Orbital selective phase transitions——a unique phenomenon in multi-orbital systems

The metal-to-insulator transition (MIT) is an extensively studied phenomenon in condensed matter physics, which has wide applications in industry since it is usually accompanied by huge resistivity changes. Various factors like disorder or electronic interaction, etc. can lead to localization of itinerant electrons and consequently the MIT. However, in real materials, such a transition becomes complicated because orbital degrees of freedom are involved. In this paper, we review one kind of MIT, named orbital selective phase transition, where orbital degrees of freedom are taken into consideration and localized and itinerant electrons coexist in one atomic shell due to the interactions between electrons. Different mechanisms and related experiments will be discussed.