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Chiral Spin Density Wave and d+id Superconductivity in the Magic-Angle-Twisted Bilayer Graphene

Date: 2019-01-24
Time: 10:00
Venue: M830
Speaker: 杨帆



We model the newly synthesized magic-angle-twisted bilayer graphene superconductor with two px,y -like Wannier orbitals on the superstructure honeycomb lattice, where the hopping integrals are constructed via the Slater-Koster formulism by symmetry analysis. The characteristics exhibited in this simple model are well consistent with both the rigorous calculations and experiment observations. A van Hove singularity and Fermi-surface (FS) nesting are found in the doping levels relevant to the correlated insulator and unconventional superconductivity revealed experimentally, based on which we identify the two phases as weak-coupling FS instabilities. Then, with repulsive Hubbard interactions turned on, we performed random-phase-approximation based calculations to identify the electron instabilities. As a result, we find chiral d + id topological superconductivity bordering the correlated insulating state near half-filling, identified as noncoplanar chiral spin-density wave ordered state, featuring the quantum anomalous Hall effect. The phase diagram obtained in our approach is qualitatively consistent with experiments.

参考文献:Cheng-Cheng Liu, Li-Da Zhang, Wei-Qiang Chen, and Fan Yang, Phys. Rev. Lett. 121, 217001(2018).

个人简介:杨帆是北京理工大学物理学院教授。2002年北京大学物理学院博士毕业,2004年清华大学高等研究中心博士后出站后加入北理。主要研究方向为强关联电子系统和超导理论。曾研究高温超导和铁基超导机理和物性理论,以及量子自旋液体理论。近期重点关注在各类模型或实际材料中由于电子关联而导致的非常规超导和拓扑超导,以及掺杂的Mott绝缘体中可能出现的奇异超导态。在包括Phys. Rev. Lett在内的国际一流期刊发表论文多篇。