General mechanism for orbital selective phase transitions

Based on the analysis of a two-orbital Hubbard model within a mean-field approach, we propose a mechanism for an orbital selective phase transition (OSPT) where coexistence of localized and itinerant electrons can be realized. We show that this OSPT exists both at and near half-filling even in the absence of crystal-field splittings or when bandwidths, orbital degeneracies, and magnetic states are equal for both orbitals, provided the orbitals have different band dispersions. Such conditions should generally be satisfied in many materials. We find that this OSPT is not sensitive to the strength of Hund's rule coupling and that heavy doping favors the collinear antiferromagnetic state over the OSPT. We discuss our results in relation to the iron pnictides.