Edge-driven transition between extended quantum anomalous Hall crystal and fractional Chern insulator in rhombohedral graphene multilayers

TL;DR

This paper explains the transition between extended quantum anomalous Hall crystals and fractional Chern insulators in rhombohedral graphene multilayers through edge mode entropy considerations, influenced by disorder, temperature, and current.

Contribution

It proposes a topological edge mode-based scenario to understand the EQAH to FCI transition driven by entropy effects and edge velocity differences.

Findings

Edge mode velocity in FCI is smaller than in EQAH.

Disorder induces a network of domains with opposite Hall conductance.

Edge entropy influences phase transitions via temperature and current.

Abstract

Fractional Chern insulators (FCI) with fractionally quantized Hall conductance at fractional fillings and an extended quantum anomalous Hall (EQAH) crystal with an integer quantized Hall conductance over an extended region of doping were recently observed in pentalayer graphene. One particularly puzzling observation is the transition between the EQAH and FCI regimes, driven either by temperature or electrical current. Here we propose a scenario to understand these transitions based on the topologically protected gapless edge modes that are present in both the FCI and EQAH phases and should be most relevant at temperature scales below the energy gap. Our consideration is based on the simple assumption that the edge velocity in FCI is smaller than that in EQAHE and thus contributes to a higher entropy. We further argue that domains with opposite fractionally quantized Hall conductance are ubiquitous in the devices due to disorder, which gives rise to a network of edge modes. The velocity of the edge modes between domains is further reduced due to edge reconstruction. The edge velocity can also be reduced by current when the occupation of the edge mode approaches the gap edge. The edge entropy therefore drives the transition from EQAH to FCI either by temperature or current at a nonzero temperature.