Dynamical mean-field theory for the exciton Mott transition in electron–hole systems

In electron–hole (e–h) systems in photoexcited insulators, the exciton Mott transition may take place, which is a typical photoinduced phase transition. To understand how the exciton Mott transition depends on the e–h carrier density and the Coulomb correlation, we study an e–h two-band Hubbard model by means of the dynamical mean-field theory assuming that electron–hole pairs do not condense. The phase diagram on the plane of interactions at zero temperature is obtained. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott–Hubbard insulator and the biexciton-like insulator. Away from half-filling we find another insulating phase, the exciton-like insulator, which is followed by the formation of incoherent (not condensed) e–h pairs, while the Mott–Hubbard insulator phase disappears. The exciton Mott transition is found to be the first-order transition. Linear optical susceptibilities are also discussed.