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The defence will be in French.
Abstract: The discovery of superconductivity in a neodymium nickelate in thin film under a critical temperature of 15 K in 2019 has triggered great excitement around these materials. Made of an infinite pattern of alternating nickel oxide layers and rare earth layers, they present interesting analogies with cuprates, the archetype of unconventional superconductors, whose underlying mechanism has not yet been understood. In particular, the nickel atom presents an electronic configuration d 9 similar to the one of copper in the latter, suggesting that the mechanism responsible for superconductivity in these so-called “infinite-layer” nickelates and in cuprates might be analogous. Superconductivity was then observed in other nickelates, including some which share less similarities with cuprates. In particular, the bilayer and trilayer compounds of the Ruddlesden-Popper serie present far higher critical temperatures, respectively 80 K and 30 K, and different electronic configurations. This is the real interest of these materials: they offer a new opportunity of understanding the phenomenon of unconventional superconductivity, with a different perspective. In this thesis, electronic and structural properties of some of these nickelates have been investigated by the means of ab initio calculations. Two methods have been used, namely density functional theory (DFT) and many-body perturbation theory in the GW approximation, the latter being more cumbersome but more adapted to describe electronic correlations.
During my defense, I will present a first work which has shown that the conventional electron-phonon interaction cannot explain the critical temperatures observed in the infinite-layer compounds, a fact that was suggested in the literature precisely by the means of GW calculations. I will then present a second work which has investigated the GW electronic structure of the bilayer lanthanum nickelate, under pressure and epitaxial strain. This work has highlighted some key differences between the DFT and GW electronic structures, knowing that the description of the superconducting instability strongly depends on the details of the latter.
