Soutenance de thèse de Fei He

Fei He, doctorant dans les équipes Minéralogie, pétrologie et physique planétaire (MP³) et Physique des systèmes simples en conditions extrêmes (PHYSIX) de l'IMPMC soutient sa thèse le mardi 18 décembre 2018 à 14 h

IMPMC - Sorbonne Université - 4 place Jussieu, 75005 Paris, tour 23, 4e étage, couloir 22-23, salle 401

Microscopic approach to the thermal transport in model non conducting oxides under extreme conditions

Abstract
Lattice dynamics and vibrational properties of materials at low and moderate temperatures can be, in most of the cases, accurately explained on the basis of an harmonic model, which assumes that phonons are independent of each other. However, at high temperatures this model finds its limitations, as the coupling between phonons become increasingly important. An improved understanding of the anharmonic interactions between lattice modes not only has a fundamental interest, but is also necessary on a more applied basis, as phonons play a major role in many physical properties such as thermal and electrical conductivity, or superconductivity.
This thesis aims at investigating the anharmonic lattice dynamics of magnesium oxide (MgO) as a function of pressure and temperature, considering up to three-phonons scattering processes. Phonon energies and line widths will be discussed in relation to the dielectric and thermal transport properties. The simple rock-salt structured MgO represents a first ideal target for both experiments and calculations, with the further advantage of being of primary interest in geoscience, being an archetypal mineral for planetary mantles.
We combined infrared spectroscopy and inelastic X-ray scattering measurements to probe phonon energies and line widths across the entire Brillouin zone. Results of high-pressure and high-temperature experiments are complemented by density functional perturbation theory calculations. The direct comparison of measurements and calculations allows to get insight on the anharmonic dynamics of the system and to test the validity of the theoretical approach.
Keywords: [phonon, thermal conductivity, anharmonicity, linewidth, MgO, inelastic X-ray scattering, infrared spectroscopy]