Evidence for pairing fluctuations in the Coulomb drag resistance of a GaAs/Graphene electron-hole bilayer

We report on experiments in a novel double-layer system composed by an ordinary two-dimensional electron gas (2DEG) in a GaAs heterostructure and a two-dimensional hole gas in a graphene monolayer placed on top of GaAs. Owing to the relatively short distance between the two layers we were able to measure the Coulomb drag in both the graphene and 2DEG layers. We discuss, in particular, the temperature evolution of the measured drag resistivity in the 2DEG. While the drag follows the expected quadratic temperature dependence at values above $T\approx 8K$, at lower temperature it displays a remarkable logarithmic increase. These data suggests the occurrence of electron-hole excitonic fluctuations as the double layer approaches the condensation temperature.