  The absorption coefficient for surface acoustic waves in a piezoelectric
insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional
electron mobility $\mu= 1.3\times 10^5 cm^2/V\cdot s)$ at T=4.2K) via a small
gap has been investigated experimentally as a function of the frequency of the
wave, the width of the vacuum gap, the magnetic field, and the temperature. The
magnetic field and frequency dependencies of the high-frequency conductivity
(in the region 30-210 MHz) are calculated and analyzed. The experimental
results can be explained if it assumed that there exists a fluctuation
potential in which current carrier localization occurs. The absorption of the
surface acoustic waves in an interaction with two-dimensional electrons
localized in the energy "tails" of Landau levels is discussed.
