Correlated electrons in the La₃Ni₂O_7-y system

In recent years, the condensed matter community has witnessed the discovery of superconducting properties of layered nickel oxides. In fact, these systems have been in the focus since the early days of cuprate high-Tc superconductivity, as possible additional representants of unconventional superconductors with a high transition temperature. But only 2019, a stable electron-pairing phase has been identified in thin-films of Sr-doped NdNiO₂ with a formal 3d^9-x valence on Ni was found. In 2023, the bilayer nickelate La₃Ni₂O₇ with formal 3d^8-x valence was detected superconducting under pressure, and a resemblance of this finding was revealed in compressively-strained thin-films in 2024. These two discoveries provided the seed for a growing number of superconducting nickelate systems associated with either of the two families.

Low-energy modelings as well as the advanced combination of density functional theory (DFT) and dynamical mean-field theory (DMFT) provide access to this novel playground of realistic interacting electrons prone to superconductivity in a competing Mott-Hubbard vs. charge-transfer scenario. In this talk, we will first focus on the second family of superconducting nickelates. It will be shown that the whole bilayer La₃Ni₂O_7-y system with varying apical-oxygen content shares intriguing correlation physics, building up from a Ni-e_g multiorbital low-energy regime, where the interplay of flat-band characteristics and (orbital-selective) Mott-critical mechanisms plays an essential role. Our studies open up possibilities to eventually connect both so far distinct superconducting nickelate families.