Ultrasonic attenuation in a superconducting vanadium-tantalum alloy

Ultrasonic-attenuation measurements have been carried out on the magnetically reversible, type-II superconductor V - 5.6-at.% Ta with both a high Ginzburg-Landau parameter (G5) and the Bardeen, Cooper, and Schrieffer (BCS) coherence length 5 times the electronic mean free path. The electronic attenuation in zero magnetic field below Tc is BCS-like, giving a zero-temperature energy gap 20(0,0)3. 6kBTc, and in the normal state has the theoretically predicted dependence on q and l. The upper critical field Hc2 could be deduced from the attenuation data to an accuracy better than 1%, and its temperature dependence agrees well with type-II theory without p-wave scattering of electrons. Use of the measured normal-state resistivity and ultrasonically determined values of Tc, Hc2(T), and H at H=Hc2 inserted in an ultrasonic-attenuation theory due to Maki enabled the calculation of an Abrikosov-Maki parameter 2, which agreed roughly with the 2 obtained in previous magnetic and calorimetric measurements. Thus, for H close to Hc2, the data agree with Maki's dirty-limit (0l1) theory, which predicts a linear dependence of on H. At fields farther from Hc2, (H) deviates from linearity and appears to fit empirically a parabolic function of H. At very low fields HHc1, the H and T dependence of is in apparent agreement with another theory. Physical insight into the behavior in this region could be obtained with a model in which vortices are replaced by cylinders of completely normal material imbedded in a superconducting matrix.