  We have calculated the frictional drag between spatially separated quantum
wells with parallel two-dimensional electron gases due to interlayer
electron-electron interaction mediated by virtual exchange of acoustic phonons
due to piezoelectric and deformation potential interaction. It is shown that
the frictional drag is dominated by the piezoelectric coupling. According to
our calculations temperature dependence of the drag scattering rate divided by
squared temperature T exhibits a pronounced peak which for the experimental
situation and in agreement with the finding of T. J. Gramila, et al., Phys.
Rev. B 47, (1993) 12957 is obtained at about T=2.1 K. We ascribe the appearance
of this peak to a change from small to large angle scattering in the virtual
phonon exchange.
