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.