Sign Reversal of Electron-Hole Coulomb Drag in Double Graphene Bilayers*

Closely coupled two-dimensional electron-hole sheets are attracting great interest as they should generate novel quantum phases driven by the strong Coulomb attractions between the sheets. Coulomb drag of carriers in one sheet by carriers moving in the other is a powerful tool to study Fermi liquid properties and identify the formation of novel phases. Two independent Coulomb drag experiments on electron-hole sheets in graphene double bilayers have reported an unexplained and puzzling sign reversal of the Coulomb drag signal. We demonstrate that this unusual effect can be explained by the multiband character of bilayer graphene and the temperature dependence of effective mass at low densities caused by the electron-electron interactions. Our theory produces excellent agreement with the observed structure in the Coulomb drag resistance, capturing the key features of the recent experiments over the full reported range of temperatures.