Observation of Magnetoplasmons in Bi₂Se₃ Topological Insulator

Both the collective (plasmon) and the single particle (Drude) excitations of an electron gas can be controlled and modified by an external magnetic field <i>B</i>. At finite <i>B</i>, plasmon gives rise to a magnetoplasmon mode and the Drude term to a cyclotron resonance. These magnetic effects are expected to be extremely strong for Dirac electrons with a linear energy-momentum dispersion, like those present in graphene and topological insulators (TIs). Here, we investigate both the plasmon and the Drude response versus <i>B</i> in Bi₂Se₃ topological insulator. At low <i>B</i>, the cyclotron resonance is still well separated in energy from the magnetoplasmon mode; meanwhile, both excitations asymptotically converge at the same energy for increasing <i>B</i>, consistently with a dynamical mass for Dirac carriers of <i>m</i>_D^* = 0.18 ± 0.01 m_<i>e</i>. In TIs, one then achieves an excellent magnetic control of plasmonic excitations and this could open the way toward plasmon controlled terahertz magneto-optics