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Electron-phonon interactions and resonant multiphonon scattering in α - Fe2O3
Dufresne, William
Dufresne, William
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Abstract
Hematite's ubiquity in natural redox processes and society's growing interest in it for solar fuel production calls for an improved understanding of the phonon-coupled optical transitions that impact its charge transport. The strong electron-phonon interactions that trap carriers and produce unusual resonance Raman phenomena in hematite have yet to be characterized. Additionally, hematite possesses five bands outside of those predicted by group theory, the origins of which are long-debated or ignored altogether. These modes appear at approximately 660, 820, 1050, 1100, and 1320 cm\textsuperscript{-1}. The 660 and 1320 cm\textsuperscript{-1} bands are currently understood as IR E\textsubscript{u} (1LO) and 2E\textsubscript{u} (2LO) bands. The 820 cm\textsuperscript{-1} band is loosely assigned to a one-magnon (1M) mode from inelastic neutron scattering experiments that show hematite’s spin-wave curve crossing the Brillouin zone center at ca. 820 cm\textsuperscript{-1}, but no Raman work has been done to assign this as a magnon conclusively. Lastly, the bands at 1050 cm\textsuperscript{-1} and 1100 cm\textsuperscript{-1} remain unassigned and are largely ignored or attributed to impurities. Given that Raman-forbidden LO modes are induced by Fröhlich interactions in several polar metal oxides, Raman spectroscopy can be used to simultaneously investigate the nature of the electron-phonon interactions and the origins of forbidden modes. In this thesis, we employed conventional, resonance, polarized, and temperature-dependent Raman spectroscopy on single-crystal hematite to characterize the resonance enhancement mechanism and assign previously unresolved bands. Polarized Raman measurements showed that the 1LO and 2LO modes are induced and resonantly enhanced by Fröhlich interactions. Through literature comparison, 1LO mode is likely dominated by extrinsic Fröhlich scattering, while the 2LO was reliably attributed to intrinsic Fröhlich interactions. Resonant Raman spectra revealed third- and fourth-order LO modes characteristic of Fröhlich scattering. The electron-phonon coupling strength was calculated and was consistent with other polar single-crystal semiconductors. The 820 cm\textsuperscript{-1} band was inconsistent with a 1M assignment through symmetrical polarization behavior and weak temperature dependence. The 820, 1050, and 1100 cm\textsuperscript{-1} bands showed similar excitation energy dependence as the 1LO mode in resonance Raman measurements and were only active in parallel polarization configurations. These measurements, supported by DFT-calculated two phonon density-of-states, show that the modes are LO overtones induced by Fröhlich interactions. The 1100 cm\textsuperscript{-1} was tentatively assigned as a LO+TO combination through comparison with density functional theory (DFT) calculations. Simple group theoretical analysis was applied to investigate potential Raman-allowed overtones, but the number of possible processes across the Brillouin zone prevented unambiguous assignment to specific symmetry processes.
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Date
2022-12-31
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University of Kansas
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Keywords
Mineralogy, Materials Science, Physical chemistry, Hematite, Phonons, Resonance Raman