Interplay between Zhang-Rice singlets and high-spin states in a model for doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">NiO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> planes

Abstract

Superconductivity found in doped $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{2}$ is puzzling as two local symmetries of doped $\mathrm{Ni}{\mathrm{O}}_{2}$ layers compete, with presumably far-reaching implications for the involved mechanism: A cupratelike regime with Zhang-Rice singlets is replaced by local triplet states at realistic values of charge-transfer energy, which would suggest a rather different superconductivity scenario from high-${T}_{c}$ cuprates. We address this competition by investigating ${\mathrm{Ni}}_{4}{\mathrm{O}}_{8}$ clusters with periodic boundary conditions in the parameter range relevant for the superconducting nickelates. With increasing value of charge-transfer energy we observe upon hole doping the expected crossover from the cuprate regime dominated by Zhang-Rice singlets to the local triplet states. We find that smaller charge-transfer energy $\mathrm{\ensuremath{\Delta}}$ is able to drive this change in the ground-state character when realistic values for nickel-oxygen repulsion ${U}_{dp}$ are taken into account. For large values of the charge-transfer energy, oxygen orbitals are less important than in superconducting cuprates as their spectral weight is found only at rather high excitation energies. However, a second $\mathrm{Ni}(3d$) orbital can easily become relevant, with either the $xy$ or the $3{z}^{2}\ensuremath{-}{r}^{2}$ orbitals contributing in addition to the ${x}^{2}\ensuremath{-}{y}^{2}$ orbital to the formation of triplet states. In addition, our result that ${U}_{dp}$ (acting between Ni and O) favors on-site triplets implies that correlation effects beyond purely on-site interactions should be taken into account when obtaining effective two-band models.

Topics & Concepts