Peculiar features of acoustic energy transmission from liquid helium to metals (Review Article)
K. N. Zinov`eva
Kapitza Institute of Physical Problems RAS, 2 Kosygina st., Moscow 117334, Russia
(Submitted November 18, 1996 )
Experimental and theoretical studies of the coefficient alpha of the phonon energy transmission from liquid helium to cubic-symmetry metals are reviewed briefly. Three of these metals (tungsten, copper and aluminum) are monocrystals and one - gold - is polycrystal. It is shown that transmission coefficients for a monocrystal with a clean perfect surface are well described by the Acoustic Mismatch Model taking into account the absorption of phonons by conducting electrons. Andreev's theory of electron resonant absorption of the Rayleigh wave by the monocrystal surface alpha~1 is confirmed. In a strongly anisotropic crystal (copper) a resonant pseudosurface absorption peak is also observed. It is shown for aluminum that at the superconducting transition the phonon dissipation decreases and the height of the resonant peak of the Rayleigh wave absorption goes down correspondingly. It is established that the main mechanism of phonon scattering in a polycrystal is the Rayleigh scattering on the grain boundaries (~omega 4). This kind of scattering is much greater than the scattering due to conducting electrons. Thus, the form of the alpha(theta) spectrum changes sharply.