Evidence for 2k_F electron-electron scattering processes in Coulomb drag

Measurements and calculations of Coulomb drag between two low density, closely spaced, two-dimensional electron systems are reported. The experimentally measured drag exceeds that calculated in the random phase approximation by a significant, and density dependent, factor. Studies of the dependence of the measured drag on the difference in density between the two layers clearly demonstrate that previously ignored q = 2 k_F scattering processes can be very important to the drag at low densities and small layer separations.