Phase diagram of the Hubbard chain with two atoms per cell - art. no. 121105

We obtain the quantum phase diagram of the Hubbard chain with alternating o n-site energy at half filling. The model is relevant for the ferroelectric perovskites and organic mixed-stack donor-acceptor crystals. For any values of the parameters, the band insulator is separated from the Mott insulator by a dimer phase. The boundaries are determined accurately by crossing of excited levels with particular discrete symmetries. We show that these cros sings coincide with jumps of charge and spin Berry phases with a clear geom etrical meaning.