报告题目(Title)：Moiré exciton condensate: nonlinear Dirac point, broken-symmetry Bloch waves and unusual optical selection rules（莫尔激子凝聚：非线性狄拉克点，破坏对称性的布洛赫波和非平庸的光选择定则）
报告人(Speaker)：朱起忠 副研究员（华南师范大学物理与电信工程学院）
报告时间(Time)：2022年7月21日(周四) 14:00
报告地点(Place)：腾讯会议979-268-459
邀请人(Inviter)：张永平 教授
主办部门：理学院物理系
摘要(Abstract)：
Moiré exciton features tunable Dirac dispersion and spatially dependent optical selection rules. With the long lifetime due to its interlayer nature, it is promising to realize a Bose-Einstein condensation of moiré excitons. Here we study the properties of moiré exciton condensate within the mean-field theory, with special focus on exciton-exciton interaction effect on the nonlinear Bloch band and Bloch waves of exciton condensate in moiré potential. We find the nonlinear dispersion of the moiré exciton condensate exhibits complex loop structure induced by exciton-exciton interaction. A nonlinear Dirac cone emerges at Γ point of the Bloch band, with a three-fold degenerate nonlinear Dirac point. Each degenerate Bloch wave at nonlinear Dirac point spontaneously breaks C3 rotational symmetry of the moiré potential, and themselves differ by C3 rotations. Since they reside in the light cone, these nontrivial Bloch band structure and broken-symmetry Bloch waves can be experimentally detected by examining light emission from those states. Symmetry breaking of Bloch states implies unusual optical selection rules compared with single exciton case: moiré exciton condensate at Γ point emits light with all three components of polarization instead of only left or right circular polarization. We further propose that, by applying in-plane electric field on one layer to drive an initially optically dark exciton condensate towards light cone, the light polarization of final states as well as their dependence on field direction serves as the smoking gun for experimental observation.
