  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.
