Surface acoustic-wave attenuation by a two-dimensional electron gas in a strong magnetic field

The propagation of a surface acoustic wave (SAW) on GaAs/AlxGa1-xAs heterostructures is studied in the case where a two-dimensional electron gas (2DEG) is subject to a strong magnetic field and a smooth random potential with correlation length Lambda and amplitude Delta. The electron wave functions are described in a quasiclassical picture using results of percolation theory for two-dimensional systems. In accordance with the experimental situation, Lambda is assumed to be much smaller than the sound wavelength 2 pi/q. This restricts the absorption of surface phonons at a filling factor <(nu)over bar> approximate to 1/2 to electrons occupying extended trajectories of fractal structure. Both piezoelectric and deformation potential interactions of surface acoustic phonons with electrons are considered and the corresponding interaction vertices are derived. These vertices are found to differ from those valid for three-dimensional bulk phonon systems with respect to the phonon wave-vector dependence. We derive the appropriate dielectric function epsilon(omega,q) to describe the effect of screening on the electron-phonon coupling. In the low-temperature, high-frequency regime T much less than Delta(omega(q) Lambda/nu(D))(alpha/2 nu), where omega(q) is the SAW frequency and nu(D) is the electron drift velocity, both the attenuation coefficient F and epsilon(omega,q) are independent of temperature. The classical percolation indices give alpha/2 nu = 3/7. The width of the region where a strong absorption of the SAW occurs is found to be given by the scaling law \Delta<(nu)over bar>\ approximate to (omega(q) Lambda/nu(D))(alpha/2 nu). The dependence of the electron-phonon coupling and the screening due to the 2DEG on the filling factor leads to a double-peak structure for Gamma (<(nu)over bar>).