The mean field approximation for a system of triplet bosons in nickelates

Rare-earth orthonickelates RNiO$_3$ are Jahn–Teller magnets, unstable with respect to the anti-Jahn–Teller disproportionation reaction with the formation of a system equivalent to a system of effective spin-triplet composite bosons moving in a non-magnetic lattice. Within the framework of the two-sublattice approximation, we have developed a mean field theory for a model nickelate with competition between phases of charge ordering, an antiferromagnetic insulator and a spin-triplet superconductor, and constructed phase diagrams taking into account phase separation.