ATTENUATION OF ULTRASONIC WAVES OF ARBITRARY POLARIZATION AND PROPAGATION DIRECTION IN SUPERCONDUCTORS

Abstract

The attenuation by electrons of ultrasonic waves of arbitrary polarization and propagation direction is computed for superconductors on the BCS model, for the limiting condition that the electronic mean free path is much longer than the sound wavelength. The ratio of the superconducting to the normal attenuation is deduced. When the fermi surface has a high symmetry about the propagation direction, the attenuation of longitudinal and quasi-longitudinal waves follows the standard BCS expression. When symmetry is lacking, a sharp drop in longitudinal attenuation occurs within a small range of temperatures just below the transition temperature; the remaining attenuation follows the BCS result. The transverse attenuation behaves in a similar manner, regardless of the symmetry of the fermi surface. In all cases the residual attenuation depends on that part of the fermi surface for which electron velocities are normal to the propagation direction. These results are consistent with experimental data on attenuation in tin.