We study phonon-mediated frictional drag between two two-dimensional e
lectron gases. We find two different regimes depending on whether the
phonon mean free path l(ph) is larger or smaller than a characteristic
value l(ph, crit). For l(ph) > l(ph, crit), the drag is dominated by
coupling through an electron-phonon collective mode, whereas for l(ph)
< l(ph, crit), the drag is mediated by damped phonons. In the latter
case, the drag varies logarithmically with interlayer distance. l(ph,
crit) depends on the electron densities, and it is therefore possible
to experimentally tune a given system between the two regimes. (C) 199
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