Strange Metals from Melting Correlated Insulators in Twisted Bilayer Graphene

TL;DR

This paper investigates the mysterious metallic state in twisted bilayer graphene, attributing its linear-in-temperature resistivity and high entropy to fluctuations of correlated insulating states caused by frustrated interactions, leading to mesoscale domain formations.

Contribution

It introduces a model linking order-parameter fluctuations and frustration-induced mesoscale domains to strange metal behavior in twisted bilayer graphene.

Findings

Fluctuations of order-parameters cause linear-in-temperature resistivity.

Frustration leads to mesoscale stripe-like domains.

Enhanced entropy is linked to mesoscopic order-parameter structures.

Abstract

Even as the understanding of the mechanism behind correlated insulating states in magic-angle twisted bilayer graphene converges towards various kinds of spontaneous symmetry breaking, the metallic "normal state" above the insulating transition temperature remains mysterious, with its excessively high entropy and linear-in-temperature resistivity. In this work, we focus on the effects of fluctuations of the order-parameters describing correlated insulating states at integer fillings of the low-energy flat bands on charge transport. Motivated by the observation of heterogeneity in the order-parameter landscape at zero magnetic field in certain samples, we conjecture the existence of frustrating extended range interactions in an effective Ising model of the order-parameters on a triangular lattice. The competition between short-distance ferromagnetic interactions and frustrating extended range antiferromagnetic interactions leads to an emergent length scale that forms stripe-like mesoscale domains above the ordering transition. The gapless fluctuations of these heterogeneous configurations are found to be responsible for the linear-in-temperature resistivity as well as the enhanced low temperature entropy. Our insights link experimentally observed linear-in-temperature resistivity and enhanced entropy to the strength of frustration, or equivalently, to the emergence of mesoscopic length scales characterizing order-parameter domains.