Superconductivity and quantized anomalous Hall in rhombohedral graphene

TL;DR

This paper demonstrates the coexistence of superconductivity and quantized anomalous Hall effect in rhombohedral graphene, enabling reconfigurable topological edge modes and topologically ordered states at zero magnetic field.

Contribution

It reports the first observation of simultaneous quantized anomalous Hall and superconducting states in rhombohedral graphene, with tunable chirality and fractional Chern insulator phases.

Findings

Quantized anomalous Hall state at superlattice filling ν=-1.

Superconductivity observed at ν≈-3.5 without magnetic field.

Gate-controlled nonvolatile switching of quantum Hall chirality.

Abstract

Inducing superconducting correlations in chiral edge states is predicted to generate topologically protected zero energy modes with exotic quantum statistics. Experimental efforts to date have focused on engineering interfaces between superconducting materials typically amorphous metals and semiconducting quantum Hall or quantum anomalous Hall (QAH) systems. However, the interfacial disorder inherent in this approach can prevent the formation of isolated topological modes. An appealing alternative is to use low-density flat band materials where the ground state can be tuned between intrinsic superconducting and quantum anomalous Hall states using only the electric field effect. However, quantized transport and superconductivity have not been simultaneously achieved. Here, we show that rhombohedral tetralayer graphene aligned to a hexagonal boron nitride substrate hosts a quantized anomalous Hall state at superlattice filling $\nu=-1$ as well as a superconducting state at $\nu-3.5$ at zero magnetic field. Remarkably, gate voltage can also be used to actuate nonvolatile switching of the chirality in the quantum anomalous Hall state, allowing, in principle, arbitrarily reconfigurable networks of topological edge modes in locally gated devices. Thermodynamic compressibility measurements further reveal a topologically ordered fractional Chern insulator at $\nu=2/3$-also stable at zero magnetic field-enabling proximity coupling between superconductivity and fractionally charged edge modes. Finally, we show that, as in rhombohedral bi- and trilayers, integrating a transition metal dichalcogenide layer to the heterostructure nucleates a new superconducting pocket, while leaving the topology of the $\nu=-1$ quantum anomalous Hall state intact. Our results pave the way for a new generation of hybrid interfaces between superconductors and topological edge states in the low-disorder limit.