2025/06/19 by Chaloupka, Jiří, Khaliullin, Giniyat
#FOS: Physical sciences #Strongly Correlated Electrons (cond-mat.str-el)
paper · doi:10.48550/arxiv.2506.16362
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 hole-like charge carriers and is identified as a powerful source of Cooper pairing among them.