主讲人：杨帆 教授（北京理工大学）

题目：Charge-4e SC and Chiral Metal Phases in the 45 degree-twisted bilayer cuprates and similar materials

时间：2023年08月07日（周一）上午10：00

地点：理工楼1226报告厅

联系人：钟寅

报告摘要：

The charge-4e superconductivity (SC) formed via condensation of electron quartets, characterized by half flux quantization, has attracted many interests recently. A previously proposed mechanism for the formation of this exotic type of SC is to emerge as the vestigial phase above the $T_c$ of a multi-component pairing state. Currently, its material realization is a big challenge. Here we propose that the charge-4e SC, in together with the chiral metal, can emerge as the vestigial phases above the chiral topological SC (TSC) ground state in the twisted-bilayer quasicrystals (TB-QC).The TB-QC represents a class of materials made through stacking a homo-bilayer with the largest twist angle, exampled by the 45 degree-twisted bilayer cuprates and 30 degree-twisted bilayer graphene. When each mononlayer hosts a pairing state with the largest pairing angular momentum, e.g. $d$-wave for the cuprates or $f$-wave for some members in the graphene family, our combined Ginzburg-Landau theory and microscopic calculations reveal

that the second-order interlayer Josephson coupling (IJC) would drive $d+id$ or $f+if$ chiral TSC in the TB-QC. Further more, above the $T_c$ of the chiral TSC, either the total- or relative- pairing phase of the two layers can be unilateral quasi- ordered or ordered. In the form case, a Cooper pair from the top layer pairs with a Cooper pair from the bottom layer to form the charge-4e SC; in the latter case, a time-reversal symmetry breaking chiral metal phase is formed. Based on a thorough symmetry analysis, we arrive at the low-energy effective Hamiltonian describing the pairing- phase fluctuations. Our combined renormalization group and Monte-Carlo studies reveal the presence of the charge-4e SC and chiral metal phases in certain regimes in the phase diagram. These vestigial phases are characterized by various temperature-dependent quantities and spatial-dependent correlations.

个人简介：

Fan Yang is a professor of physics in Beijing Institute of Technology.He got the Ph.D of physics from Peking University in 2002, and afterwards became post doctor at the Institute of Advanced Study in Tsinghua University. He joined Beijing Institute of Technology in 2004, and was promoted as full professor in 2013.Fan Yang's research area is theoretical strongly-correlated systems and superconductivity.He has contributed in the research areas including the high-temeperature cuprates superconductors, the iron-based superconductors, the twisted-bilayer graphene and the quantum spin liquid.