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In monolayer transition metal dichalcogenides, tightly bound excitons havebeen discovered with a valley pseudospin that can be optically addressedthrough polarization selection rules. Here, we show that this valley pseudospinis strongly coupled to the exciton center-of-mass motion through electron-holeexchange. This coupling realizes a massless Dirac cone with chirality index I=2for excitons inside the light cone, i.e. bright excitons. Under moderatestrain, the I=2 Dirac cone splits into two degenerate I=1 Dirac cones, andsaddle points with a linear Dirac spectrum emerge in the bright excitondispersion. Interestingly, after binding an extra electron, the charged excitonbecomes a massive Dirac particle associated with a large valley Hall effectprotected from intervalley scattering. Our results point to uniqueopportunities to study Dirac physics, with exciton's optical addressability atspecifiable momentum, energy and pseudospin. The strain-tunable valley-orbitcoupling also implies new structures of exciton condensates, newfunctionalities of excitonic circuits, and possibilities for mechanical controlof valley pseudospin.

Bright excitons in monolayer transition metal dichalcogenides: from Dirac cones to Dirac saddle points