Hydrodynamic theory of electron transport in double-layer graphene

We study the transport properties of double-layer graphene within the hydrodynamic regime, taking into account the slow carrier population imbalance relaxation due to interlayer electron Coulomb interaction and electron--optical-phonon scattering. Assuming fast intralayer equilibration we derive the hydrodynamic theory from the Boltzmann equation, incorporating weak quenched disorder, electron-phonon scattering, and magnetic field. The system is described by three macroscopic currents carrying electron charge, energy, and carrier population, that are the response of electrochemical field, temperature gradient, and gradient of particle-hole imbalance chemical potential. We apply our hydrodynamic theory to the Coulomb drag experiments in graphene-hBN heterostructures and focus on the strong drag regime about charge neutrality where the drag resistance behaves anomalously. In the limit of infinite imbalance relaxation of various quantities, our theory reproduces existing drag theories such as those based on the momentum and energy mechanism. We also propose a diagnostics for different transport regimes base on nonlocal drag measurement.