Strange metal phase of disordered magic-angle twisted bilayer graphene at low temperatures: from flatbands to weakly coupled Sachdev-Ye-Kitaev bundles

TL;DR

This paper investigates how strong Coulomb disorder in magic-angle twisted bilayer graphene induces a transition to a strange metal phase characterized by quantum chaos and absence of quasiparticles, using stochastic expansion and exact diagonalization.

Contribution

It demonstrates that substrate-induced disorder drives TBG into a strange metal phase with weakly coupled SYK bundles, revealing a disorder-driven quantum phase transition.

Findings

Disorder induces a transition from superconducting/insulating to strange metal phase.

Long-time dynamics follow Gaussian orthogonal ensemble statistics.

Short-time dynamics show rapid quantum scrambling.

Abstract

We use stochastic expansion and exact diagonalization to study the magic-angle twisted bilayer graphene (TBG) on a disordered substrate. We show that the substrate-induced strong Coulomb disorder in TBG with the chemical potential at the level of the flatbands drives the system to a network of weakly coupled Sachdev-Ye-Kitaev (SYK) bundles, stabilizing an emergent quantum chaotic strange metal (SM) phase of TBG that exhibits the absence of quasiparticles. The Gaussian orthogonal ensemble dominates TBG's long-time chaotic dynamics at strong disorder, whereas fast quantum scrambling appears in the short-time dynamics. In weak disorder, gapped phases of TBG exhibit exponentially decaying specific heat capacity and exponential decay in out-of-time-ordered correlators (OTOC). This is the system behavior in correlated insulator and superconducting phases, in agreement with the corresponding Larkin-Ovchinnikov result for correlators. The result suggests a low-temperature transition from the superconducting and correlated insulating phases into the strange metal upon increasing the disorder strength. We propose a finite-temperature phase diagram for Coulomb-disordered TBG and discuss the experimental consequences of the emergent SM phase.