Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene

We describe the realization of van der Waals (vdW) heterostructures with accurate rotational alignment of individual layer crystal axes. We illustrate the approach by demonstrating a Bernal-stacked bilayer graphene formed using successive transfers of monolayer graphene flakes. The Raman spectra of this artificial bilayer graphene possess a wide 2D band, which is best fit by four Lorentzians, consistent with Bernal stacking. Scanning tunneling microscopy reveals no moiré pattern on the artificial bilayer graphene, and tunneling spectroscopy as a function of gate voltage reveals a constant density of states, also in agreement with Bernal stacking. In addition, electron transport probed in dual-gated samples reveals a band gap opening as a function of transverse electric field. To illustrate the applicability of this technique to realize vdW heterostructuctures in which the functionality is critically dependent on rotational alignment, we demonstrate resonant tunneling double bilayer graphene heterostructures separated by hexagonal boron-nitride dielectric.

Heterostructures formed by stacking layered materials require atomically clean interfaces. However, contaminants are usually trapped between the layers, aggregating into randomly located blisters, incompatible with scalable fabrication processes. Here we report a process to remove blisters from fully formed heterostructures. Our method is over an order of magnitude faster than those previously reported and allows multiple interfaces to be cleaned simultaneously. We fabricate blister-free regions of graphene encapsulated in hexagonal boron nitride with an area ~ 5000 μm2, achieving mobilities up to 180,000 cm2 V−1 s−1 at room temperature, and 1.8 × 106 cm2 V−1 s−1 at 9 K. We also assemble heterostructures using graphene intentionally exposed to polymers and solvents. After cleaning, these samples reach similar mobilities. This demonstrates that exposure of graphene to process-related contaminants is compatible with the realization of high mobility samples, paving the way to the development of wafer-scale processes for the integration of layered materials in (opto)electronic devices.