Le lundi 22 juin 2026,  Louis BASTOGNE présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Physique) sous la direction de Philippe GHOSEZ.

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Structural Phase Transitions and Complex Polar Orderings in Oxide Perovskites: Insights and Control from First- and Second-Principles ».

Le Jury sera composé de :

M. M. VERSTRAETE (Président), MM. E. BOUSQUET (Secrétaire), B. DUPE, P. GHOSEZ (Promoteur), M. HADJIMICHAEL (University of Warwick), J. JUNQUERA (Universidad de Cantabria).

Abstract

Oxide perovskites constitute one of the most versatile families of functional materials, exhibiting a broad spectrum of physical phenomena including ferroelectricity, antiferroelectricity, metal-insulator transitions, and the emergence of complex topological textures. Understanding these properties at the microscopic level remains challenging, as they emerge from the collective behavior of multiple coupled structural contributions that are computationally demanding to simulate at finite temperature and large scale. This thesis addresses these challenges through the systematic development, validation, and application of second-principles interatomic potentials for a series of representative perovskite oxides, and exploits these models to investigate the nature, stability, and dynamical control of topological polar textures in bulk crystals and heterostructures. A rich variety of such textures is identified and characterized, and their stability and tunability are systematically assessed. The dynamical manipulation of these textures is then addressed through two complementary strategies. Spatially modulated acoustic phonon excitations are shown to enable deterministic and reversible engineering of a broad family of polar topologies, while inhomogeneous, time-dependent electric fields allow precise control and displacement of individual topological objects, including the reversible switching between states of opposite topological charge in the same material. Together, these results establish a framework for the active in situ control of polar textures in bulk ferroelectrics. Finally, an initially non-polar perovskite is shown to harbor a hidden ferroelectric phase whose metastability is governed by a layer-by-layer nucleation mechanism, and an electric-field-driven first-order polarization switching process is identified as an alternative pathway to antiferroelectricity. The lattice dynamics of a correlated oxide across its metal-insulator transition are also examined, reproducing the triggered instability mechanism at the origin of the structural phase transition.

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