Electric potential of the electron sound wave: Sharp disappearance in   the superconducting state
  We study the ac electric potential induced by the electron sound wave (a perturbation of the electron distribution function propagating with the Fermi velocity) in single crystals of high purity gallium. The potential and the elastic components of the electron sound demonstrate qualitatively different dependencies on the electron relaxation rate: while the phase of the potential increases with temperature, the phase of elastic displacement decreases. This effect is explained within the multiband model, in which the potential is attributed to the ballistic quasiwave, while the elastic component is associated with the zero-sound wave. We observed a mysterious property of the superconducting state: all manifestations of the potential accompanying the lattice deformations, including usual sound wave, disappear below T_c in almost jumplike manner. 
Yu. A. Avramenko, E. V. Bezuglyi, N. G. Burma, V. D. Fil
cond-mat.supr-con cond-mat.mtrl-sci