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 MoSe2 or WSe2, most optoelectronic phenomena are described well by labelling electron and hole states with the spin projection along the normal to the layer (Sz). In contrast, for WSe2/MoSe2 interfaces recent experiments show that taking Sz 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 Sz, but the z-component of the total angular momentum -- Jz = Lz + Sz -- associated to the C3 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.
@article{arxiv.2002.11997,
title = {Flipping exciton angular momentum with chiral phonons in MoSe$_2$/WSe$_2$ heterobilayers},
author = {A. Delhomme and D. Vaclavkova and A. Slobodeniuk and M. Orlita and M. Potemski and D. M. Basko and K. Watanabe and T. Taniguchi and D. Mauro and C. Barreteau and E. Giannini and A. F. Morpurgo and N. Ubrig and C. Faugeras},
journal= {arXiv preprint arXiv:2002.11997},
year = {2020}
}