FRICTIONAL DRAG MEDIATED BY ACOUSTIC PHONONS

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 8 Elsevier Science B.V. All rights reserved.