Why polar excitons stay sharp: parity protection of the center-of-mass recoil channel in exciton-phonon scattering
Standard treatments fail to explain sharp excitonic resonances in polar semiconductors because they ignore a crucial center-of-mass recoil degree of freedom. Restoring this degree of freedom reveals an open absorption channel controlled by electron-hole interference. This channel is protected by parity for symmetric excitons, while mass asymmetry dictates the linewidth, leading to new design rules for coherent materials.