Intervalley excitonic hybridization, optical selection rules, and imperfect circular dichroism in monolayer hBN*

We perform first-principles GW plus Bethe-Salpeter equation calculations to investigate the photophysics of monolayer hexagonal boron nitride (hBN), revealing excitons with novel k-space characteristics. The excitonic states forming the first and third peaks in its absorption spectrum are s-like, but those of the second peak is notably p-like, a first finding of strong co-occurrence of bright s-like and bright p-like states in an intrinsic 2D material. Moreover, even though the k-space wavefunction of these excitonic states are centered at the K- and K'-valleys like in monolayer transition metal dichalcogenides, the envelope functions of the basis excitons at one valley have significant extents to the basin of the other valley. As a consequence, the optical response of monolayer hBN exhibits a lack of circular dichroism, as well as a coupling that induces an intervalley mixing between s- and p-like states.