Using the mean-field approximation, we study a model for quasi-two-dimensional superconductors. The interlayer coupling, assumed to be mediated by a small electron-hopping term, is found to leave Tc practically unaffected. Thus, a three-dimensional pairing mechanism is required to explain the observed rise in Tc with decreasing average layer spacing in the Bi and Tl compounds. Taking the inhomogeneities of intrinsic or extrinsic nature into account, we find, in the dirty limit, corrections to the conventional anisotropic Ginzburg-Landau behavior—an upward curvature of the upper critical fields which appears to be a universal feature of layered superconductors.
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