Direct Photon Scattering by Plasmons in BiTeI

Abstract

We use polarization resolved Raman spectroscopy to show that for 3D giant Rashba system the bulk plasmon collective mode directly couples to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit although the standard theory predicts that the plasmon spectral weight should scale as the square of its quasi-momentum and hence be negligibly weak in the Raman spectra. Such plasmon coupling to the Raman response at $\mathbf q \rightarrow 0$ arises for a polar system with spin-orbit coupling when the incoming photon excitation is turned to a resonance with Rashba-split intermediates states involved in the resonant Raman process. As an example, we identify special features of BiTeI's chiral band structure that enable the appearance of plasmon mode in the Raman spectrum