Extended Quantum Anomalous Hall States in Graphene/hBN Moir\'e Superlattices

TL;DR

This study reports the discovery of extended quantum anomalous Hall states and multiple fractional quantum anomalous Hall states in graphene/hBN moire superlattices at very low temperatures, revealing new topological phases driven by electron correlations.

Contribution

It provides experimental evidence of extended quantum anomalous Hall states and multiple fractional states in rhombohedral graphene/hBN moire superlattices, expanding understanding of topological flat band systems.

Findings

Observation of two additional FQAH states in penta-layer devices.

Detection of FQAHE at v=3/5 and 2/3 in tetra-layer devices at 300 mK.

Identification of a new extended quantum anomalous Hall state with magnetic hysteresis.

Abstract

Electrons in topological flat bands can form novel topological states driven by the correlation effects. The penta-layer rhombohedral graphene/hBN moire superlattice has been shown to host fractional quantum anomalous Hall effect (FQAHE) at ~400 mK, triggering discussions around the underlying mechanism and the role of moire effects. In particular, novel electron crystal states with non-trivial topology have been proposed. Here we report DC electrical transport measurement in rhombohedral penta- and tetra-layer graphene/hBN moire superlattices at electronic temperatures down to ~40 mK. We observed two more FQAH states in the penta-layer devices than previously reported. In a new tetra-layer device, we observed FQAHE at filling factors v = 3/5 and 2/3 at 300 mK. With a small bias current and the lowest temperature, we observed a new extended quantum anomalous Hall (EQAH) state and magnetic hysteresis, where Rxy = h/e2 and vanishing Rxx span a wide range of moire filling factor v from 0.5 to up to 1.3. By increasing the temperature or current, FQAHE can be recovered -- suggesting the break-down of the EQAH states and a phase transition into the fractional quantum Hall liquid. Furthermore, we observed displacement field-induced quantum phase transitions from the EQAH states to Fermi liquid, FQAH liquid and the likely composite Fermi liquid. Our observation establishes a new topological phase of electrons with quantized Hall resistance at zero magnetic field, and enriches the emergent quantum phenomena in materials with topological flat bands.