Ultrasonic Attenuation in Superconducting Mercury and Mercury-Cadmium Single Crystals

This paper presents the results of low-temperature ultrasonic-attenuation studies in mercury single crystals, polycrystals, and mercury single crystals with varying concentrations of cadmium impurity. The frequency ranged from 10 to 130 MHz and the doping concentration from 0.01 to 0.10% by weight. The major emphasis is a systematic study of the deviations of $\frac{{\alpha }_s}{{\alpha }_n}$ for longitudinal ultrasound from that predicted by the BCS theory. These deviations are a drop in $\frac{{\alpha }_s}{{\alpha }_n}$ with decreasing temperature near $T_c$ which is greater than that predicted by BCS. The rapidity of the drop increased with increasing frequency in all pure samples, was frequency-independent, and decreased with doping concentration in the doped samples. The deviations observed cannot be explained adequately by dislocation attenuation or by multiple anisotropic energy gaps.