Raman resonances mediated by excitonic polarons in BiVO$_4$

TL;DR

This study uses resonant Raman spectroscopy to investigate excitonic polarons in BiVO4, revealing two optical resonances and demonstrating the technique's effectiveness in probing quasiparticles in oxide materials.

Contribution

It introduces resonant Raman spectroscopy as a novel method to probe excitonic polarons and their coupling in BiVO4, highlighting two distinct optical resonances associated with excitons and polarons.

Findings

Identification of two optical resonances at 1.94 eV and 2.45 eV.

Observation of anisotropy in free excitons at high energy.

Detection of excitonic polaron resonance with strong exciton-phonon coupling.

Abstract

Excitonic polarons are quasiparticles formed by a Coulomb-bound electron-hole pair with strong coupling to lattice vibrations. Despite high fundamental interest in excitonic polarons, the experimental investigation of these particles remains challenging. In this work, we exploit the resonant Raman effect to probe the excitonic polarons in bismuth vanadate. We track enhancement of Raman modes as a function of excitation energy and reveal two optical resonances: one inside the band gap at 1.94 eV and another one near the optical absorption edge at 2.45 eV. The high-energy resonance originates from free excitons, which exhibit a characteristic 40 meV anisotropy between polarizations parallel and perpendicular to the c axis. Remarkably, the low-energy resonance shows no contrast in the optical absorption spectra. We attribute this resonance to an excitonic polaron formed through strong exciton-phonon coupling, making excitonic and excitonic polaron Raman resonances similar in strength. We probe the energy level of the excitonic polaron and compare its coupling strength to the different vibrational modes. Our results establish resonant Raman spectroscopy as a unique and powerful tool for probing quasiparticles of polaronic and excitonic nature in oxide materials.