Propagation of longitudinal sound in superconductors

We investigate the influence of electron-impurity scattering on the propaga tion of longitudinal ultrasound in superconductors, The role of nonmagnetic impurities for arbitrary mean free path (l) and the collective oscillation s are considered. It is shown that the effect of superconductivity and impu rity on the relative sound velocity shift, beta (s), is strongly dependent on the choice of a model. In particular. we find that in the two-band model , impurities supress the fluctuations of superconducting gaps more effectiv ely than in the one-band model. Consequently, the value of beta (s) increas es for all temperatures below the transition. This behavior is in qualitati ve agreement with experiments. (C) 2001 Elsevier Science B.V. All rights re served.