Soutenance de thèse / PhD Defense - Saadi Chabane

Saadi Chabane, doctorant soutient sa thèse le lundi 16 septembre 2024 à 14 h.

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

Limite de l’approximation quasi-harmonic: conséquences sur le transport thermique en condition extrêmes

Abstract

The lattice dynamics and vibrational properties of materials can be accurately described using the harmonic approximation at low and moderate temperatures. This approximation assumes phonons behave independently. However, at high temperatures, this assumption falls short as the interactions between phonons become significant. Understanding these anharmonic interactions between lattice modes is not only fundamentally interesting but also crucial for practical applications as they influence physical properties such as thermal transport and elastic properties. The understanding of phonon interactions is not only a fundamental question, as it allows us to gain insights on the history and evolution of Earth. 

Thermal conductivity of the minerals constituting planetary interiors play a key-role in controlling the heat transfer within the planets and hence the planetary dynamics and thermo-chemical history (\cite{Samuel2021Apr}). The heat flux from the core to mantle directly reflects in the degree of basal heating of the mantle, the rate of core cooling and inner core growths, which in turn influence the outer core dynamics and the magnetic field generation and evolution.

The aim of this thesis is to investigate how the anharmonic terms behave at extreme temperature and pressure conditions of Earth’s lower mantle considering Magnesium Oxide (MgO).  This will be conducted through Density Functional Theory (DFT) based methods, namely Density Functional Perturbation Theory (DFPT) and the Stochastic Self-Consistent Harmonic Approximation (SSCHA). This methodology permits to study the anharmonicity up to fourth-order (four-phonon interactions); it will be benchmarked with Inelastic X-ray Scatterings (IXS) and Infra-Red spectroscopy (IR) measurements performed in our group at varying temperature. Followed by a discussion on thermal conductivity (TC) obtained through the Boltzmann transport equation as a function of the temperature that highlights the importance of considering presence of defects in analyzing TC. 

We then move to the extreme conditions of Earth’s lower mantle, temperature of the order of thousands of Kelvin and pressure at Mbar scale, where the behavior of anharmonicity in MgO is identified throughout the region. Our theoretical treatment permitted to establish the microscopic origin of the TC drop at the bottom of the lower mantle. This drop is attributed to the interplay between three and four-phonon interactions. 

Jury

• Konstantinos Termenitzidis (Directeur de recherche CNRS, INSA Lyon) - Rapporteur
• Razvan Caracas (Directeur de recherche CNRS, IPGP Paris) - Rapporteur                                  
• Fabio Fgnocchi (Directeur de recherche CNRS, INSP - Examinateur
• Jelena Sjakste (Chargée de recherche CNRS, LSI Palaiseau) - Examinatrice
• Julia Contreras-Garcia (Directrice de recherche, LCT Paris)               
• Examinatrice
• Paola Giura (Maître de conférence, IMPMC Paris) - Directrice de thèse 
• Lorenzo Paulatto(Ingénieur de recherche, IMPMC Paris) - Directeur de thèse
• Daniele Antonnangeli (Directeur de recherche, IMPMC Paris) - Directeur de thèse