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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 moiré superlattices. The pronounced screening effects observed in the moiré 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 moiré superlattice as a unique 3D system for exploring correlated surface states.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: Flat-band systems, ferromagnetism, rhombohedral graphene

Date published: 2024-03-22

Publisher: Springer Science and Business Media LLC

Journal:

• Nature Communications (ISSN: 20411723) vol. 15 issue. 1 2597

Funding:

• National Natural Science Foundation of China 12274354
• Natural Science Foundation of Zhejiang Province XHD23A2001
• Natural Science Foundation of Zhejiang Province LR24A040003

Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1038/s41467-024-46913-5

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Updated at: 2025-02-07 16:30:55 +0900

Published on MDR: 2025-02-07 16:30:55 +0900