Coupled electronic and structural instabilities in complex oxides from first-principles, many-body perturbation theory and machine learning

Quintin Meier – Institut Néel (Grenoble)

Complex oxides are a broad class of materials showing a plethora of functional properties ranging from ferroelectricity and multiferroicity to superconductivity. These properties arise from new phases that emerge from the rich interplay between structural or electronic degrees of freedom in the materials. In this talk, I will present our efforts to obtain an accurate first-principles description of both structural and superconducting phase transitions. At a structural phase transition, a material changes its shape and/or atomic structure at a critical temperature. Phenomenologically, this can be explained by a low frequency, or  “soft”, mode, which condenses at the phase transition. Above the phase transition, it is entropically stabilized by anharmonic interactions. I will introduce a newly developed method to calculate phonon anharmonicity fully ab initio using a combination of DFT and machine learning potentials. I show, using the example of the quantum paraelectric perovskite KTaO3 that we not only can accurately calculate the temperature dependence of the soft phonon modes, but also reproduce the correct behavior of the dielectric constant when approaching the quantum limit at 0K.[1] Then, I will point out an intimate link between phonon-mediated superconductivity and soft modes, and will present our recent work studying the possibility of phonon-mediated superconducting instabilities in the novel family of Nickelate superconductors based on combining first-principles calculations and many-body perturbation theory [2] [1] QN Meier, N Mingo, A van Roekeghem, arXiv:2206.08296 [2] QN Meier, JB de Vaulx, et al, arXiv:2309.05486