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Ballistic miniband conduction in a graphene superlattice
Rational design of artificial lattices yields effects unavailable in simple
solids, and vertical superlattices of multilayer semiconductors are already
used in optical sensors and emitters. Manufacturing lateral superlattices
remains a much bigger challenge, with new opportunities offered by the use of
moire patterns in van der Waals heterostructures of graphene and hexagonal
crystals such as boron nitride (h-BN). Experiments to date have elucidated the
novel electronic structure of highly aligned graphene/h-BN heterostructures,
where miniband edges and saddle points in the electronic dispersion can be
reached by electrostatic gating. Here we investigate the dynamics of electrons
in moire minibands by transverse electron focusing, a measurement of ballistic
transport between adjacent local contacts in a magnetic field. At low
temperatures, we observe caustics of skipping orbits extending over hundreds of
superlattice periods, reversals of the cyclotron revolution for successive
minibands, and breakdown of cyclotron motion near van Hove singularities. At
high temperatures, we study the suppression of electron focusing by inelastic
scattering
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