时间：2023年04月20日（星期四）上午10:30

地点：国科大中关村校区7号楼401会议室

报告人：陈帅 (HKSTU)

摘要：

Superconductivity in flat-band systems, such as twisted bilayer graphene, exhibits unconventional behaviours which cannot be described by the BCS theory. In this work, we derive an effective Lagrangian from a microscopic flat-band Hamiltonian which allows us to establish a Ginzburg-Landau (GL) theory that includes the quantum geometric effects in flat-band superconductors. First of all, we deduce the critical temperature of a flat-band superconductor within the mean-field description which is determined by the quantum metric. Secondly, going beyond the mean-field approximation by taking into account the fluctuations of the order parameter, we determine the superfluid weight, superconducting coherent length, and the upper critical field and their dependence on the quantum metric of the flat bands. Thirdly, we apply the GL to twisted bilayer graphene (TBG). By calculating the quantum metric of flat bands of TBG, the GL theory allows the estimation of the superconducting coherence length and the upper critical field. The results of the superconducting coherence length and upper critical field match the experimental results incredibly well without the fine tuning of parameters. The GL theory with quantum metric provides a new and general theoretical understanding of unconventional behaviors of flat-band superconductors.

报告人简介：

本科毕业天津大学物理系，2021年博士毕业于清华大学高等研究院，获理论物理学博士学位。现在香港科技大学做博士后。主要研究方向是强关联物理，高温超导，和非传统超导理论。