Negative longitudinal resistance of monolayer graphene in the quantum Hall regime

TL;DR

This paper reports the observation of negative longitudinal resistance in monolayer graphene within the quantum Hall regime, revealing unconventional edge state interactions and conditions for negative resistance.

Contribution

It provides the first systematic study and numerical analysis of negative longitudinal resistance in monolayer graphene, highlighting the role of local disorder and edge state intersections.

Findings

Negative longitudinal resistance observed in graphene Hall bar

Numerical calculations identify conditions for negative r_xx

Macroscopic disorder excluded as primary cause

Abstract

In the quantum Hall regime the charge current is carried by ideal one-dimensional edge channels where the backscattering is prohibited by topology. This results in the constant potential along the edge of the Hall bar leading to zero 4-terminal longitudinal resistance r_xx. Finite scattering between the counter-propagating edge states, when the topological protection is broken, commonly results in r_xx > 0. However, a local disorder, if allowing intersection of the edge states, can result in a counter-intuitive scenario when r_xx<0. In this work we report the observation and a systematic study of such unconventional negative longitudinal resistance seen in an encapsulated monolayer graphene Hall bar device measured in the quantum Hall regime. We supplement our findings with the numerical calculations which allow us to outline the conditions necessary for the appearance of negative r_xx and to exclude the macroscopic disorder (contamination bubble) as the main origin of it.