Spin-Mediated Direct Photon Scattering by Plasmons in BiTeI

TL;DR

This study demonstrates that in a giant Rashba system, bulk plasmons can directly couple to Raman scattering even at long wavelengths, revealing a sharp in-gap plasmon mode enabled by spin-orbit coupling and resonant photon excitation.

Contribution

It provides the first experimental evidence of direct plasmon-Raman coupling in a 3D Rashba system without phonon assistance, challenging conventional theory.

Findings

Observation of a sharp in-gap plasmon mode in BiTeI

Direct coupling of plasmons to Raman response at long wavelengths

Resonance with Rashba-split states enables this coupling

Abstract

We use polarization resolved Raman spectroscopy to demonstrate that for a 3D giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasi-momentum and thus negligibly weak in the Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of BiTeI below the Rashba continuum. This coupling, in a polar system with spin-orbit coupling, occurs without assistance from phonons when the incoming photon excitation is resonant with Rashba-split intermediate states. We discuss the distinctive features of BiTeI's giant Rashba system band structure that enable the direct observation of plasmon in Raman scattering.