Density imbalance effect on the Coulomb drag upturn in an undoped electron-hole bilayer

Abstract

A low-temperature upturn of the Coulomb drag resistivity measured in an undoped electron-hole bilayer (uEHBL) device, possibly manifesting from exciton formation or condensation, was recently observed. The effects of density imbalance on this upturn are examined. Measurements of drag as a function of temperature in an uEHBL with a 20-nm-wide ${\text{Al}}_{0.90}{\text{Ga}}_{0.10}\text{As}$ barrier layer at various density imbalances are presented. The results show drag increasing as the density of either two-dimensional system was reduced, both within and above the upturn temperature regime and with a stronger density dependence than weak-coupling theory predicts. A comparison of the data with numerical calculations of drag in the presence of electron-hole pairing fluctuations, which qualitatively reproduce the drag upturn behavior, is also presented. The calculations predict a peak in drag at matched densities, which is not reflected by the measurements.