Phase diagram for the exciton Mott transition in infinite-dimensional   electron-hole systems
  To understand the essence of the exciton Mott transition in three-dimensional electron-hole systems, the metal-insulator transition is studied for a two-band Hubbard model in infinite dimensions with interactions of electron-electron (hole-hole) repulsion U and electron-hole attraction -U'. By using the dynamical mean-field theory, the phase diagram in the U-U' plane is obtained (which is exact in infinite dimensions) assuming that electron-hole pairs do not condense. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott-Hubbard insulator for U > U' and the biexciton-like insulator for U < U'. Even when away from half-filling, we find the phase transition between the exciton- or biexciton-like insulator and a metallic state. This transition can be assigned to the exciton Mott transition, whereas the Mott-Hubbard transition is absent. 
Yuh Tomio, Tetsuo Ogawa