OPTIMAL SUPERCONDUCTING CRITICAL-TEMPERATURE IN THE 2-DIMENSIONAL ATTRACTIVE HUBBARD-MODEL

The critical temperature for superconductivity for the two-dimensional attractive (negative-U) Hubbard model on a square lattice is calculated as a function of the strength of the interaction and of the density of electrons. The critical temperature is obtained by calculating the helicity modulus associated with a wavelike distortion of the gap parameter in the BCS approximation as a function of temperature and subsequent comparison with the Kosterlitz-Thouless relation between critical temperature and helicity modulus. The adequacy of the approximations is discussed and a comparison with recent Monte Carlo calculations is made.