Reentrant Correlated Insulators in Twisted Bilayer Graphene at 25T ($2\pi$ Flux)

TL;DR

This paper investigates the behavior of twisted bilayer graphene under a 25T magnetic field, revealing reentrant flat bands with nontrivial topology and correlated insulating states at $2 extpi$ flux, with implications for superconductivity.

Contribution

The study introduces a gauge-invariant formalism to analyze TBG at $2 extpi$ flux, showing reemergence of flat bands with Chern numbers and predicting reentrant correlated insulators and potential superconductivity.

Findings

Reentrant flat bands reappear at $2 extpi$ flux with Chern numbers $ extpm 1$.

Correlated insulating states emerge at integer fillings under magnetic field.

Predicted Landau fans from excitation spectrum and possible reentrant superconductivity.

Abstract

Twisted bilayer graphene (TBG) is remarkable for its topological flat bands, which drive strongly-interacting physics at integer fillings, and its simple theoretical description facilitated by the Bistritzer-MacDonald Hamiltonian, a continuum model coupling two Dirac fermions. Due to the large moir\'e unit cell, TBG offers the unprecedented opportunity to observe reentrant Hofstadter phases in laboratory-strength magnetic fields near $25$T. This Letter is devoted to magic angle TBG at $2\pi$ flux where the magnetic translation group commutes. We use a newly developed gauge-invariant formalism to determine the exact single-particle band structure and topology. We find that the characteristic TBG flat bands reemerge at $2\pi$ flux, but, due to the magnetic field breaking $C_{2z} \mathcal{T}$, they split and acquire Chern number $\pm1$. We show that reentrant correlated insulating states appear at $2\pi$ flux driven by the Coulomb interaction at integer fillings, and we predict the characteristic Landau fans from their excitation spectrum. We conjecture that superconductivity can also be re-entrant at $2\pi$ flux.