Unusual magnetotransport in twisted bilayer graphene.

SignificanceWhen two sheets of graphene are twisted to the magic angle of 1.1, the resulting flat moiré bands can host exotic correlated electronic states such as superconductivity and ferromagnetism. Here, we show transport properties of a twisted bilayer graphene device at 1.

Tunable Orbital Ferromagnetism at Noninteger Filling of a Moiré Superlattice.

The flat bands resulting from moiré superlattices exhibit fascinating correlated electron phenomena such as correlated insulators, ( 2018, 556 (7699), 80-84), ( 2019, 15 (3), 237) superconductivity, ( 2018, 556 (7699), 43-50), ( 2019, 572 (7768), 215-219) and orbital magnetism. ( 2019, 365 (6453), 605-608), ( 2020, 579 (7797), 56-61), ( 2020, 367 (6480), 900-903) Such magnetism has been observed only at particular integer multiples of , the density corresponding to one electron per moiré superlattice unit cell. Here, we report the experimental observation of ferromagnetism at noninteger filling (NIF) of a flat Chern band in a ABC-TLG/hBN moiré superlattice.

Evidence of Orbital Ferromagnetism in Twisted Bilayer Graphene Aligned to Hexagonal Boron Nitride.

We have previously reported ferromagnetism evinced by a large hysteretic anomalous Hall effect in twisted bilayer graphene (tBLG). Subsequent measurements of a quantized Hall resistance and small longitudinal resistance confirmed that this magnetic state is a Chern insulator. Here, we report that when tilting the sample in an external magnetic field, the ferromagnetism is highly anisotropic.

Publisher Correction: Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice.

Studies of two-dimensional electron systems in a strong magnetic field revealed the quantum Hall effect, a topological state of matter featuring a finite Chern number C and chiral edge states. Haldane later theorized that Chern insulators with integer quantum Hall effects could appear in lattice models with complex hopping parameters even at zero magnetic field. The ABC-trilayer graphene/hexagonal boron nitride (ABC-TLG/hBN) moiré superlattice provides an attractive platform with which to explore Chern insulators because it features nearly flat moiré minibands with a valley-dependent, electrically tunable Chern number.

Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene.

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 ([Formula: see text]) 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 [Formula: see text], we observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as 10.

Signatures of tunable superconductivity in a trilayer graphene moiré superlattice.

Understanding the mechanism of high-transition-temperature (high-T) superconductivity is a central problem in condensed matter physics. It is often speculated that high-T superconductivity arises in a doped Mott insulator as described by the Hubbard model. An exact solution of the Hubbard model, however, is extremely challenging owing to the strong electron-electron correlation in Mott insulators.

Zero-field edge plasmons in a magnetic topological insulator.

Incorporating ferromagnetic dopants into three-dimensional topological insulator thin films has recently led to the realisation of the quantum anomalous Hall effect. These materials are of great interest since they may support electrical currents that flow without resistance, even at zero magnetic field. To date, the quantum anomalous Hall effect has been investigated using low-frequency transport measurements.