Jannik Gondolf

Ruhr-Universität Bochum

Electronic correlations and superconductivity in low-dimensional nickelates

Motivated by the report of superconductivity in bilayer La₃Ni₂O₇ at 80 K under high pressure, we examine the interacting electrons in this system. First-principles many-body theory is utilized to study the normal-state electronic properties. In a model Hamiltonian picture, spin fluctuations originating mostly from the Ni-dz² orbital give rise to strong tendencies towards a superconducting instability with a B_1g or B_2g order parameter. The dramatic enhancement of T_c in pressurized La₃Ni₂O₇ is due to stronger Ni-dz² correlations compared to those in the infinite-layer nickelates. We further study the dependence of the leading instability on interaction strengths and variations of the band structure.

On a related basis, we investigate the structural metal-to-metal transition in the layered delafossite AgNiO₂ at 365 K, which is accompanied by a tripling of the unit cell. The transition can be attributed to a site-selective Mott transition. Intrinsic electronic contributions can be seen at the level of unrenormalized quasiparticles. This is evidenced by features of the bare susceptibility that coincide with the ordering wave vectors of the symmetry-broken phase.