We analyze the effects of the on-site Coulomb repulsion $U$ on a band insulator using dynamical mean field theory (DMFT). We find the surprising result that the gap is suppressed to zero at a critical $U_{c1}$ and remains zero within a metallic phase. At a larger $U_{c2}$ there is a second transition from the metal to a Mott insulator, in which the gap increases with increasing $U$. These results are qualitatively different from Hartree-Fock theory which gives a monotonically decreasing but nonzero insulating gap for all finite $U$.

• Received 14 November 2005

DOI:https://doi.org/10.1103/PhysRevLett.97.046403