Spin-orbit coupling-enhanced valley ordering of malleable bands in twisted bilayer graphene on WSe2

TL;DR

This study demonstrates how spin-orbit coupling enhances valley polarization and stabilizes novel correlated phases in twisted bilayer graphene on WSe2, revealing tunable anomalous Hall effects and magnetic properties linked to moiré band singularities.

Contribution

It uncovers the role of spin-orbit coupling in stabilizing ordered phases and valley polarization in twisted bilayer graphene coupled to WSe2, with detailed experimental observations.

Findings

Observation of anomalous Hall effect and Lifshitz transitions.

Sign change in magnetization indicating orbital nature.

High coercive fields suggesting Stoner instability.

Abstract

New phases of matter can be stabilized by a combination of diverging electronic density of states, strong interactions, and spin-orbit coupling. Recent experiments in magic-angle twisted bilayer graphene (TBG) have uncovered a wealth of novel phases as a result of interaction-driven spin-valley flavour polarization. In this work, we explore correlated phases appearing due to the combined effect of spin-orbit coupling-enhanced valley polarization and large density of states below half filling ($\nu \lesssim 2$) of the moir\'e band in a TBG coupled to tungsten diselenide. We observe anomalous Hall effect, accompanied by a series of Lifshitz transitions, that are highly tunable with carrier density and magnetic field. Strikingly, the magnetization shows an abrupt sign change in the vicinity of half-filling, confirming its orbital nature. The coercive fields reported are about an order of magnitude higher than previous studies in graphene-based moir\'e systems, presumably aided by a Stoner instability favoured by the van Hove singularities in the malleable bands. While the Hall resistance is not quantized at zero magnetic fields, indicative of a ground state with partial valley polarization, perfect quantization and complete valley polarization are observed at finite fields. Our findings illustrate that singularities in the flat bands in the presence of spin-orbit coupling can stabilize ordered phases even at non-integer moir\'e band fillings.