  We report on a fully self-consistent determination of a phase transition to a
superconducting state in a conserving approximation. The transition temperature
calculated for a two-dimensional Hubbard model with an attractive interaction
in the fluctuation exchange approximation agrees with quantum Monte Carlo
calculations. The temperature dependences of the superfluid density and of the
specific heat near the transition temperature indicate that the phase
transition in this model of coupled collective degrees of freedom and
electronic degrees of freedom is consistent with neither mean-field theory,
Gaussian fluctuations about a mean field order parameter, nor unbinding
Kosterlitz-Thouless vortices.
