Viscous Hall effect in graphene superlattice enabled via proximity screening

In electronic systems with strong electron-electron interactions, charge carriers can exhibit fluidlike behavior governed by viscosity. While such hydrodynamic regimes have been observed in pristine graphene, realizing similar behavior in moir\'e superlattices has been challenging due to enhanced momentum-relaxing umklapp electron-electron scattering. Here, we show that placing a graphene/hBN superlattice in close proximity to a conductive screening layer suppresses umklapp momentum relaxation, creating favorable conditions for the observation of viscous electron flow. The hydrodynamic response near the first Dirac point remains largely insensitive to the moir\'e potential, allowing clear observation of the viscous Hall effect and extraction of the electron-electron scattering length. These results identify proximity screening as a practical route to tune scattering processes and enable quantitative measurement of electron viscosity in moir\'e superlattices. In contrast, transport near the secondary Dirac points is strongly affected by narrow bandwidth, which hinders reliable measurements of the viscous Hall effect in this regime. This limitation highlights the need for multicomponent hydrodynamic frameworks to describe narrow-bandwidth moir\'e systems.