Excitonic instability in Kondo insulator.

Kondo insulators (KI) are well known example of topological insulators. They are strongly interacting electronic systems where the hybridization between itinerant and localized electrons opens up a gap. KIs have been extensively studied for more than five decades. However, the study of excitonic phases in these materials is a relatively new research direction. Here, using a Bethe-Salpeter approach we study the exciton spectrum in these materials and we show that in some interaction strength regimes there can be an excitonic instability which leads to the formation of an excitonic insulator (EI) phase. Correspondingly, we calculate the gap and critical temperature of the EI phase using a mean-field approach.