Propagation of longitudinal sound in clean superconductors

A general treatment of propagation of longitudinal ultrasound in one- and two-band models of superconductors is given at arbitrary temperatures taking into account the terms proportional to s 2 v −2 ( s, v are the velocities of the sound and of the electrons on the Fermi surface, respectively). The qualitative agreement with experimental data on the change of the sound velocity and of the absorption coefficient of ultrasound has been achieved for certain choice of the two-band parameters.