物性セミナー/2019-6

2019-6-28

2019年 夏学期 第6回 物性セミナー

講師 横川 大輔 氏 (東大総合文化)

題目 積分方程式理論を用いた溶媒和の理解

日時 2019年 6月 28日(金) 午後4時50分

場所 16号館 827

アブストラクト

溶液中での化学現象を理論的に解析する手法は、これまでにいくつも提案されている。最も有名な手法として、各分子を構成する原子個々の運動方程式を解く分子動力学法が挙げられる。この手法は、用いる式のシンプルさから世界中で様々な系に対して用いられている。これとは別に、原子個々の位置・速度情報を得るのではなく、分子間の距離に関する分布に着目する積分方程式理論というものがある。この積分方程式理論では、統計力学に基づき導かれた連立方程式を解くことにより分布を解析的に得る。そのため積分方程式理論は、溶媒和自由エネルギーなどの熱力学量を解析的に計算することができ、分子動力学法で抱えるサンプリングの問題を克服できる可能性を秘めている。本セミナーでは、積分方程式理論の中で最も有名なReference Interaction Site Model(RISM)[1]について紹介し、RISM法の長所と短所について説明する。さらに、その短所を克服できる可能性について、過去の研究[2]を紹介しながら説明する。

[1] Hansen, J. P.; McDonald, I. R. Theory of Simple Liquids, 2nd ed.; Academic: London, 1986.

[2] Yokogawa, D. Toward Accurate Solvation Free Energy Calculation with the Reference Interaction Site Model Self-Consistent Field: Introduction of a New Bridge Function. J. Chem. Theory Comput. 2018, 14, 3272−3278.

宣伝用ビラ

KMB20190628.pdf(196)

物性セミナーのページ

http://park.itc.u-tokyo.ac.jp/KMBseminar/wiki.cgi/BusseiSeminar

2019-6-21

2019年 夏学期 第5回 物性セミナー

講師 孫 悦 (Sun Yue) 氏 (青山学院大学 理工)

題目 Probing the superconducting gap structure and nematicity by angle-resolved specific heat

日時 2019年 6月 21日(金) 午後4時50分

場所 16号館 827

アブストラクト

The gap structures of some unconventional superconductors such high-Tc cuprates, iron based superconductors, and heavy-fermion materials contain nodes or gap minima, which are directly related to their pairing mechanism. On the other hand, the electronic nematic phases, which break the rotational symmetry, e.g. the 2-fold electronic system observed in the tetragonal FeSe, have been proposed in many unconventional superconductors. The relation between nematicity and superconductivity is a major unsolved problem in condensed matter physics. To study the gap structure and nematicity, a space-resolved technique is required. The angle-resolved specific heat (ARSH) measurement not only has the space-resolution, but also probes the quasi-partial in bulk, which is ideal for studying both the superconducting gap structure and the nematicity.

In this seminar, I will introduce the study of gap structure and nematicity by ARSH measurements based on three examples. The first one is the FeSe, which attracts much attention due to its intriguing properties including the unexpected high Tc (over 60 K) in the monolayer thin film, a nematic state without long-range magnetic order, and a crossover from BCS to BEC. Our ARSH study on the high-quality FeSe reveals a small gap with two vertical-line nodes or gap minima along the kz–direction [1]. Such symmetry-unprotected nodes or gap minima are found to be smeared out by a small amount of disorder, and the gap becomes isotropic in the sample of lower quality. Our study reveals that the reported controversy on the gap structure of FeSe is due to the disorder-sensitive nodelike small gap [2]. The second example is the SrxBi2Se3, which is a topological superconductor with Tc ~ 3.2 K. By ARSH measurements, we report bulk quasi-particle evidence of nematicity in the topological superconductor SrxBi2Se3. The specific heat exhibits a clear 2-fold symmetry despite the 3-fold symmetric lattice. Most importantly, the 2-fold symmetry appears in the normal state above Tc. Such results highlight the interrelation between nematicity and unconventional superconductivity [3]. The last example is the PbTaSe2, which is reported to be a topological nodal-line semimetal with Tc ~ 3.7 K. Our ARSH results combined with the angle-dependent measurements of resistivity and point-contact found out the nematic superconductivity on the topological surface state of PbTaSe2 despite its isotropic bulk gap structure [4].

[1] Y. Sun et al., Phys. Rev. B 96, 220505(R) (2017)

[2] Y. Sun et al., Phys. Rev. B 98, 064505 (2018)

[3] Y. Sun et al., Phys. Rev. Lett. in press (arXiv:1902.08903)

[4] T. Le, Y. Sun et al., arXiv:1905.11177 (2019)

宣伝用ビラ

KMB20190621.pdf(248)

物性セミナーのページ

http://park.itc.u-tokyo.ac.jp/KMBseminar/wiki.cgi/BusseiSeminar