  We investigate the superfluid phase transition in a gas of Fermi atoms loaded
on a three-dimensional optical lattice. When the lattice potential is strong,
this system can be well described by an attractive Hubbard model. In this
model, we calculate the superfluid phase transition temperature Tc, including
both superfluid and (spin and charge) density fluctuations within the
self-consistent t-matrix theory and fluctuation exchange approximation,
respectively. Since we treat these fluctuations in a consistent manner, our
theory satisfies the required particle-hole symmetry over the entire BCS-BEC
crossover region. We show that charge density fluctuations compete against
superfluid fluctuations near the half-filling, leading to the suppression of
Tc. As a result, the maximum Tc is obtained away the half-filling. Since the
strong density fluctuations originate from the nesting property of the Fermi
surface at the half filling (which is absent in a uniform gas with no lattice
potential), our results would be useful in considering lattice effects on
strong-coupling superfluidity.
