Characteristics of the electric field accompanying a longitudinal   acoustic wave in a metal. Anomaly in the superconducting phase
  The temperature dependence of the amplitude and phase of the electric potential arising at a plane boundary of a conductor when a longitudinal acoustic wave is incident normally on it is investigated theoretically and experimentally. The surface potential is formed by two contributions, one of which is spatially periodic inside the sample, with the period of the acoustic field; the second is aperiodic and arises as a result of an additional nonuniformity of the electron distribution in a surface layer of the metal. In the nonlocal region the second contribution is dominant. The phases of these contributions are shifted by approximately \pi /2. For metals in the normal state the experiment is in qualitative agreement with the theory. The superconducting transition is accompanied by catastrophically rapid vanishing of the electric potential, in sharp contrast to the theoretical estimates, which predict behavior similar to the BCS dependence of the attenuation coefficient for a longitudinal sound. 
Yu. A. Avramenko, E. V. Bezuglyi, N. G. Burma, I. G. Kolobov, V. D.   Fil', O. A. Shevchenko, and V. M. Gokhfeld
cond-mat.supr-con cond-mat.other