title: 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.
authors.affiliation: Department of Physics and Center of Theoretical and Computational Physics, The University of Hong Kong, Hong Kong, China.
authors.forename: Hongyi
authors.initials: H
authors.lastname: Yu
authors.affiliation: 1] Department of Physics and Center of Theoretical and Computational Physics, The University of Hong Kong, Hong Kong, China [2] School of Physics, Beijing Institute of Technology, 100081 Beijing, China.
authors.forename: Gui-Bin
authors.initials: GB
authors.lastname: Liu
authors.affiliation: Department of Physics and Center of Theoretical and Computational Physics, The University of Hong Kong, Hong Kong, China.
authors.forename: Pu
authors.initials: P
authors.lastname: Gong
authors.affiliation: 1] Department of Physics, University of Washington, Seattle, Washington 98195, USA [2] Department of Material Science and Engineering, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Xu
authors.affiliation: Department of Physics and Center of Theoretical and Computational Physics, The University of Hong Kong, Hong Kong, China.
authors.forename: Wang
authors.initials: W
authors.lastname: Yao