Flipping exciton angular momentum with chiral phonons in MoSe${}_2$/WSe${}_2$ heterobilayers

Abstract

Identifying quantum numbers to label elementary excitations is essential for the correct description of light-matter interaction in solids. In monolayer semiconducting transition metal dichalcogenides (TMDs) such as MoSe${}_2$ or WSe${}_2$, most optoelectronic phenomena are described well by labelling electron and hole states with the spin projection along the normal to the layer (S${}_z$). In contrast, for WSe${}_2$/MoSe${}_2$ interfaces recent experiments show that taking S${}_z$ as quantum number is not a good approximation, and spin mixing needs to be always considered. Here we argue that the correct quantum number for these systems is not S${}_z$, but the $z$-component of the total angular momentum -- J${}_z$ = L${}_z$ + S${}_z$ -- associated to the C${}_3$ rotational lattice symmetry, which assumes half-integer values corresponding modulo 3 to distinct states. We validate this conclusion experimentally through the observation of strong intervalley scattering mediated by chiral optical phonons that -- despite carrying angular momentum 1 -- cause resonant intervalley transitions of excitons, with an angular momentum difference of 2.

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