Deviations from the BCS expression for the acoustic attenuation in an anisotropic superconductor

It is shown, both theoretically and experimentally, that during the superconducting transition in a metal with an anisotropic dispersion curve for the electrons regions of a rapid drop of the acoustic absorption may be preceded by its increase. Such a qualitative difference from the behavior characteristic of the well known BCS expression is due to an energy dependence of the velocity of the excitations in a superconductor if the Fermi surface contains flattened spots. In that case the temperature dependence of the acoustic velocity is signficantly modified as well.

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