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

TL;DR

This paper demonstrates the observation of a ratchet effect in bilayer graphene, showing how frequency dependence of the generated current reveals the transition to a hydrodynamic electron transport regime at low temperatures.

Contribution

It provides experimental evidence linking the frequency dependence of the ratchet current to hydrodynamic electron behavior in bilayer graphene.

Findings

Ratchet current decreases as 1/ω² at high T

Ratchet current decreases as 1/ω⁶ at low T

Strong frequency dependence indicates hydrodynamic regime

Abstract

We report on the observation of the ratchet effect -- generation of direct electric current in response to external terahertz (THz) 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, $T = 150~\textrm{K}$, the ratchet current decreases at high frequencies as $ \propto 1/\omega^2$, while at low temperature, $T = 4.2~\textrm{K}$, the frequency dependence becomes much stronger $\propto 1/\omega^6$. 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.