The frictional drag between coupled two-dimensional electronic systems in magnetic fields

The coupling between systems of two spatially separated two-dimensional (2D) electron gases and between systems of a separated 2D electron gas and a 2D hole gas is studied as a function of magnetic field B. The coupled electron gases with a barrier of 30 nm showed a transition from a phonon-dominated interaction at B = 0 to a Coulomb dominated one at quantising fields. For larger barriers (190 nm), phonons have been found to be the dominant coupling mechanism both at zero and finite fields. However, for all barriers investigated, we could observe a suppression of the coupling at half-filled Landau-levels probably due to novel screening effects. In the case of the coupled electron-hole gas samples with large barriers (>190 nm) we have investigated, the enhanced screening effects could not be observed possibly due to the less pronounced quantisation of the hole gas. (C) 1998 Elsevier Science B.V. All rights reserved.