  Unidirectional surface plasmon polaritons (SPPs) at the interface between a
gyrotropic medium and a simple medium are studied in a newly-recognized
frequency regime wherein the SPPs form narrow, beam-like patterns due to
hyperbolic dispersion. The SPP beams are steerable by controlling parameters
such as the cyclotron frequency (external bias) or the frequency of operation.
The bulk band structure along different propagation directions is examined to
ascertain a common bandgap, valid for all propagation directions, which the
SPPs cross. The case of a finite-thickness gyrotropic slab is also considered,
for which we present the Green function and examine the thickness and loss
level required to maintain a unidirectional SPP.
