  We study the topological edge plasmon modes between two "diatomic" chains of
identical plasmonic nanoparticles. Zak phase for longitudinal plasmon modes in
each chain is calculated analytically by solutions of macroscopic Maxwell's
equations for particles in quasi-static dipole approximation. This
approximation provides a direct analogy with the Su-Schrieffer-Heeger model
such that the eigenvalue is mapped to the frequency dependent
inverse-polarizability of the nanoparticles. The edge state frequency is found
to be the same as the single-particle resonance frequency, which is insensitive
to the separation distances within a unit cell. Finally, full electrodynamic
simulations with realistic parameters suggest that the edge plasmon mode can be
realized through near-field optical spectroscopy.
