Interplay between Zhang-Rice singlets and high-spin states in a model for doped NiO2 planes

Superconductivity found in doped NdNiO2 is puzzling as two local symmetries of doped NiO2 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-Tc cuprates. We address this competition by investigating Ni4O8 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 Δ is able to drive this change in the ground-state character when realistic values for nickel-oxygen repulsion Udp 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 Ni(3d) orbital can easily become relevant, with either the xy or the 3z2−r2 orbitals contributing in addition to the x2−y2 orbital to the formation of triplet states. In addition, our result that Udp (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.