2025/08/10 by Moskvin, A. S.
#FOS: Physical sciences #Materials Science (cond-mat.mtrl-sci) #Strongly Correlated Electrons (cond-mat.str-el)
paper · doi:10.48550/arxiv.2508.07377
The insulator-bad metal transition observed in the Jahn-Teller (JT) magnets orthonickelates RNiO3 (R = rare earth or yttrium Y) is considered to be a canonical example of the Mott transition, traditionally described in the framework of the Hubbard U-t-model and the density functional theory. However, actually the real insulating phase of nickelates is the result of charge disproportionation (CD) with the formation of a system of spin-triplet (S=1) electron [NiO6]10- and spinless (S=0) hole [NiO6]8- centers, equivalent to a system of effective spin-triplet composite bosons moving in a nonmagnetic lattice. Taking account of only charge degree of freedom we develop a novel minimal U-V-tb-model for nickelates making use of the charge triplet model with the pseudospin formalism and effective field approximation. We show the existence of two types of CD-phases, high-temperature classical CO-phase with the G-type charge ordering of electron and hole centers, and low-temperature quantum CDq-phase with charge and spin density transfer between electron and hole centers, uncertain valence and spin value for NiO6 centers. Model T-R phase diagram reproduces main features of the phase diagram found for RNiO3.