Soutenance de thèse de Guilherme Ribeiro

Guilherme Ribeiro, doctorant dans l'équipe TQM, soutient sa thèse mardi 5 décembre 2017 à 14 h.

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

Strong anharmonicity in the phonon spectra of PbTe and SnTe evaluated via the stochastic self-consistent harmonic approximation

Abstract

At room temperature, PbTe and SnTe are efficient thermoelectrics with a cubic structure. At low temperature, SnTe undergoes a ferroelectric transition with a critical temperature strongly dependent on the hole concentration, while PbTe is an incipient ferroelectric. In this thesis by using the stochastic self-consistent harmonic approximation, we investigate the anharmonic phonon spectra  and  the occurrence of a ferroelectric transition in both systems. We find that vibrational spectra strongly depends on the approximation used for the exchange-correlation kernel in density functional theory. If gradient corrections and the theoretical volume are employed, then  the calculation of the free energy hessian leads to phonon spectra in good agreement with experimental data for both systems. In PbTe, we reproduce the transverse optical mode phonon satellite detected in inelastic neutron scattering and the crossing between the transverse optical and the longitudinal acoustic modes along the gamma-x direction. In the case of SnTe,  we describe the occurrence of a ferroelectric transition from the high temperature  fm3m structure to the low temperature r3m one.