KEIO RESEARCHERS INFORMATION SYSTEM

Career 【 Display / hide 】

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 】

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 】

Academic Degrees 【 Display / hide 】

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

Books 【 Display / hide 】

Books 【 Display / hide 】

• Encyclopedia of Condensed Matter Physics, 2nd edition

  Y. Ohashi, Elsevier, 2022

  Scope: BEC-BCS CROSSOVER IN ULTRACOLD ATOMIC GASES AND NEUTRON STARS , 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

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Papers 【 Display / hide 】

Papers 【 Display / hide 】

• 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, 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.

  • Access to Document (DOI)

• 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

  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.

  • Access to Document (DOI)

• 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

  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.

  • Access to Document (DOI)

• 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

  submitted to Physical Review A （American Physical Society APS)   2022

  Research paper (scientific journal), Joint Work, Accepted

  　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.

• 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.

  • Access to Document (DOI)

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

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

• Theoretical study on neutron-star cooling mechanism by using non-equilibrium Fermi gas superfluids

  Ōhashi, Yōji

  科学研究費補助金研究成果報告書 2018

• Theoretical studies on the realization of a p-wave superfluid fermi gas by using a synthetic gauge field

  Ohashi, Yoji

  科学研究費補助金研究成果報告書 2015

• Superfluid fluctuations in the BCS-BEC crossover regime of a trapped Fermi gas

  Ohashi, Yoji

  科学研究費補助金研究成果報告書 2012

Reviews, Commentaries, etc. 【 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

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Presentations 【 Display / hide 】

Presentations 【 Display / hide 】

• Non-equilibrium BCS-BEC crossover in a strongly interacting driven-dissipative Fermi gas: Part 1: Exotic Fermi superfluid of a driven-dissipative ultracold Fermi gas

  Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji Ohashi

  International Winter School on Open Quantum Many-Body Systems ( Evangelische Adademie in Tutzing (Germany)) , 

  2023.02

  , 

  Oral presentation (invited, special),  Open System Control of Atomic and Photonic Matter (OSCAR)

• Non-equilibrium BCS-BEC crossover in a strongly interacting driven-dissipative Fermi gas: Part 2: Non-equilibrium BCS-BEC crossover

  Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji Ohashi

  International Winter School on Open Quantum Many-Body Systems ( Evangelische Adademie in Tutzing (Germany)) , 

  2023.02

  , 

  Oral presentation (invited, special),  Open System Control of Atomic and Photonic Matter (OSCAR)

• Thermodynamics of a strongly-interacting Bose-Fermi mixture: Emergent deconfinement crossover and QCD-like phase diagram

  Koki Manabe, Yoji Ohashi

  第8回クラスター階層領域研究会 (大阪大学吹田キャンパス接合科学研究所、荒田記念館（ハイブリッド）) , 

  2023.02

  , 

  Oral presentation (invited, special), 科研費新学術領域「量子クラスターで読み解く物質の階層構造」

• 奇周波数クーパーペア波動関数の時空間構造

  岩﨑舜平, 真辺幸喜, 大橋洋士

  第８回クラスター階層領域研究会 (大阪大学吹田キャンパス接合科学研究所、荒田記念館) , 

  2023.02

  , 

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

• Odd-frequency Fermi superfuid in the BCS-BEC crossover region

  Shunpei Iwasaki, Koki Manabe, and Yoji Ohashi

  International symposium on Clustering as a Window on the Hierarchical Structure of Quantum Systems (CLUSHIQ2022) (Sendai-international Center, Sendai, Japan) , 

  2022.10

  -

  2022.11

  , 

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

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

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)

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Works 【 Display / hide 】

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 】

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]

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Courses Taught 【 Display / hide 】

Courses Taught 【 Display / hide 】

• THERMODYNAMICS AND STATISTICAL MECHANICS

  2022

• STATISTICAL MECHANICS 1

  2022

• PHYSICS D

  2022

• PHYSICS C

  2022

• LITERATURE OF PHYSICS

  2022

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Courses Previously Taught 【 Display / hide 】

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

Memberships in Academic Societies 【 Display / hide 】

Memberships in Academic Societies 【 Display / hide 】

• 日本物理学会, 

  1991.09

  -

  Present

Committee Experiences 【 Display / hide 】

Committee Experiences 【 Display / hide 】

• 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

• 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

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