Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides

Abstract

In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I=2 for excitons inside the light cone, that is, bright excitons. Under moderate strain, the I=2 Dirac cone splits into two degenerate I=1 Dirac cones, and saddle points with a linear Dirac spectrum emerge. After binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton’s optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits and mechanical control of valley pseudospin. It has been show that monolayer transition metal dichalcogenides exhibit tightly bound excitons with a valley pseudospin that can be optically addressed by polarization selection rules. Here, the authors show the valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange.