Odd integer quantum Hall states with interlayer coherence in twisted bilayer graphene

TL;DR

This paper reports the observation of odd-integer quantum Hall states in twisted bilayer graphene, attributed to Coulomb interaction-induced interlayer coherence and exciton condensation, revealing complex phase transitions influenced by density and tunneling strength.

Contribution

It demonstrates the emergence of odd-integer quantum Hall states driven by interlayer coherence in twisted bilayer graphene, a phenomenon not anticipated in traditional models.

Findings

Observation of odd-integer quantum Hall states with interlayer coherence.

Reentrant behavior due to phase transition from coherent to incompressible states.

Suppression of interlayer coherence with increased tunneling at higher densities.

Abstract

We report on the quantum Hall effect in two stacked graphene layers rotated by 2 degree. The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the density. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densities, yet it is observed. We interpret this as a signature of Coulomb interaction induced interlayer coherence and Bose Einstein condensation of excitons that form at half filling of each layer. A density imbalance gives rise to reentrant behavior due to a phase transition from the interlayer coherent state to incompressible behavior caused by simultaneous condensation of both layers in different quantum Hall states. With increasing overall density, magnetic breakdown gains the upper hand. As a consequence of the enhanced interlayer tunneling, the interlayer coherent state and the phase transition vanish.