  Topological domain walls in dual-gated gapped bilayer graphene host edge
states that are gate- tunable and valley polarized. Here we predict that
plasmonic collective modes can propagate along these topological domain walls
even at zero bulk density, and possess a markedly different character from that
of bulk plasmons. Strikingly, domain wall plasmons are extremely long-lived,
with plasmon lifetimes that can be orders of magnitude larger than the
transport scattering time in the bulk. While most pronounced at low
temperatures, long domain wall plasmon lifetimes persist even at room
temperature with values up to a few picoseconds. Domain wall plasmons possess a
rich phenomenology including a wide range of frequencies (up to the
mid-infrared), tunable sub-wavelength electro-magnetic confinement lengths, as
well as a valley polarization for forward/backward propagating modes. Its
unusual features render them a new tool for realizing low-dissipation
plasmonics that transcend the restrictions of the bulk.
