Recovery of massless Dirac fermions at charge neutrality in strongly interacting twisted bilayer graphene with disorder

TL;DR

This paper proposes that disorder and interactions in twisted bilayer graphene can lead to either insulating or semimetallic behavior at charge neutrality, depending on sample size and domain structure, revealing the complex role of randomness.

Contribution

It introduces a model where weak disorder stabilizes a network of gapped quantum valley Hall domains, explaining diverse experimental observations in strongly correlated twisted bilayer graphene.

Findings

Small samples with a single domain show insulating behavior.

Multi-domain samples exhibit gapless domain-wall modes and semimetallic behavior.

Disorder influences ground-state properties and can induce massless Dirac fermions.

Abstract

Stacking two graphene layers twisted by the 'magic angle' $\theta \approx 1.1^\circ$ generates flat energy bands, which in turn catalyzes various strongly correlated phenomena depending on filling and sample details. At charge neutrality, transport measurements reveal superficially mundane semimetallicity (as expected when correlations are weak) in some samples yet robust insulation in others. We propose that the interplay between interactions and disorder admits either behavior, even when the system is strongly correlated and locally gapped. Specifically, we argue that strong interactions supplemented by weak, smooth disorder stabilize a network of gapped quantum valley Hall domains with spatially varying Chern numbers determined by the disorder landscape--even when an entirely different order is favored in the clean limit. Within this scenario, sufficiently small samples that realize a single domain display insulating transport characteristics. Conversely, multi-domain samples exhibit re-emergent massless Dirac fermions formed by gapless domain-wall modes, yielding semimetallic behavior except on the ultra-long scales at which localization becomes visible. We discuss experimental tests of this proposal via local probes and transport. Our results highlight the crucial role that randomness can play in ground-state selection of twisted heterostructures, an observation that we expect to have further ramifications at other fillings.