Ultrasonic attenuation in niobium I: Purity dependence in normal and Meissner states, measurements and theory

Abstract

The attenuation of 30-70 MHz longitudinal sound waves has been measured in both the normal and superconducting Meissner states from 1.2 K to 10 K for several crystals of niobium with resistance ratios 83-3880. This corresponds to measurements with ql<1 (q is the sound wavevector and l the electron mean free path). In the normal state an approximately T-3 term is observed in the electron mean free path, which can be caused by interband scattering. On entering the superconducting state this contribution is modified and leads to deviations from the simple BCS behaviour. However, it is shown that the observed deviations are too large to be explained by this alone and theoretical arguments are developed which explain the deviations in terms of a mechanism which will lead to an apparently purity dependent energy gap near Tc.