Valley-Controlled Viscosity of Two-Dimensional Dirac Fluids

TL;DR

This paper explores how valley imbalance influences the viscosity of two-dimensional Dirac fluids, revealing a nonmonotonic response and proposing valley control as a means to tune hydrodynamic transport.

Contribution

It demonstrates that shifting Dirac cones controls viscosity and analyzes the effects across different 2D materials, expanding understanding of valley-dependent hydrodynamics.

Findings

Viscosity exhibits a nonmonotonic response to valley splitting.

Valley control enables tuning of hydrodynamic transport in Dirac materials.

Monolayer graphene's viscosity decreases monotonically with temperature.

Abstract

Motivated by recent experiments in weakly hybridized small-angle twisted bilayer graphene, we investigate how valley imbalance affects the viscosity of two-dimensional Dirac fluids. We show that shifting the two low-energy Dirac cones relative to one another provides a direct knob to control the viscosity of the electron fluid. As the splitting is increased, the system passes through distinct transport regimes associated with valley depletion, charge-neutrality crossover, and the onset of electron-hole scattering, producing a pronounced nonmonotonic response. To place this result in context, we also analyze the viscosity in monolayer graphene (MLG) and two-dimensional electron gas (2DEG). We show that, due to the strong dependence of its inertial mass density on temperature, the kinematic viscosity of MLG is a monotonically decreasing function of temperature. Our results identify valley control as a route to tuning hydrodynamic transport in Dirac materials and clarify the interplay between band structure, scattering phase space, and screening in setting the viscous response.