Spin-charge separation of plasmonic excitations in thin topological insulators

We discuss plasmonic excitations in a thin slab of topological insulators. In the limit of no hybridization of the surface states and same electronic density of the two layers, the electrostatic coupling between the top and bottom layers leads to optical and acoustic plasmons which are purely charge and spin collective oscillations. We then argue that a recent experiment on the plasmonic excitations of Bi sub(2) Se sub(3) [Di Pietro et al., Nat. Nanotechnol. 8. 556 (2013) (http://dx.doi.org/10.1038/nnano.2013.134)] must be explained by including the charge response of the two-dimensional electron gas of the depletion layer underneath the two surfaces. We also present an analytic formula to fit their data.