Ratchet effect in spatially modulated bilayer graphene: Signature of hydrodynamic transport

We report on the observation of the ratchet effect-generation of direct electric current in response to external terahertz radiation-in bilayer graphene, where inversion symmetry is broken by an asymmetric dual-grating gate potential. As a central result, we demonstrate that at high temperature, T = 150 K, the ratchet current decreases at high frequencies as proportional to 1/omega 2, while at low temperature, T = 4.2 K, the frequency dependence becomes much stronger proportional to 1/omega 6. The developed theory shows that the frequency dependence of the ratchet current is very sensitive to the ratio of the electron-impurity and electron-electron scattering rates. The theory predicts that the dependence 1/omega 6 is realized in the hydrodynamic regime, when electron-electron scattering dominates, while 1/omega 2 is specific for the drift-diffusion approximation. Therefore, our experimental observation of a very strong frequency dependence reveals the emergence of the hydrodynamic regime.