Superconductivity in the spin-state crossover materials: Nickelates with planar-coordinated low-spin Ni2+ ions

Jiří Chaloupka; Giniyat Khaliullin

doi:10.1103/s5b6-crzc

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Superconductivity in the spin-state crossover materials: Nickelates with planar-coordinated low-spin Ni2+ ions

Jiří Chaloupka, Giniyat Khaliullin

DOI 10.1103/s5b6-crzc · Physical Review B

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Abstract

We theoretically study quasi-two-dimensional nickel compounds, where the nickel ions assume Ni2+d8 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 Ni3+d7 ionic configurations with effective spins S=1/2, and analyze their interactions with the d8 singlet-triplet background. The interplay with the triplet excitations in the d8 sector is found to have a deep impact on the propagation of the doped holelike charge carriers and is identified as a powerful source of Cooper pairing among them.