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Twisted double bilayer graphene (tDBG) comprises two Bernal-stacked bilayer graphene sheets with a twist between them. Gate voltages applied to top and back gates of a tDBG device tune both the flatness and topology of the electronic bands, enabling an unusual level of experimental control. Broken spin/valley symmetry metallic states have been observed in tDBG devices with twist angles ∼ 1.2-1.3◦, but the topologies and order parameters of these states have remained unclear. We report the observation of an anomalous Hall effect in the correlated metal state of tDBG, with hysteresis loops spanning 100s of mT in out-of-plane magnetic field that demonstrate spontaneously broken time-reversal symmetry. The B⊥ hysteresis persists for in-plane fields up to several Tesla, suggesting valley (orbital) ferromagnetism. At the same time, the resistivity is strongly affected by even mT-scale values of in-plane magnetic field, pointing to spin-valley coupling or to a direct orbital coupling between in-plane field and valley symmetry.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: wisted double bilayer graphene, anomalous Hall effect, ferromagnetism

Date published: 2022-10-29

Publisher: Springer Science and Business Media LLC

Journal:

• Nature Communications (ISSN: 20411723) vol. 13 issue. 1 6468

Funding:

• Canada Foundation for Innovation
• Gouvernement du Canada | Natural Sciences and Engineering Research Council of Canada

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

MDR DOI:

First published URL: https://doi.org/10.1038/s41467-022-34192-x

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Updated at: 2025-02-27 08:31:02 +0900

Published on MDR: 2025-02-27 08:31:02 +0900