Giant fluctuations of Coulomb drag

We have observed reproducible fluctuations of the Coulomb drag resistivity, originating from coherent scattering of electrons in two layers. The fluctuations are observed as functions of both the carrier density of each layer, and perpendicular magnetic field. The magnitude of the fluctuations is much larger than expected from the theory of ‘diffusive’ drag and the temperature dependence is stronger than theoretically predicted. We account for this enhancement by considering the ‘ballistic’ nature of the drag in our system. We also present results on the fluctuations at large magnetic fields, where the lowest Landau level is half filled so that coherent drag occurs between composite fermions. The magnitude of fluctuations is seen to be greatly enhanced compared with the small field case, although fluctuations of composite fermion drag show much better agreement with the theory developed for the ‘diffusive’ drag regime.