Crossover between Mott-insulator and band-insulator in the two-orbital Hubbard model

Electronic states of the two-orbital Hubbard model are investigated by means of the composite opeator method. In addition to the transfer within the same kind of orbital, we introduce the off-diagonal transfer t', which provides the mixing of orbitals. In the t' = 0 case, the system shows the orbital selective Mott transition at U = 4. Upon adding t', the band gap goes wider. This increase of the gap originates from the crossover between the Mott-insulator and the band-insulator.