Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure

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Science  16 May 2013:
DOI: 10.1126/science.1237240

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Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically thin planar crystals. Here, we demonstrate band structure engineering in a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally aligned hexagonal boron nitride substrate. The spatially varying interlayer atomic registry results both in a local breaking of the carbon sublattice symmetry and a long-range moiré superlattice potential in the graphene. In our samples, this interplay between short- and long-wavelength effects resulted in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality. This picture is confirmed by our observation of fractional quantum Hall states at ±5/3 filling and features associated with the Hofstadter butterfly at ultrahigh magnetic fields.

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