  We consider spin-1/2 fermions of mass m with interactions near the unitary
limit. In an applied periodic potential of amplitude V and period a, and with a
density of an even integer number of fermions per unit cell, there is a
second-order quantum phase transition between superfluid and insulating ground
states at a critical V=Vc. We compute the universal ratio Vc m a^2 / h^2 at
N=infinity in a model with Sp(2N) spin symmetry. The insulator interpolates
between a band insulator of fermions and a Mott insulator of fermion pairs. We
discuss implications for recent experiments.
