▎ 摘　　要

We present numerical and analytical results for the optical and acoustic plasmon collective modes of coupled massless-Dirac two-dimensional electron systems. Our results apply to topological insulator (TI) thin films and to two graphene sheets separated by a thin dielectric barrier layer. We find that, because of strong bulk dielectric screening, TI acoustic modes are locked to the top of the particle-hole continuum and, therefore, probably unobservable.