Zhang Xiaoou, Shan Wen-Yu, Xiao Di
Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
Phys Rev Lett. 2018 Feb 16;120(7):077401. doi: 10.1103/PhysRevLett.120.077401.
We show that the exciton optical selection rule in gapped chiral fermion systems is governed by their winding number w, a topological quantity of the Bloch bands. Specifically, in a C_{N}-invariant chiral fermion system, the angular momentum of bright exciton states is given by w±1+nN with n being an integer. We demonstrate our theory by proposing two chiral fermion systems capable of hosting dark s-like excitons: gapped surface states of a topological crystalline insulator with C_{4} rotational symmetry and biased 3R-stacked MoS_{2} bilayers. In the latter case, we show that gating can be used to tune the s-like excitons from bright to dark by changing the winding number. Our theory thus provides a pathway to electrical control of optical transitions in two-dimensional material.
我们表明，带隙手性费米子系统中的激子光学选择规则由其缠绕数w决定，w是布洛赫能带的一个拓扑量。具体而言，在具有Cₙ对称性的手性费米子系统中，亮激子态的角动量由w±1 + nN给出，其中n为整数。我们通过提出两个能够容纳暗s类激子的手性费米子系统来证明我们的理论：具有C₄旋转对称性的拓扑晶体绝缘体的带隙表面态和偏置的3R堆叠MoS₂双层。在后一种情况下，我们表明通过改变缠绕数，门电压可用于将s类激子从亮态调谐到暗态。因此，我们的理论为二维材料中光学跃迁的电控制提供了一条途径。
