ELECTRON-PHONON INTERACTIONS AND RESONANT MULTIPHONON SCATTERING IN HEMATITE (Invited Presentation)

Hematite (α-Fe₂O₃) is a ubiquitous, naturally-occurring semiconductor with a central role in global geochemical cycles and significant promise for photocatalysis and photovoltaic technology. First-row transition metal oxide semiconductors, like hematite, regularly possess strong electron-phonon interactions that limit charge transport. These interactions can be probed via Raman spectroscopy, providing insight into how these materials move electrons in natural and engineered systems. Electron-phonon interactions additionally govern resonant Raman scattering in semiconductors, where the excitation energy is near the band gap energy. Resonance Raman spectroscopy can thus be used to study electron-phonon interactions, and an understanding of electron-phonon interactions can, in turn, elucidate details about the Raman spectrum itself. Despite decades of study, hematite still possesses puzzling resonance behavior and several Raman bands outside of those predicted by group theory, some of which are unassigned.

In this work, we report the effects of electron-phonon interactions on hematite's Raman spectrum through polarized, resonant, and temperature-dependent Raman spectroscopy. Resonant conditions give rise to the forbidden 1LO band, up to fourth-order LO overtones, and previously unassigned multiphonon bands at 820, 1050, and 1100 cm^-1. For the first time, 3LO and 4LO phonons are reported in hematite, showing that a series of 1-4 LO phonons is activated near the band gap, potentially offering a new method to study radiative or nonradiative recombination pathways. The symmetry and resonance behavior of the induced Raman bands confirms that long-range electron-phonon interactions called Fröhlich interactions are the source of the resonance enhancement. Our findings also show that the putative one magnon (1M) mode at 820 cm^-1 is inconsistent with a 1M assignment through symmetrical polarization behavior and weak temperature dependence, in agreement with isotopically substituted hematite spectra [1]. Comparison to the calculated two-phonon density of states (DOS) substantiated our experimental data and allowed for proposed band assignments of the second-order modes.

References

[1] Massey, M. J., Baier, U., Merlin, R., & Weber, W. H. 1990. Phys Rev B, 41(11), p.7822.