Correlated insulator behaviour at half-filling in magic-angle graphene superlattices

When the two graphene sheets in a van der Waals heterostructure are twisted relative to each other by a specific amount, Mott-like insulating phases are observed at half-filling. Strong interactions between electrons are behind many exotic phenomena in condensed matter, such as the fractional quantum Hall effect and Mott insulator transitions. Of the many different classes of materials that exhibit strong electronic interactions, some of the most intensively studied are oxides with complex structures. However, their structure is often difficult to tune. Pablo Jarillo-Herrero and colleagues now show that when two graphene sheets are twisted by a certain angle a new insulating state emerges that appears to be driven by strong electronic interactions, consistent with a Mott-like insulator phase. This system can easily be tuned through both the twist angle and electric fields, and could provide a new two-dimensional platform for probing strongly correlated phases such as high-temperature superconductivity. Elsewhere in this issue, the same team reports that twisting graphene sheets in this manner also makes them exhibit unconventional superconductivity, with features similar to high-temperature superconducting cuprates. A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials1. One such property is the ‘twist’ angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analogue in other types of heterostructure, such as semiconductors grown using molecular beam epitaxy. For small twist angles, the moiré pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concentrated mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride2,3,4,5. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show experimentally that, as predicted theoretically6, when this angle is close to the ‘magic’ angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moiré pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through electrical tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liquids.