Superfluid characteristics of the attractive Hubbard model for various lattice structures

We study the basic thermodynamic and electromagnetic properties of the superconductor described by the negative-U Hubbard model (gap parameter Delta, critical temperature T-c, London penetration depth lambda, thermodynamic critical field H-c and Ginzburg-Landau correlation length xi(GL)) The calculations are performed for square (SQ, d = 2), simple cubic (SC, d = 3) and face-centered cubic (FCC, d = 3) lattices. We analyze the results as a function of electron density n and interaction U using the Hartree-Fock approximation in the weak-to-intermediate U case combined with the conclusions from the perturbation theory valid in the strong coupling limit. From the equality: lambda/xi(GL) = 1/root 2 we find the boundaries between local and nonlocal electronic behaviour of the U < 0 Hubbard model in the U-n parameter space. Using the calculated values of T-c and lambda we compare the ''universal plot'' of T-c/T-c(m) versus [lambda(0)/lambda(m)(0)](2) (where: T-c(m) denotes maximum T-c as a function of n and lambda(m)(0) corresponds to T-c(m)) with the recent experimental data for various families of nonconventional superconductors and conclude that the best agreement can be obtained for intermediate values of the local attraction.