  Topological materials may exhibit Hall-like currents flowing transversely to
the applied electric field even in the absence of a magnetic field. In graphene
superlattices, which have broken inversion symmetry, topological currents
originating from graphene's two valleys are predicted to flow in opposite
directions and combine to produce long-range charge neutral flow. We observe
this effect as a nonlocal voltage at zero magnetic field in a narrow energy
range near Dirac points at distances as large as several microns away from the
nominal current path. Locally, topological currents are comparable in strength
to the applied current, indicating large valley-Hall angles. The long-range
character of topological currents and their transistor-like control by gate
voltage can be exploited for information processing based on the valley degrees
of freedom.
