Plasmons in Two-Dimensional Topological Insulators

We analyze collective excitations in models of two-dimensional topological insulators, revealing characteristic signatures that distinguish different topological regimes by their plasmonic response. Using the random phase approximation in real and reciprocal space, we show how topology influences both bulk and surface plasmon properties. Bulk features are controlled by the overlap of the single particle wave functions, which in turn lead to a strong hardening and softening of inter- and intra-band plasmonic branches, respectively, in the topologically non-trivial regimes. Plasmonic surface modes are found to be governed by electronic transitions involving the mid-gap topological edge states, resulting in characteristic charge-density distributions localized at the boundaries of the system.