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Massive Dirac fermions and Hofstadter butterfly in a van der Waals heterostructure

Published June 21st in Science: our new paper on the behavior of graphene Dirac electrons in the presence of a moiré superlattice potential.

http://www.sciencemag.org/content/early/2013/05/15/science.1237240

See this nice perspective in the same issue from Michael Fuhrer.

Also, see MIT News for a nice description of the work.

Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically-thin planar crystals. Here, we demonstrate band structure engineering of 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 moire' superlattice potential in the graphene. This interplay between short- and long-wavelength effects results in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality, both of which can be tuned by varying the interlayer alignment. Magnetocapacitance measurements reveal previously unobserved fractional quantum Hall states reflecting the massive Dirac dispersion that results from broken sublattice symmetry. At ultra-high fields, integer conductance plateaus are observed at non-integer filling factors due to the emergence of the Hofstadter butterfly in a symmetry-broken Landau level.