Negative longitudinal resistance of monolayer graphene in the quantum Hall regime

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.