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Inequalities for Δ(T)/kBTc: A test for weak-coupling theory applied to high-Tc superconductors

R. Combescot

DOI 10.1103/PhysRevB.37.2235 · Physical Review B

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Abstract

High-Tc superconductors should be strong or weak-coupling superconductors depending on the mechanism responsible for superconductivity. The answer will allow one to discriminate between various theories. In order to have a definite criterion independent of band-structure effects, we investigate the effect of a rapidly varying density of states in weak-coupling Bardeen-Cooper-Schrieffer (BCS) theory. We show that, whatever the density of states, Δ(0)/kBTc≤2. At finite temperature, this is generalized into Δ(T)≤1.13ΔBCS(T) where ΔBCS is the standard result for a constant density of states. If the gap is anisotropic, the same inequalities must be satisfied by the minimum of the gap over the Fermi surface. These exact results allow one to test whether high-Tc superconductors are weak or strong coupling.

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