  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 Uc1 and remains zero within a
metallic phase. At a larger Uc2 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 non-zero insulating gap for all finite U.

Strongly Correlated Electrons