Anomalous Hall effect at half filling in twisted bilayer graphene

TL;DR

This paper reports the unexpected observation of the anomalous Hall effect at half filling in twisted bilayer graphene slightly away from the magic angle, suggesting a valley-polarized ground state in an intermediate coupling regime.

Contribution

It demonstrates the emergence of an anomalous Hall effect at half filling in tBLG with twist angles near the magic angle, revealing a new ground state in the intermediate coupling regime.

Findings

AHE observed at half filling ($ u=2$) in devices with twist angles 0.96° and 1.20°

Valley-polarized phase can be the ground state at half filling

Highlights the importance of intermediate coupling regime in tBLG

Abstract

Magic-angle twisted bilayer graphene (tBLG) has been studied extensively owing to its wealth of symmetry-broken phases, correlated Chern insulators, orbital magnetism, and superconductivity. In particular, the anomalous Hall effect (AHE) has been observed at odd integer filling factors ($\nu=1$ and $3$) in a small number of tBLG devices, indicating the emergence of a zero-field orbital magnetic state with spontaneously broken time-reversal symmetry. However, the AHE is typically not anticipated at half filling ($\nu=2$) owing to competing intervalley coherent states, as well as spin-polarized and valley Hall states that are favored by an intervalley Hund's coupling. Here, we present measurements of two tBLG devices with twist angles slightly away from the magic angle (0.96$^{\circ}$ and 1.20$^{\circ}$), in which we report the surprising observation of the AHE at $\nu=+2$ and $-2$, respectively. These findings imply that a valley-polarized phase can become the ground state at half filling in tBLG rotated slightly away from the magic angle. Our results reveal the emergence of an unexpected ground state in the intermediately-coupled regime ($U/W \sim 1$, where $U$ is the strength of Coulomb repulsion and $W$ is the bandwidth), in between the strongly-correlated insulator and weakly-correlated metal, highlighting the need to develop a more complete understanding of tBLG away from the strongly-coupled limit.