  The Landauer "residual resistivity dipole" is a well-known concept in
electron transport through a disordered medium. It is formed when a
defect/scatterer reflects an impinging electron causing negative charges to
build up on one side of the scatterer and positive charges on the other. This
results in the formation of a microscopic electric dipole that affects the
resistivity of the medium. Here, we show that an equivalent entity forms in
spin polarized electron transport through the surface of a disordered
topological insulator (TI). When electrons reflect from a scatterer on the TI
surface, a spin imbalance forms around the scatterer, resulting in a spin
current that flows either in the same or the opposite direction as the injected
spin current and hence either increases or decreases the spin resistivity. It
also destroys spin-momentum locking and produces a magnetic field around the
scatterer. The latter will cause transiting spins to precess as they pass the
scatterer, thereby radiating electromagnetic waves and implementing an
oscillator. If an alternating current is passed through the TI instead of a
static current, the magnetic field will oscillate with the frequency of the
current and radiate electromagnetic waves of the same frequency, thus making
the scatterer act as a miniature antenna.
