Robust Interlayer-Coherent Quantum Hall States in Twisted Bilayer Graphene

TL;DR

This paper demonstrates that twisted bilayer graphene with a large twist angle can host high-temperature interlayer-coherent quantum Hall states, showing significant energy gaps and phase transitions, making it a promising platform for excitonic condensation.

Contribution

The study introduces a new system with ultrastrong interlayer interactions and suppressed tunneling, leading to the observation of high-temperature interlayer-coherent quantum Hall states at large twist angles.

Findings

Observation of odd-integer quantum Hall states with interlayer coherence at N=1

Energy gaps of about 1 K for these states, much larger than in GaAs

Experimental phase transitions consistent with phenomenological models

Abstract

We introduce a novel two-dimensional electronic system with ultrastrong interlayer interactions, namely twisted bilayer graphene with a large twist angle, as an ideal ground for realizing interlayer-coherent excitonic condensates. In these systems, subnanometer atomic separation between the layers allows significant interlayer interactions, while interlayer electron tunneling is geometrically suppressed due to the large twist angle. By fully exploiting these two features we demonstrate that a sequence of odd-integer quantum Hall states with interlayer coherence appears at the second Landau level (N = 1). Notably the energy gaps for these states are of order 1 K, which is several orders of magnitude greater than those in GaAs. Furthermore, a variety of quantum Hall phase transitions are observed experimentally. All the experimental observations are largely consistent with our phenomenological model calculations. Hence, we establish that a large twist angle system is an excellent platform for high-temperature excitonic condensation.