Double-layer graphene and topological insulator thin-film plasmons
  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. 
Rosario E. V. Profumo, Reza Asgari, Marco Polini, A. H. MacDonald
cond-mat.mes-hall