Electronic Raman scattering in materials with strong spin-orbit coupling*

Polarization resolved Raman spectroscopy is used extensively to study spin and charge collective excitations in solids. Electronic Raman scattering in the fully symmetric channel couples to charge excitations including the plasmons which are longitudinal oscillations of charge density in the system. However, it is well known that the spectral weight of a plasmon in Raman response is typically suppressed by the smallness of q² pre-factor, where q is the momentum imparted by the photon to the plasmon. Because the smallness of q, researchers have coupled plasmons to polar phonons (phonon-plasmon-polaritons) or use periodic structures at microscopic scale (gratings) to induce a larger quasi-momentum to make plasmons observable.

We will show in this talk that for inversion broken systems which allow Rashba type spin-orbit interaction, there is an additional resonant Raman coupling process that results in plasmon spectral weight which scales with the strength of the spin-orbit interaction, and which is present even in the long-wavelength q=0 limit without any help from phonons. We demonstrate that direct purely electronic resonant Raman coupling to plasmon for giant Rashba systems could be significant. We discuss our result in the context of recent observation in BiTeI where strong and sharp plasmon below the Rashba continuum of single particle excitations was observed [1-3].