Courses Taught 【 Display / hide

  • THERMODYNAMICS AND STATISTICAL MECHANICS

    2021

  • STATISTICAL MECHANICS 1

    2021

  • PHYSICS D

    2021

  • PHYSICS C

    2021

  • LITERATURE OF PHYSICS

    2021

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Courses Previously Taught 【 Display / hide

  • 凝縮系物理学特論

    Keio University, 2014, Autumn Semester, Major subject, Lecture, Within own faculty

  • 熱統計力学第1

    Keio University, 2014, Spring Semester, Major subject, Lecture, Within own faculty

 

Memberships in Academic Societies 【 Display / hide

  • 日本物理学会, 

    1991.09
    -
    Present

Committee Experiences 【 Display / hide

  • 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

  • 2020.04
    -
    2021.03

    令和2年度知識集約型社会を支える人材育成委員会, 日本学術振興会、文部科学省

  • 2019.04
    -
    2020.03

    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

  • 2018.11
    -
    2019.10

    大学設置・学校法人審議会(大学設置分科会)専門委員, 文部科学省

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