Details of a Researcher - Ohashi, Yoji

Ohashi, Yoji

写真a

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

Position: Professor

Related Websites

http://www.phys.keio.ac.jp/faculty/yohashi/index-jp.html

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）

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

Papers 【 Display / hide 】

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)
Emergence of Larkin-Ovchinnikov-type superconducting state in a voltage-driven superconductor

Taira Kawamura, Yoji Ohashi, H. T. C. Stoof

Physical Review B （American Physical Society) 109 （ 10 ） 104502(1-19) 2024.03

Research paper (scientific journal), Joint Work, Accepted, ISSN 24699950

　View Summary

We theoretically investigate a voltage-biased normal metal-superconductor-normal metal (NSN) junction. Using the nonequilibrium Green's function technique, we derive a quantum kinetic equation to determine the superconducting order parameter self-consistently. The derived equation is an integral-differential equation with memory effects. We solve this equation by converting it into a system of ordinary differential equations with the use of a pole expansion of the Fermi-Dirac function. When the applied voltage exceeds the critical value, the superconductor switches to the normal state. We find that when the voltage is decreased from the normal phase, the system relaxes to a Larkin-Ovchinnikov-type (LO) inhomogeneous superconducting state, even in the absence of a magnetic Zeeman field. We point out that the emergence of the LO-type state can be attributed to the nonequilibrium energy distribution of electrons due to the bias voltage. We also point out that the system exhibits bistability, which leads to hysteresis in the voltage-current characteristic of the NSN junction.
- Access to Document (DOI)
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.
- Access to Document (DOI)
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.
- 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

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.
- Access to Document (DOI)

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 】

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

Presentations 【 Display / hide 】

2次元ー3次元混合フェルミ原子気体に対する強結合効果

武田遥、岩崎舜平、大橋洋士

日本物理学会 (北海道大学札幌キャンパス) ,

2024.09
,
Oral presentation (general)
奇周波数超伝導の核-スピン格子緩和率1/T_1

岩崎舜平、大橋洋士

日本物理学会 (北海道大学札幌キャンパス) ,

2024.09
,
Oral presentation (general)
Specific heat of spin-orbit coupled ultracold Fermi gas and effects of rashbon bound states II

岡嶋佑典, 竹本功貴, 岩崎舜平, 大橋洋士

日本物理学会 (北海道大学札幌キャンパス) ,

2024.09
,
Oral presentation (general)
スピン軌道相互作用を有する冷却フェルミ原子気体におけるrashbonと熱解離の効果

岡嶋佑典, 竹本功貴, 岩崎舜平, 大橋洋士

熱場の量子論とその応用 (京都大学基礎物理学研究所パナソニックホール) ,

2024.09
,
Poster presentation
Emergence of an inhomogeneous superfluid state in a driven-dissipative two-dimensional Fermi condensate

Taira Kawamura, Yoji Ohashi, and Henk Stoof

Minicolloquium MC8 "Collective and nonlinear phenomena in confined quantum systems” in the 31th General Conference of the Condensed Matter Division of the European Physical Society CMD31 (NH Conference Centre Koningshof, The Netherlands) ,

2024.09
,
Oral presentation (invited, special), European Physical Society

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

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
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
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)

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

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]

Courses Taught 【 Display / hide 】

THERMODYNAMICS AND STATISTICAL MECHANICS

2024
STATISTICAL MECHANICS 1

2024
PHYSICS D

2024
PHYSICS C

2024
LITERATURE OF PHYSICS

2024

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 】

2022.04

-

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
2021.04

-

2022.03

令和3年度知識集約型社会を支える人材育成委員会, 大橋洋士