Sudden disappearance of a bright exciton in solid helium

Abstract

Excitons, bound electron-hole pairs, are investigated extensively in optical spectra, where their wave vector is considered to be vanishing. Excitonic spectra as a function of wave vector are much less explored, both theoretically and with measurements of the loss function. Here, we uncover a striking angular dependence of excitonic spectra for the prototypical case of solid helium, where a prominent excitonic peak is completely suppressed for a momentum transfer in certain directions. The effect is entirely due to the electron-hole exchange interaction. This phenomenon leads to a counter-intuitive effect close to Bragg points, where an infinitesimal change of direction of wave vector is sufficient to change the spectrum dramatically. We explain in which way the nature of excitons makes such a scenario possible. We discuss the pressure dependence, point to experimental evidence for our findings, and predict under which circumstances dramatic angular dependencies may also be found in other materials.