  Smooth interfaces of topological systems are known to host massive surface
states along with the topologically protected chiral one. We show that in Weyl
semimetals these massive states, along with the chiral Fermi arc, strongly
alter the form of the Fermi-arc plasmon, Most saliently, they yield further
collective plasmonic modes that are absent in a conventional interfaces. The
plasmon modes are completely anisotropic as a consequence of the underlying
anisotropy in the surface model and expected to have a clear-cut experimental
signature, e.g. in electron-energy loss spectroscopy.
