, Volume 12, Issue 3-4, pp 387-411

Quasiparticle phenomenology for thermodynamics of strong-coupling superconductors

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We show that reduced critical fields, entropies, and specific heats of superconductors, regardless of “coupling-strength,” can be fitted essentially within experimental errors by curves appropriate to a system of independent fermion quasiparticles. The analysis proceeds from the formula for the entropy of a system of independent fermionsS=−k B Σ [f lnf+(1−f) ln (1−f)], where we take the quasiparticle spectrum to be the same as in the BCS (weak-coupling) theoryE k 2 =(ε k 2 2). The temperature dependence of the energy gap is also taken to be the same as in the BCS theory, the only adjustable parameter being the gap ratio Δ(0)/k B T c . The necessary values of this ratio are found to be in reasonable agreement with the experimental values deduced from electron tunneling and infrared absorption. Some speculations are offered for the paradoxical success of this simple model, based on analogies with the effects of strong coupling in the normal state.

Supported in part by the U.S. Atomic Energy Commission under Contract No. AT(30-1)-3972.