  When a surface acoustic wave propagates on the surface of a GaAs
semiconductor, coupling between electrons in the two-dimensional electron gas
beneath the interface and the elastic host crystal through piezoelectric
interaction will attenuate the SAW. The coupling coefficient is calculated for
the SAW propagating along an arbitrary direction. It is found that the coupling
strength is largely dependent on the propagating direction. When the SAW
propagates along the [011] direction, the coupling becomes quite weak.
