Collective excitonic and plasmonic excitations on a surface of 3D topological insulator

Abstract

Properties of collective excitonic and plasmonic excitations on the surface of three-dimensional topological insulator are reviewed. Due to spin-momentum locking for electrons populating the surface states plasmonic excitation manifests itself as coupled spin- and density-wave and can be called "spin-plasmon". Its excitation induces spin polarization of topological insulator surface that is perpendicular to the spin-plasmon momentum. Excitons appear on the surface when the gap in the surface spectrum is magnetically induced. Excitons correspond to the set of subgap levels with unusual dependence of the energy on orbital angular momentum quantum number and can be called "chiral". Contrary to conventional excitons chiral ones resonantly contribute to Hall component of optical conductivity tensor and play significant role in magneto-optical Faraday and Kerr effects. They resonantly enhance Faraday effect and reduce Kerr effect.