Uncovering the spin ordering in magic-angle graphene via edge state equilibration

TL;DR

This paper introduces a new technique to measure spin polarization in magic-angle twisted bilayer graphene, revealing that certain quantum Hall states are spin-unpolarized, thus providing insights into the symmetry breaking in correlated quantum phases.

Contribution

The study presents a novel method using edge state equilibration in twist-decoupled layers to determine spin polarization in MATBG, advancing understanding of its quantum Hall states.

Findings

Broken-symmetry quantum Hall states are spin-unpolarized at even integer fillings.

Correlated Chern insulator from half filling is spin-unpolarized.

Conduction band of the Chern insulator may be spin-polarized.

Abstract

Determining the symmetry breaking order of correlated quantum phases is essential for understanding the microscopic interactions in their host systems. The flat bands in magic angle twisted bilayer graphene (MATBG) provide an especially rich arena to investigate such interaction-driven ground states, and while progress has been made in identifying the correlated insulators and their excitations at commensurate moire filling factors, the spin-valley polarizations of the topological states that emerge at high magnetic field remain unknown. Here we introduce a new technique based on twist-decoupled van der Waals layers that enables measurements of their electronic band structure and, by studying the backscattering between counter-propagating edge states, determination of relative spin polarization of the their edge modes. Applying this method to twist-decoupled MATBG and monolayer graphene, we find that the broken-symmetry quantum Hall states that extend from the charge neutrality point in MATBG are spin-unpolarized at even integer filling factors. The measurements also indicate that the correlated Chern insulator emerging from half filling of the flat valence band is spin-unpolarized, but suggest that its conduction band counterpart may be spin-polarized. Our results constrain models of spin-valley ordering in MATBG and establish a versatile approach to study the electronic properties of van der Waals systems.