Microwave ultrasonic studies in a highly coupled superconductor

Abstract

Measurements of the ultrasonic attenuation in niobium crystals at frequencies from 290 to 1280 Mc/s over the temperature range 1.4 to 25 °K are presented. The sound was propagated in the (100), (110) and (111) directions. The equipment used and the techniques of measure­ment are described. The electronic attenuation in superconducting niobium at temperatures just below its transition temperature (9.9 °K) is found to be less than that predicted by the B. C. S. theory, but the results can be correlated with recent heat capacity data. Similar measurements of ultrasonic attenuation and heat capacity in vanadium show a normal B. C. S. behaviour. The apparent anomaly in niobium is shown to be related to the high value of its coupling ratio T c /θ . The energy gap parameter, 2∆(0), in niobium is shown to be anisotropic, having minima in the directions of high symmetry, but the anisotropy is much less than that found in tin.