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The flat electronic bands in magic-angle twisted bilayer graphene (MATBG) host a variety of correlated insulating ground states, many of which are predicted to support charged excitations with topologically non-trivial spin and/or valley skyrmion textures. However, it has remained challenging to experimentally address their ground state order and excitations, both because some of the proposed states do not couple directly to experimental probes, and because they are highly sensitive to spatial inhomogeneities in real samples. Here, using a scanning single-electron transistor, we observe thermodynamic gaps at even integer moiré filling factors at low magnetic fields. We find evidence of a field-tuned crossover from charged spin skyrmions to bare particle-like excitations, and use this to establish that the underlying ground state is a Kramers intervalley-coherent insulator. From the spatial dependence of these states and the chemical potential variation within the flat bands, we infer a link between the stability of the correlated ground states and local twist angle and strain. Our work advances the microscopic understanding of the correlated insulators in MATBG and their unconventional excitations, giving insight into the potential mechanisms by which they seed superconductivity upon doping.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: Magic-angle twisted bilayer graphene, correlated insulating states, spin skyrmions

Date published: 2023-10-21

Publisher: Springer Science and Business Media LLC

Journal:

• Nature Communications (ISSN: 20411723) vol. 14 issue. 1 6679

Funding:

• U.S. Department of Energy DE-SC0021238
• Alfred P. Sloan Foundation
• Stanford Unversity Terman Fellowship

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

MDR DOI:

First published URL: https://doi.org/10.1038/s41467-023-42275-6

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Updated at: 2025-02-15 12:31:04 +0900

Published on MDR: 2025-02-15 12:31:04 +0900