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, 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

  • Access to Document (DOI)

• 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 】

• Nuclear Spin-Lattice Relaxation Rate in Odd-Frequency Superconductivity

  Shumpei Iwasaki,Yoji Ohashi

  J. Phys. Soc. Jpn. (JPSJ)  94 ( 3 ) 034707(1-11) 2025.03

  Research paper (scientific journal), Joint Work, Last author, Accepted,  ISSN  00319015

  　View Summary

  We theoretically investigate the temperature dependence of nuclear spin–lattice relaxation rate T1-1 in bulk odd-frequency superconductivity. For a model odd-frequency pairing interaction, we first evaluate the superconducting order parameter, within the framework of the combined path-integral formalism with the saddle-point approximation. We then calculate T1-1 below the superconducting phase transition temperature Tc, to see how the odd-frequency pairing affects this physical quantity. In the odd-frequency p-wave state, while the so-called coherence peak is suppressed as in the even-frequency p-wave case, T1-1 is found to exhibit the Korringa-law-like behavior (T1-1 / T) except just below Tc, even without impurity scatterings. In the odd-frequency s-wave case, the behavior of T1-1 is found to be sensitive to the detailed spin structure of the superconducting order parameter: In a case, T1-1 is enhanced far below Tc, being in contrast to the conventional (even-frequency) s-wave BCS case, where the coherence peak appears just below Tc. We also show that the calculated T1-1 in the odd-frequency p-wave case well explains the recent experiment on CeRh0.5Ir0.5In5, where the possibility of odd-frequency p-wave superconductivity was recently suggested experimentally.

  • Access to Document (DOI)

• Equation of State of Quantum Cluster Matter

  Munekazu Horikoshi, Yoji Ohashi, and Daichi Kagamihara

  European Physical Journal A (Springer)   2025

  Accepted

• Engineering nonequilibrium superconducting phases in a voltage-driven superconductor under an external magnetic field

  Taira Kawamura, Yoji Ohashi

  Annalen der Physik (Wiley)   2025

  Accepted

• Nonequilibrium BCS-BEC crossover and unconventional FFLO superfluid in a strongly interacting driven-dissipative Fermi gas

  Taira Kawamura, Yoji Ohashi

  AAPPS Bulletin (AAPPS)  34 ( 1 ) 31(1-3) 2024.11

  Research paper (scientific journal), Joint Work, Accepted,  ISSN  02182203

  　View Summary

  We present a theoretical review of the recent progress in non-equilibrium BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover physics. As a paradigmatic example, we consider a strongly interacting driven-dissipative two-component Fermi gas where the non-equilibrium steady state is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. As a powerful theoretical tool to deal with this system, we employ the Schwinger-Keldysh Green’s function technique. We systematically evaluate the superfluid transition, as well as the single-particle properties, in the non-equilibrium BCS-BEC crossover region, by adjusting the chemical potential difference between the reservoirs and the strength of an s-wave pairing interaction associated with a Feshbach resonance. In the weak-coupling BCS side, the chemical potential difference is shown to imprint a two-step structure on the particle momentum distribution, leading to an anomalous enhancement of pseudogap, as well as the emergence of exotic Fulde-Ferrell-Larkin-Ovchinnikov-type superfluid instability. Since various non-equilibrium situations have recently been realized in ultracold Fermi gases, the theoretical understanding of non-equilibrium BCS-BEC crossover physics would become increasingly important in this research field.

  • Access to Document (DOI)

• Pairing properties of an odd-frequency superfluid Fermi gas

  Shumpei Iwasaki, Taira Kawamura, Koki Manabe, Yoji Ohashi

  arXiv:2402.02222 submitted to PRA (Physical Review A)  109 ( 6 ) 063309(1-15) 2024.06

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

  　View Summary

  We theoretically investigate strong-coupling properties of an odd-frequency Fermi superfluid. This pairing state has the unique property that Cooper pairs are formed between fermions, not at the same time, but at different times. To see whether or not such unequal-time pairs still exhibit bosonic behavior, we examine the space-time structure of the odd-frequency Cooper-pair wave function at T=0, by employing the combined path-integral formalism with the BCS-Eagles-Leggett-type superfluid theory. In the strong-coupling regime, the odd-frequency pair wave function still has different space-time structure from that in the ordinary even-frequency s-wave superfluid state; their magnitudes are found to become close to each other, except for the equal-time pairing component. In this regime, we also evaluate the superfluid phase transition temperature Tc within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink. The calculated Tc in the strong-coupling regime of the odd-frequency system is found to be well described by the Bose-Einstein condensation of tightly bound Bose molecules. Our results indicate that, in spite of vanishing equal-time pairing, odd-frequency Cooper pairs still behave like bosons in the strong-coupling regime, as in the even-frequency s-wave superfluid case.

  • Access to Document (DOI)

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

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

• Research to elucidate the formation process of strongly correlated many-body systems using high tunability of ultracold Fermi gas

  Ohashi, Yoji

  科学研究費補助金研究成果報告書 2021

• 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 】

• 室温超伝導の研究開発動向と実現の可能性

  大橋 洋士

  車載テクノロジー (（株）技術情報協会)   2024.04

  Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media), Single Work, Lead author, Last author, Corresponding author

• 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

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Presentations 【 Display / hide 】

Presentations 【 Display / hide 】

• Nuclear spin-lattice relaxation rate and ultrasound attenuation rate in odd-frequency superconductivity

  Shumpei Iwasaki and Yoji Ohashi

  30th International Conference on Low Temperature Physics (LT30) (Bilbao Exhibition Centre, Spain ) , 

  2025.08

  , 

  Poster presentation,  International Union of Pure and Applied Physics (IUPAP)

• Strong-Coupling Properties of a 2D-3D mixed dimensional Fermi atomic gas

  Haruka Takeda, Shumpei Iwasaki, and Yoji Ohashi

  30th International Conference on Low Temperature Physics (LT30) (Bilbao Exhibition Centre, Spain ) , 

  2025.08

  , 

  Poster presentation,  International Union of Pure and Applied Physics (IUPAP)

• BCS-rashbon BEC crossover in a superfluid gas of Fermi atoms with an anti-centrosymmetric spin-orbit interaction

  Yusuke Okajima, Koki Takemoto, Shumpei Iwasaki, and Yoji Ohashi

  30th International Conference on Low Temperature Physics (LT30) (Bilbao Exhibition Centre, Spain ) , 

  2025.08

  , 

  Poster presentation,  International Union of Pure and Applied Physics (IUPAP)

• Ultrasound attenuation in odd-frequency superconductivity

  岩崎舜平, 大橋洋士

  日本物理学会 (オンライン開催) , 

  2025.03

  , 

  Oral presentation (general), 日本物理学会

• The non-centrosymmetric superfluidity of a spin-orbit coupled ultracold Fermi gas

  Yusuke Okajima, Koki Takemoto, Shumpei Iwasaki, Yoji Ohashi

  日本物理学会 (オンライン開催) , 

  2025.03

  , 

  Oral presentation (general), 日本物理学会

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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

  2025

• STATISTICAL MECHANICS 1

  2025

• PHYSICS D

  2025

• PHYSICS C

  2025

• LITERATURE OF PHYSICS

  2025

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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

Media Coverage 【 Display / hide 】

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 】

Memberships in Academic Societies 【 Display / hide 】

• 日本物理学会, 

  1991.09

  -

  Present

Committee Experiences 【 Display / hide 】

Committee Experiences 【 Display / hide 】

• 2023.04

  -

  2024.03

  令和5年度知識集約型社会を支える人材育成委員会, 大橋洋士

• 2022.04

  -

  2023.03

  令和4年度知識集約型社会を支える人材育成委員会, 大橋洋士

• 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

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