Orbital selective phase transition*

Orbital selective phase transition (OSPT), proposed to be responsible for the coexistence of localized and itinerant electrons, has attracted extensive interest from both experimentalists and theoreticians, since the observation of an anomalous behavior with a Curie-Weiss-like local spin in the metallic phase of Ca$_{2-x}$Sr$_x$RuO$_4$ at $0.2 \leq x \leq 0.5$. Recently, even more attentions have been paid to OSPT since the coexistence of localized and itinerant electrons may reconcile the strong debates on how to understand the origin of magnetism in various iron-based superconductors. Here, various mechanisms for OSPT are reviewed and a new mechanism will be proposed. The distinct band dispersion of different orbitals, which should be generally satisfied in various materials, is identified to be the crucial point for OSPT with magnetic order. Such an OSPT are not sensitive to the strength of Hund's rule coupling. Heavy doping favors collinear antiferromagnetic state over the OSPT. Discussions are made related to the pnictides.