On the absorption of a surface acoustic wave in a bulk superconductor in the presence of a supercurrent

Surface acoustic wave absorption in bulk superconductors in the current state at low temperatures is discussed. In this situation a group of quasiparticles are localized near the surface due to the inhomogeneous supercurrent distribution; the rest of the quasiparticles have infinite trajectories. It is shown that it is possible for the main contribution to the acoustic wave absorption to be due to this group, depending on the sonic frequency. If this frequency is large, the localized quasiparticles are the most important in the absorption. In this case the absorption coefficient can be larger than in the absence of the supercurrent. In addition, there is a dependence of the absorption on the angle between the directions of the acoustic wave propagation and of the supercurrent. In the case of lower acoustic frequency, the quasiparticles with infinite trajectories dominate in the absorption. In this situation the absorption coefficient is smaller than in the zero-current state, because of the Andreev reflection of a fraction of these quasiparticles. It is shown that by studying the absorption dependence on the direction and on the absolute value of the current it is possible to obtain information on the law of current decrease near the surface of the sample, on the angular dependence of the deformational potential tensor, etc.