Quantum critical behavior in magic-angle twisted bilayer graphene

TL;DR

This study reveals a quantum critical phase in magic-angle twisted bilayer graphene, characterized by strange-metal behavior with linear resistivity and Planckian scattering, linked to quantum fluctuations and superconductivity.

Contribution

It provides direct transport evidence of a quantum critical phase in MATBG, distinguishing it from Fermi liquid behavior and connecting quantum fluctuations to superconductivity.

Findings

Strange-metal behavior with linear resistivity over three decades.

Quantum critical phase dominated by fluctuations across doping levels.

Transition from superconductivity to strange metal upon suppression.

Abstract

The flat bands of magic-angle twisted bilayer graphene (MATBG) host strongly-correlated electronic phases such as correlated insulators, superconductors and a strange-metal state. The latter state, believed to be key for understanding the electronic properties of MATBG, is obscured by various phase transitions and thus could not be unequivocally differentiated from a metal undergoing frequent electron-phonon collisions. Here, we report transport measurements in superconducting MATBG in which the correlated insulator states are suppressed by screening. The uninterrupted metallic ground state shows resistivity that is linear in temperature over three decades and spans a broad range of doping including those where a correlation-driven Fermi surface reconstruction occurs. This strange-metal behavior is distinguished by Planckian scattering rates and a linear magnetoresistivity. In contrast, near charge neutrality or a fully-filled flat band, as well as for devices twisted away from the magic angle, we observe the archetypal Fermi liquid behavior. Our measurements demonstrate the existence of a quantum critical phase whose fluctuations dominate the metallic ground state throughout a continuum of doping. Further, we observe a transition to the strange metal upon suppression of the superconducting order, suggesting a relationship between quantum fluctuations and superconductivity in MATBG.