报告题目：Anomalous Coherence Length in Superconductors with Quantum Metric 

报告人：陈帅 研究助理教授（香港科技大学）

报告时间：2023年10月20日（星期五）14：00-15：30

报告地点：北洋园校区 49 教 210 室（腾讯会议：516-8729-1333）

报告摘要：

        Recent experimental study unveiled highly unconventional phenomena in the superconducting twisted bilayer graphene (TBG) with ultra flat bands, which cannot be described by the conventional BCS theory. For example, given the small Fermi velocity of the flat bands, the superconducting coherence length predicted by BCS theory is more than 20 times shorter than the measured values. A new theory is needed to understand many of the unconventional properties of flat band superconductors. In this work, we establish a Ginzburg-Landau (GL) theory from a microscopic flat band Hamiltonian. The GL theory shows how the properties of the physical quantities such as the critical temperature, the superconducting coherence length, the upper critical field and the superfluid density are governed by the quantum metric of the Bloch states. One key conclusion is that the superconducting coherence length is not determined by the Fermi velocity but by the size of the optimally localized Wannier functions which is limited by quantum metric. We also confirm the GL theory by a microscopic mean-field theory. Applying the theory to TBG, we calculated the superconducting coherence length and the upper critical fields. The results match the experimental ones well without fine-tuning of parameters. The established GL theory provides a new and general theoretical framework for understanding flat band superconductors with quantum metric. 

报告人简介：

      陈帅于2016年从天津大学获得学士学位，并于2021年在清华大学高等研究院完成了他的博士学位。目前，他是香港科技大学的研究助理教授，主要关注非常规超导体和量子多体系统中的量子几何效应。他的研究兴趣还包括掺杂莫特物理、高温超导体。

参考文献：

[1] Shuai Chen, KT Law, arXiv:2303.15504.

[2] J.-X. Hu, Shuai Chen, KT Law arXiv: 2308.05686.