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

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.

13 pages, 6 figures, extended Conclusions section

References in corpus (8)

• Ambient-pressure superconductivity onset above 40 K in bilayer nickelate ultrathin films
• Phase separation in a two-band model for strongly correlated electrons
• Microscopic origin of diagonal stripe phases in doped nickelates
• Bulk superconductivity in pressurized trilayer nickelate Pr4Ni3O10 single crystals
• Highly frustrated magnetism in relativistic ${\mathbf{d}}^4$ Mott insulators: Bosonic analog of Kitaev honeycomb model
• The origin of strong correlations and superconductivity in Na${}_x$CoO${}_2$
• Spin polaron theory for the photoemission spectra of layered cobaltates
• Equilibrium and non-equilibrium dynamics of a hole in a bilayer antiferromagnet