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Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene

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Science  09 Aug 2019:
Vol. 365, Issue 6453, pp. 605-608
DOI: 10.1126/science.aaw3780

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Twisted bilayer graphene goes magnetic

When two layers of graphene in a bilayer are twisted with respect to each other by just the right, “magic,” angle, the electrons in the system become strongly correlated. As the electronic density is tuned by gating, the system goes through several exotic phases, including superconductivity. Now, Sharpe et al. show that, at a particular electronic density, magic-angle graphene becomes magnetic (see the Perspective by Pixley and Andrei). The finding is supported by the observation of a large anomalous Hall effect.

Science, this issue p. 605; see also p. 543

Abstract

When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interactions. Here, we present evidence that near three-quarters (34) filling of the conduction miniband, these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. In a narrow density range around an apparent insulating state at 34, we observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as 10.4 kilohms and indications of chiral edge states. Notably, the magnetization of the sample can be reversed by applying a small direct current. Although the AH resistance is not quantized, and dissipation is present, our measurements suggest that the system may be an incipient Chern insulator.

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