Tunable Phases of Moire Excitons in van der Waals Heterostructures

Stacking monolayers of transition metal dichalcogenides into a heterostructure with a finite twist-angle gives rise to artificial moire superlattices with a tunable periodicity. As a consequence, excitons experience a periodic potential, which can be exploited to tailor optoelectronic properties of these materials. Whereas recent experimental studies have confirmed twist-angle-dependent optical spectra, the microscopic origin of moire exciton resonances has not been fully clarified yet. Here, we combine first-principles calculations with the excitonic density matrix formalism to study transitions between different moire exciton phases and their impact on optical properties of the twisted MoSe2/WSe2 heterostructure. At angles smaller than 2 degrees, we find flat, moire-trapped states for inter- and intralayer excitons. This moire exciton phase changes into completely delocalized states at 3 degrees. We predict a linear and quadratic twist-angle dependence of excitonic resonances for the moire-trapped and delocalized exciton phases, respectively.