Geometric control of universal hydrodynamic flow in a two dimensional electron fluid

TL;DR

This paper demonstrates control over hydrodynamic electron flow in a 2D system by engineering device geometry and boundary conditions, enabling direct measurement of intrinsic viscosity and scattering properties without fitting parameters.

Contribution

It introduces a method to eliminate boundary roughness effects, allowing direct measurement of intrinsic electron fluid viscosity and scattering lifetime in 2D materials.

Findings

Observation of transition from ballistic to hydrodynamic flow

Direct measurement of viscosity and scattering lifetime

Viscosity depends strongly on electron density

Abstract

Fluid dynamics is one of the cornerstones of modern physics and has recently found applications in the transport of electrons in solids. In most solids electron transport is dominated by extrinsic factors, such as sample geometry and scattering from impurities. However in the hydrodynamic regime Coulomb interactions transform the electron motion from independent particles to the collective motion of a viscous `electron fluid'. The fluid viscosity is an intrinsic property of the electron system, determined solely by the electron-electron interactions. Resolving the universal intrinsic viscosity is challenging, as it only affects the resistance through interactions with the sample boundaries, whose roughness is not only unknown but also varies from device to device. Here we eliminate all unknown parameters by fabricating samples with smooth sidewalls to achieve the perfect slip boundary condition, which has been elusive both in molecular fluids and electronic systems. We engineer the device geometry to create viscous dissipation and reveal the true intrinsic hydrodynamic properties of a 2D system. We observe a clear transition from ballistic to hydrodynamic electron motion, driven by both temperature and magnetic field. We directly measure the viscosity and electron-electron scattering lifetime (the Fermi quasiparticle lifetime) over a wide temperature range without fitting parameters, and show they have a strong dependence on electron density that cannot be explained by conventional theories based on the Random Phase Approximation.