Charge excitations in resonant Raman scattering due to inversion breaking spin-orbit coupling

Raman spectroscopy has been used extensively to study spin and charge collective excitations of various quantum systems. The observed spectrum can be decomposed into excitations with symmetries described by the irreducible representations (irreps) of the point group of the system. One excitation of interest in the fully symmetric irrep is the plasmon which are longitudinal oscillations of charge density in the system. In the fully symmetric irrep, the plasmon only couples to the Raman probe with a weight of q², where q in the momentum imparted by the photon to the system during the inelastic scattering. Due to this q² suppression, one needed to couple plasmons to phonons (phonon-plasmon-polariton) or use gratings to induce larger quasi-momentum to observe the plasmons. In this work, we argue that for low density systems, with strong inversion breaking spin-orbit coupling, it is possible to couple to the plasmons without the q² suppression if we tune the incident photon energy to resonate between bands that are split due to an inter-band SOC. We list the conditions under which this effect is observable and discuss the implications for the self-doped semiconductor BiTeI.