Coulomb Drag in the Exciton Regime in Electron-Hole Bilayers

TL;DR

This study investigates Coulomb drag in electron-hole bilayers with varying barrier thicknesses, revealing signs of exciton formation and strong interlayer coupling as the barrier narrows, which could lead to exciton condensation.

Contribution

It provides experimental evidence of transition from weakly coupled to strongly coupled exciton regimes in electron-hole bilayers by measuring Coulomb drag at different barrier thicknesses.

Findings

Drag decreases with temperature at 30 nm barrier, indicating weak coupling.

Drag increases with decreasing temperature at 20 nm barrier, indicating strong coupling.

Results suggest onset of exciton formation and potential exciton condensation phenomena.

Abstract

We report electrical transport measurements on GaAs/AlGaAs based electron-hole bilayers. These systems 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. Once excitons form, phenomena such as Bose-Einstein condensation of excitons could be observed. In our devices, electrons and holes are confined in double quantum wells, and carriers in the devices are induced with top and bottom gates leading to variable density in each layer. Separate contact to each layer allows Coulomb drag transport measurements where current is driven in one layer while voltage is measured in the other. Coulomb drag is sensitive to interlayer coupling and has been predicted to provide a strong signature of exciton condensation. Drag measurement on EHBLs with a 30 nm barrier are consistent with drag between two weakly coupled 2D Fermi systems where the drag decreases as the temperature is reduced. When the barrier is reduced to 20 nm, we observe a consistent increase in the drag resistance as the temperature is reduced. These results indicate the onset of a much stronger coupling between the electrons and holes which leads to exciton formation and possibly phenomena related to exciton condensation.