Ultrasonic Attenuation in Normal and Superconducting Indium

Measurements of the electron contribution to the attenuation of longitudinal sound waves propagating in normal and superconducting indium have been made up to 330 MHz. Data have been obtained for propagation along the principal crystallographic axes as well as for a number of directions in the (010) plane. The results exhibit considerable deviation from free-electron behavior. The limiting slopes of the attenuation versus frequency are highly anisotropic, and for quasi-longitudinal modes include a significant contribution which vanishes within approximately 30 mdeg of $T_c$. This drop is in addition to the less sudden drop associated with the BCS attenuation for longitudinal waves. Comparison of the measurements with simple calculations based on a modified free-electron model suggests that the effect of the crystal potential on the Fermisurface topology is largely responsible for the observed attenuation behavior.