  Topological surface states of three-dimensional topological insulator
nanoribbons and their distinct magnetoconductance properties are promising for
topoelectronic applications and topological quantum computation. A crucial
building block for nanoribbon-based circuits are three-terminal junctions.
While the transport of topological surface states on a planar boundary is not
directly affected by an in-plane magnetic field, the orbital effect cannot be
neglected when the surface states are confined to the boundary of a nanoribbon
geometry. Here, we report on the magnetotransport properties of such
three-terminal junctions. We observe a dependence of the current on the
in-plane magnetic field, with a distinct steering pattern of the surface state
current towards a preferred output terminal for different magnetic field
orientations. We demonstrate that this steering effect originates from the
orbital effect, trapping the phase-coherent surface states in the different
legs of the junction on opposite sides of the nanoribbon and breaking the
left-right symmetry of the transmission across the junction. The reported
magnetotransport properties demonstrate that an in-plane magnetic field is not
only relevant but also very useful for the characterization and manipulation of
transport in three-dimensional topological insulator nanoribbon-based junctions
and circuits, acting as a topoelectric current switch.
