QLC Young Colloquium

The 1st QLC young colloquium

Date & Time : Friday, February 26, 2021. 13:30~14:30 (online)
Speaker:Kousuke ISHIDA (University of Tokyo), Rina TAZAI (Nagoya University), winners of the 1st QLC Young Researcher Award.
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*Zoom information will be sent on the day of this colloquium to those who registered by noon on the day of the event (Japan Time).

Speaker:Kousuke ISHIDA (University of Tokyo)
Title:Non-magnetic nematic quantum criticality in FeSe1-xTex
 Our understanding of electronic nematic phases, one of the quantum liquid crystal phases which was initially discussed in high-temperature cuprate superconductors and quantum hall systems, has been considerably improved through the extensive studies on iron-based superconductors. This phase is characterized by the spontaneous rotational symmetry breaking of the underlying crystal lattice without further breaking the translational symmetry. It has been established that the electronic nematic phase coexists with the superconductivity in the phase diagram of many iron-based superconductors, capturing the interest on the relationship between them. In particular, FeSe is an ideal compound to deepen our understanding of electronic nematic phase and their impact on superconductivity because unlike many other iron-based superconductors, it shows a nematic transition without static magnetism.
 In this seminar, I will show our research on the electronic nematic phase of FeSe1-xTex, in which nematic transition temperatures can be tuned by partial substitutions of Te for Se sites. Recently, our group has succeeded in synthesizing the single crystals of FeSe1-xTex with several Te compositions using a chemical vapor transport technique. The elastoresistivity measurements for the obtained FeSe1-xTex single crystals have revealed that its nematic transition temperature is continuously suppressed to zero by increasing Te substitution, evidencing for a nematic quantum critical point inside superconducting dome of FeSe1-xTex. Remarkably, superconducting transition temperature has been found to show an increase toward the nematic critical point, implying their intimate relation.

Speaker:Rina TAZAI (Nagoya University)
Title: Theoretical Study of Microscopic Mechanism and Thermodynamic Properties of QLC Phase
 近年、強相関電子系における「量子ネマティック相」が様々な実験で報告され注目されている。一方、発現機構や相転移に伴う熱力学的性質は未解決であり実験・理論両面からの進展が望まれる。本講演では初めに、量子ネマティック相の発現を記述するための理論的枠組みとして最近開発した、「構造因子を最適化するくりこみ群法」と「線形DW方程式」について簡単に説明する[1]。本理論を擬1次元系のハバード模型に適用した結果、「単位胞内の電荷ループ相」が発現することを見出した。加えて、磁気フラストレーションによる磁気揺らぎの抑制が 電荷ループ相の発現に重要な役割を果たすことを見出した。
 更に本研究では、Luttinger-Ward理論とGL理論を併せた「量子ネマティック転移 に伴う熱力学関数の理論的計算手法」を構築しその検証を行った[2]。この結果、ネマティック転移に伴う熱力学量の温度・空間依存性がBCS型の超伝導転移とは異なることを説明した。本理論を推し進めることで、ネマティック転移の発現機構~その特徴的な振る舞いまでを、統一の理論で説明できると考えられる。 当日は、具体的な鉄系や銅酸化物での熱力学量の振る舞いについても議論する予定である。
[1] R. Tazai et al., arXiv:2010.16109 (2020), [2] R. Tazai et al., in preparation

Committee Chair:Hiroki WADATI(University of Hyogo)