Coulomb Drag in the Exciton Regime in Electron-Hole Bilayers

Electron-hole bilayers are expected to make a transition from a pair of weakly coupled two-dimensional systems to a strongly coupled exciton system as the barrier between the layers is reduced. Coulomb drag measurements on devices with a 30 nm barrier are consistent with two weakly coupled 2D Fermi systems where the drag decreases with temperature. For a 20 nm barrier, however, we observe an increase in the drag resistance as the temperature is reduced when a current is driven in the electron layer and voltage measured in the hole layer. These results indicate the onset of strong coupling possibly due to exciton formation or phenomena related to exciton condensation.