Ballistic-Ohmic quantum Hall plateau transition in graphene pn junction

TL;DR

This paper investigates the transition from ballistic to Ohmic quantum Hall plateaus in graphene pn junctions, identifying key processes like Landau state equilibration and valley iso-spin dilution, supported by theoretical and numerical analysis.

Contribution

It reveals that two distinct processes are necessary for the ballistic-Ohmic plateau transition in graphene pn junctions, combining scattering theory and quantum magneto-transport simulations.

Findings

Disorder at the pn interface facilitates Landau state equilibration.

Edge roughness and intervalley scattering are crucial for valley iso-spin dilution.

Higher Ohmic plateaus are less observable due to these processes.

Abstract

Recent quantum Hall experiments conducted on disordered graphene pn junction provide evidence that the junction resistance could be described by a simple Ohmic sum of the n and p mediums' resistances. However in the ballistic limit, theory predicts the existence of chirality-dependent quantum Hall plateaus in a pn junction. We show that two distinctively separate processes are required for this ballistic-Ohmic plateau transition, namely (i) hole/electron Landau states equilibration and (ii) valley iso-spin dilution of the incident Landau edge state. These conclusions are obtained by a simple scattering theory argument, and confirmed numerically by performing ensembles of quantum magneto-transport calculations on a 0.1um-wide disordered graphene pn junction within the tight-binding model. The former process is achieved by pn interface roughness, where a pn interface disorder with a root-mean-square roughness of 10nm was found to suffice under typical experimental conditions. The latter process is mediated by extrinsic edge roughness for an armchair edge ribbon and by intrinsic localized intervalley scattering centers at the edge of the pn interface for a zigzag ribbon. In light of these results, we also examine why higher Ohmic type plateaus are less likely to be observable in experiments.