2022 Fiscal Year Final Research Report

Mechanims of quantum liquid crysital states in strongly correlated electron systems

Research Project

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Project/Area Number	18H01175
Research Category	Grant-in-Aid for Scientific Research (B)
Allocation Type	Single-year Grants
Section	一般
Review Section	Basic Section 13030:Magnetism, superconductivity and strongly correlated systems-related
Research Institution	Nagoya University
Principal Investigator	Kontani Hiroshi 名古屋大学, 理学研究科, 教授 (90272533)
Co-Investigator(Kenkyū-buntansha)	土射津昌久奈良女子大学, 自然科学系, 准教授 (70362225)
Project Period (FY)	2018-04-01 – 2021-03-31
Keywords	量子干渉機構 / 鉄系超伝導体 / カゴメ金属 / 銅酸化物高温超伝導体 / ネマティック秩序 / バーテックス補正
Outline of Final Research Achievements	In strongly correlated electron systems, rich variety of quantum phase transitions has been discovered in the last decade, such as the nematic and smectic orders with broken rotational symmetry and the current orders with spontaneous spin- or charge-currents. To understand the origin of these transitions microscopically, the applicant has developed (1) the density-wave equation theory, which is equivalent to the extreme value condition of the thermodynamic potential, and (2) the derivation of optimized order parameter based on the functional renormalization group theory. Based on these theories, we explained the following novel quantum phase transitions. (a) Microscopic mechanism of "Davidian order" and superconducting state realized in kagome metal AV3Sb5. (b) Nematic ordering in twisted bilayer graphene and (c) Smectic order in iron-based superconductors. (d) Microscopic mechanism of current order in frustrated metals.
Free Research Field	物性理論
Academic Significance and Societal Importance of the Research Achievements	強相関電子系における多彩な量子相転移の研究は、実験技術の急速な進歩により爆発的に進展した一方で、単純な平均場近似では理解不可能であるため、理論の発展が急務であった。申請者の理論（密度波方程式理論および汎関数くりこみ群理論）により、平均場近似を超えた量子相転移の解析が可能になり、実験事実の解析や理論的予言が可能になったことは大変意義深い。本理論を具体的物質に適用し、カゴメ金属AV3Sb5のダビデ星型秩序および超伝導発現機構の特定、ねじれ2層グラフェンのネマティック秩序や鉄系超伝導体のスメクティック秩序の機構解明など、多岐に渡る量子液晶の研究を遂行し、当該分野の理論研究の指導的役割を果たした。