  Plexcitons are polaritonic modes that result from the strong coupling between
excitons and plasmons. We consider plexcitons emerging from the interaction of
excitons in an organic molecular layer with surface plasmons in a metallic
film. We predict the emergence of Dirac cones in the two-dimensional
bandstructure of plexcitons due to the inherent alignment of the excitonic
transitions in the organic layer. These Dirac cones may open up in energy by
simultaneously interfacing the metal with a magneto-optical layer and
subjecting the whole system to a perpendicular magnetic field. The resulting
energy gap becomes populated with topologically protected one-way modes which
travel at the interface of this plexcitonic system. Our theoretical proposal
suggests that plexcitons are a convenient and simple platform for the
exploration of exotic phases of matter as well as of novel ways to direct
energy flow at the nanoscale.
