Ballistic-hydrodynamic phase transition in flow of two-dimensional electrons

TL;DR

This paper predicts a classical phase transition in 2D electron flow from ballistic to viscous fluid behavior induced by magnetic field, supported by theoretical analysis and experimental observations.

Contribution

It introduces a new classical-mechanical phase transition mechanism in 2D electron systems driven by magnetic field and size effects, linking ballistic and viscous flow regimes.

Findings

Identification of a ballistic-to-viscous flow transition in 2D electrons

Observation of a kink in resistance dependencies at critical magnetic field

Theoretical support for classical origin of the phase transition

Abstract

Phase transitions are characterized by a sharp change in the type of dynamics of microparticles, and their description usually requires quantum mechanics. Recently, a peculiar type of conductors was discovered in which two-dimensional (2D) electrons form a viscous fluid. In this work we reveal that such electron fluid in high-quality samples can be formed from ballistic electrons via a phase transition. For this purpose, we theoretically study the evolution of a ballistic flow of 2D weakly interacting electrons with an increase of magnetic field and trace an emergence of a fluid fraction at a certain critical field. Such restructuring of the flow manifests itself in a kink in magnetic-field dependencies of the longitudinal and the Hall resistances. It is remarkable that the studied phase transition has a classical-mechanical origin and is determined by both the ballistic size effects and the electron-electron scattering. Our analysis shows that this effect was apparently observed in the recent transport experiments on 2D electrons in graphene and high-mobility GaAs quantum wells.