  Light propagates symmetrically in opposite directions in most materials and
structures. This fact -- a consequence of the Lorentz reciprocity principle --
has tremendous implications for science and technology across the
electromagnetic spectrum. Here, we investigate an emerging approach to break
reciprocity that does not rely on magneto-optical effects or spacetime
modulations, but is instead based on biasing a plasmonic material with a direct
electric current. Using a 3D Green function formalism and microscopic
considerations, we elucidate the propagation properties of surface
plasmon-polaritons (SPPs) supported by a generic nonreciprocal platform of this
type, revealing some previously overlooked, anomalous, wave-propagation
effects. We show that SPPs can propagate in the form of steerable, slow-light,
unidirectional beams associated with inflexion points in the modal dispersion.
We also clarify the impact of dissipation (due to collisions and Landau
damping) on nonreciprocal effects and shed light on the connections between
inflexion points, exceptional points at band-edges, and complex modal
transitions in leaky-wave structures. We then apply these concepts to the
important area of thermal photonics, and provide the first theoretical
demonstration of drift-induced nonreciprocal near-field radiative heat transfer
between two planar bodies. Our findings may open new opportunities toward the
development of nonreciprocal magnet-free devices that combine the benefits of
plasmonics and nonreciprocal photonics for wave-guiding and energy
applications.
