报告题目：Chiral TSC, Charge-4e SC and Chiral Metal Phases in the 45 degree-twisted bilayer cuprates and similar materials.
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 (SC). In the past decades, he was focused on such research areas as the cuprate superconductors, the iron-based superconductors, the quantum spin liquid and its doping, electron-correlation driven physics in the twisted bilayer graphene and quasicrystals. Fan Yang has published over 60 papers, with citations over 2000.
The charge-4e superconductivity (SC) formed via condensation of electron quartets, characterized by half flux quantization, has attracted many interests recently. 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 [2,3]. 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. These vestigial phases are characterized by various temperature-dependent quantities and spatial-dependent correlations, revealed by our combined RG and MC calculations.
 Yu-Bo Liu, Jing Zhou, Congjun Wu and Fan Yang, arXiv: 2301.06357.
 Yu-Bo Liu, Yongyou Zhang, Wei-Qiang Chen and Fan Yang, Phys. Rev. B 107, 014501 (2023).
 Yu-Bo Liu, Jing Zhou, Yongyou Zhang, Wei-Qiang Chen and Fan Yang, arXiv: 2301.07553.