Description du sujet de thèse

Context: The study of oxygen-deficient perovskites is a fast-growing field of research which may allow the discovery of functional materials potentially interesting for a wide range of applications. It is particularly the case of RNiO3-x perovskites, which have stimulated several experimental and theoretical research efforts because of their rich phase diagrams including different crystallographic phases associated with metallic / insulating and paramagnetic / antiferromagnetic states. Nickelates have recently become even more famous because of the major breakthrough in condensed matter with the finding of a superconducting state in infinite square-planar lattices. Owing to their strong electronic correlations, their tunable atomic structures and their rich properties, Ni-based perovskites are moreover considered as promising candidates for memristive and neuromorphic applications.

Goals: During this PhD thesis, we will numerically calculate the physical properties (atomic and electronic structure, spin and orbital magnetic moments) of RNiO3 (R = La, Nd or Pr) bulk oxide. We will then study the thermodynamical stability of phases RNiO3-x as a function of the distribution of oxygen vacancies and of structural strain induced by the presence of these defects. The first goal of this work will be to understand the intricate link between the atomic distortions in the new structural phases and the induced modifications of their physical properties in regard to those calculated for the perfect RNiO3 crystal. We will then try to understand the mechanisms which would allow phase transitions between the previously-discovered metastable structures. Finally, we intend to understand how the stability of these phases and their transitions can be tuned via epitaxial strain or the electric polarization of an adjacent ferroelectric material.

Ab initio numerical calculations of the electronic structure of nickelates will be performed using the density functional theory (DFT). A few code developments may be needed for the post-processing of the DFT data. Depending on the obtained results, the study can be extended to the development of models to consider temperature effects.

Prerequisite: The candidate will have a good knowledge of condensed-matter and quantum physics, a strong interest in numerical physics, and good communication skills (oral and writing). Correct mastery of at least one programming language is also desired.

Contexte de travail

The thesis will take place at the CEMES-CNRS laboratory in Toulouse, within the framework of the ANR project ''Tailoring metastable phases in nickelates: toward neuromorphic functionalities''. The project TaMe gathers experimentalists and theoreticians from 3 national laboratories, with whom regular scientific exchanges will be maintained. The student will have access to the computational resources provided by the HPC centers CALMIP and from GENCI.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

None