Strain Disorder and Gapless Intervalley Coherent Phase in Twisted Bilayer Graphene

TL;DR

This paper investigates how random strain variations in twisted bilayer graphene suppress spectral gaps in intervalley coherent phases, explaining discrepancies between theoretical predictions and experimental measurements.

Contribution

It demonstrates that strain disorder can significantly reduce the spectral gap while preserving intervalley coherence, resolving the gap discrepancy in experiments.

Findings

Spectral gap can be strongly suppressed by strain disorder.

Intervalley coherence can persist despite gap suppression.

The measured activation gap reflects the diminished spectral gap.

Abstract

Correlated insulators are frequently observed in magic angle twisted bilayer graphene at even fillings of electrons or holes per moir\'e unit-cell. Whereas theory predicts these insulators to be intervalley coherent excitonic phases, the measured gaps are routinely much smaller than theoretical estimates. We explore the effects of random strain variations on the intervalley coherent phase, which have a pair-breaking effect analogous to magnetic disorder in superconductors. We find that the spectral gap may be strongly suppressed by strain disorder, or vanish altogether, even as intervalley coherence is maintained. We discuss predicted features of the tunneling density of states, show that the activation gap measured in transport experiments corresponds to the diminished gap, and thus offer a solution for the apparent discrepancy between the theoretical and experimental gaps.