Superconductivity in the spin-state crossover materials: Nickelates with planar-coordinated low-spin Ni$^{2+}$ ions

TL;DR

This paper theoretically investigates the electronic and magnetic properties of quasi-two-dimensional nickelates with low-spin Ni$^{2+}$ ions, revealing how their level structure and doping influence superconductivity mechanisms.

Contribution

It introduces a singlet-triplet exchange model for nickelates with square-planar coordination and explores how doping affects their magnetic excitations and potential for Cooper pairing.

Findings

Low-spin Ni$^{2+}$ ions support a quasidegenerate singlet-triplet structure.

Doping introduces mobile Ni$^{3+}$ ions with $S=1/2$, interacting with the background.

Triplet excitations significantly influence charge carrier propagation and pairing.

Abstract

We theoretically study quasi-two-dimensional nickel compounds, where the nickel ions assume Ni$^{2+}$ $d^8$ valence state and feature a low-spin $S=0$ ground state quasidegenerate with $S=1$ ionic excitations. Such a level structure is supported by square-planar coordination of nickel ions or a suitable substitution of apical oxygens. We construct the corresponding singlet-triplet exchange model and explore its phase diagram and excitation spectrum. By hole doping, we further introduce mobile Ni$^{3+}$ $d^7$ ionic configurations with effective spins $S=1/2$, and analyze their interactions with the $d^8$ singlet-triplet background. The interplay with the triplet excitations in the $d^8$ sector is found to have a deep impact on the propagation of the doped hole-like charge carriers and is identified as a powerful source of Cooper pairing among them.