Surface ferromagnetism in rhombohedral heptalayer graphene moire superlattice

TL;DR

This paper demonstrates tunable surface ferromagnetism in rhombohedral heptalayer graphene moire superlattices, revealing a new platform for exploring correlated surface states driven by flat bands and electron interactions.

Contribution

It introduces a method to isolate surface flat bands in rhombohedral heptalayer graphene using moire superlattices, enabling the study of correlated surface phenomena like ferromagnetism.

Findings

Observation of tunable surface ferromagnetism via anomalous Hall effect

Isolation of surface flat bands in rhombohedral 7L graphene

Evidence of surface state polarization using displacement fields

Abstract

The topological electronic structure of crystalline materials often gives rise to intriguing surface states, such as Dirac surface states in topological insulators, Fermi arc surface states in Dirac semimetals, and topological superconductivity in iron-based superconductors. Recently, rhombohedral multilayer graphene has emerged as a promising platform for exploring exotic surface states due to its hosting of topologically protected surface flat bands at low energy, with the layer-dependent energy dispersion. These flat bands can promote electron correlations, leading to a plethora of quantum phenomena, including spontaneous symmetry breaking, superconductivity, ferromagnetism, and topological Chern insulators. Nevertheless, the intricate connection between the surface flat bands in rhombohedral multilayer graphene and the highly dispersive high-energy bands hinders the exploration of correlated surface states. Here, we present a method to isolate the surface flat bands of rhombohedral heptalayer (7L) graphene by introducing moire superlattices. The pronounced screening effects observed in the moire potential-modulated rhombohedral 7L graphene indicate its essential three-dimensional (3D) nature. The isolated surface flat bands favor correlated states on the surface in the regions away from charge-neutrality points. Most notably, we observe tunable surface ferromagnetism, manifested as an anomalous Hall effect with hysteresis loops, which is achieved by polarizing surface states using finite displacement fields. Our work establishes rhombohedral multilayer graphene moire superlattice as a unique 3D system for exploring correlated surface states.