arXiv subjects

Christina Hill

Publications and source records attributed to Christina Hill.

4 recordsLinked to original sources

arXiv2026

Raman resonances mediated by excitonic polarons in BiVO$_4$

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.

cond-mat.mtrl-sci↗

arXiv2025

Band gap tuning by structural phase transition in Sm-substituted BiFeO3 powders

The substitution of bismuth by samarium in BiFeO3 is known to induce a structural phase transition from the polar phase to a non-polar phase, with a possible antiferroelectric intermediate structure. In this paper, we investigate the impact of this phase change on the optical properties. The optical band gap was measured by diffuse reflectance as a function of temperature for several samarium concentrations across the structural phase transition. We found that the optical band gap for each of the pure phases varies linearly with temperature and that the phase transitions are revealed by smooth transitions between those linear regimes. This allows us to quantify the contribution of the structural change in the optical absorption. We find that a difference in optical band gap of about 130meV can be attributed to the phase change. We anticipate that the same change could be obtained by applying an electric field in an antiferroelectric composition.

cond-mat.mtrl-sci↗

arXiv2023

Phonon-phonon coupling in bismuth vanadate over a large temperature range across the monoclinic phase

In this work we study phonon-phonon coupling in bismuth vanadate (BiVO4), known for its second-order transition involving a variety of coupling mechanisms. Using Raman spectroscopy as a probe, we identify two optical coupled phonon modes of the VO4 tetrahedron and study them by varying light polarization and temperature. The coupling manifests in non-Lorentzian line-shapes of Raman peaks and frequency shifts. We use theoretical framework of coupled damped harmonic oscillators to model the coupling and capture the phenomena in the temperature evolution of the coupling parameters. The coupling is negligible at temperatures below 100 K and later increases in magnitude with temperature until 400 K. The sign of the coupling parameter depends on the light polarization direction, causing either phonon attraction or repulsion. After 400 K the phonon-phonon coupling diminishes when approaching phase transition at which the phonon modes change their symmetry and the coupling is no longer allowed.

cond-mat.mtrl-sci↗

arXiv2020

Role of the ferroelastic strain in the optical absorption of BiVO4

Bismuth vanadate (BiVO4) has recently been under focus for its potential use in photocatalysis thanks to its well-suited absorption edge in the visible light range. Here, we characterize the optical absorption of a BiVO4 single crystal as a function of temperature and polarization direction by reflectance and transmittance spectroscopy. The optical band gap is found to be very sensitive to temperature, and to the monoclinic-to-tetragonal ferroelastic transition at 523K. The anisotropy, as measured by the difference in absorption edge for light polarized parallel and perpendicular to the principal axis, is reduced from 0.2 eV in the high-temperature tetragonal phase to 0.1 eV at ambient temperature. We show that this evolution is dominantly controlled by the ferroelastic shear strain. These findings provide a route for further optimization of bismuth-vanadate-based light absorbers in photocatalytic devices.

cond-mat.mtrl-sci↗