Magnetism and topology in graphene-based flat minibands

Until recently, flat bands were achieved as Landau levels at high magnetic field. Recently, Pablo Jarillo-Herrero of MIT and coworkers demonstrated flat minibands in graphene-based superlattices, discovering correlated insulators and superconductors at different fillings. We have now discovered dramatic magnetic states in such systems. Specifically, in magic-angle twisted bilayer graphene also aligned with a hexagonal boron nitride (hBN) cladding layer, we observe a giant anomalous Hall effect and signs of chiral edge states. This all occurs at zero magnetic field, near 3 electrons per moire cell in the conduction miniband [1]. Remarkably, the magnetization of the sample can be reversed by applying a small DC current. Although the anomalous Hall resistance is not quantized, and dissipation is significant, we suggest that the system is an incipient Chern insulator, similar to an integer quantum Hall state. In a different superlattice system, ABC-trilayer graphene aligned with hBN, again near 3 electrons per moire cell a Chern insulator emerges [2]. A magnetic field of order 100 mT is needed to quantize the anomalous hall signal. This trilayer system can be tuned in-situ to display superconductivity instead of magnetism [3]. We will discuss possible magnetic states, and complementary probes to examine which state actually emerges as the ground state in each system.

[1] A.L. Sharpe et al., “Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene”, Science (2019).
[2] G. Chen et al., “Tunable Correlated Chern Insulator and Ferromagnetism in Trilayer Graphene/Boron Nitride Moire Superlattice”, arXiv:1905.06535 (2019).
[3] G. Chen et al., “Signatures of tunable superconductivity in a trilayer graphene moiré superlattice”, Nature (2019).