Selective equilibration of spin and valley polarized quantum Hall edge states in graphene

TL;DR

This study demonstrates that in high-quality graphene devices, spin and valley polarized quantum Hall edge states can propagate without mixing, revealing detailed information about the ground state polarization through transport measurements.

Contribution

It provides experimental evidence of selective edge state equilibration in graphene, highlighting the role of electron correlations in spin and valley polarization at quantum Hall regimes.

Findings

Edge states with opposite spins do not mix in high-quality devices.

The first Landau level is spin-polarized at half-filling.

Conductance suppression indicates formation of an insulating stripe at PN interfaces.

Abstract

We report on transport measurements of dual-gated, single-layer graphene devices in the quantum Hall regime, allowing for independent control of the filling factors in adjoining regions. Progress in device quality allows us to study scattering between edge states when the four-fold degeneracy of the Landau level is lifted by electron correlations, causing edge states to be spin and/or valley polarized. In this new regime, we observe a dramatic departure from the equilibration seen in more disordered devices: edge states with opposite spins propagate without mixing. As a result, the degree of equilibration inferred from transport can reveal the spin polarization of the ground state at each filling factor. In particular, the first Landau level is shown to be spin-polarized at half-filling, providing an independent confirmation of a conclusion of Ref.[1]. The conductance in the bipolar regime is strongly suppressed, indicating that co-propagating edge states, even with the same spin, do not equilibrate along PN interfaces. We attribute this behavior to the formation of an insulating nu=0 stripe at the PN interface.