Destroyed quantum Hall effect in graphene with [0001] tilt grain boundaries

TL;DR

This study demonstrates that [0001] tilt grain boundaries in graphene disrupt the quantum Hall effect by connecting edge states through defect bands, explaining its suppression in CVD-grown graphene.

Contribution

It reveals how grain boundary defect states in graphene cause the destruction of quantum Hall quantization, a novel insight into material imperfections affecting electronic properties.

Findings

Grain boundaries contain defect states forming metallic bands.

Edge states are connected by defect bands, destroying Hall quantization.

Disorder can partially restore quantization by redirecting current paths.

Abstract

The reason why the half-integer quantum Hall effect (QHE) is suppressed in graphene grown by chemical vapor deposition (CVD) is unclear. We propose that it might be connected to extended defects in the material and present results for the quantum Hall effect in graphene with [0001] tilt grain boundaries connecting opposite sides of Hall bar devices. Such grain boundaries contain 5-7 ring complexes that host defect states that hybridize to form bands with varying degree of metallicity depending on grain boundary defect density. In a magnetic field, edge states on opposite sides of the Hall bar can be connected by the defect states along the grain boundary. This destroys Hall resistance quantization and leads to non-zero longitudinal resistance. Anderson disorder can partly recover quantization, where current instead flows along returning paths along the grain boundary depending on defect density in the grain boundary and on disorder strength. Since grain sizes in graphene made by chemical vapor deposition are usually small, this may help explain why the quantum Hall effect is usually poorly developed in devices made of this material.