Excitons as massless and massive Dirac particles in monolayer transition metal dichalcogenides*

In monolayer transition metal dichalcogenides, tightly bound excitons can form at +-K valleys, where optical generation of excitonic valley polarization and coherence can be realized through a polarization selection rule. Here, we show that the the electron-hole Coulomb exchange leads to the strong coupling between the valley pseudospin of bright exciton and its motion. In the light cone, the exciton dispersion exhibits a massless Dirac cone with chirality index I=2. Moderate tensile strain provides a powerful approach to tune the exciton dispersion. When the exciton binds an electron to form a negatively charged trion, the exchange interaction with the excess electron opens up a gap and the trion behaves as a massive Dirac particle. With the optical addressability at specifiable momentum and energy, excitons in monolayer transition metal dichalcogenides may provide unique opportunities to study Dirac particles.