The Study of Plasmons in Topological Insulators and Semimetals Using High Resolution Electron Energy Loss Spectroscopy

Plasmon is a fundamental collective excitation in physics related to numerous technological applications. However plasmons usually have very short lifetime in large momentum range due to strong damping, independent of their origin from normal massive electrons or from Dirac massless electrons. Then the properties of plasmons in topologically nontrivial systems that have massless electrons with longer lifetime are extremely important. Here, we report the study of the properties of plasmons in three-dimensional (3D) Topological insulators (TIs), Dirac Semimetals (DSMs) and Weyl Semimetals (WSMs). In a typical 3D TI, Bi₂Se₃, we observed an anomalous acoustic plasmon on the surface with weak damping in a large momentum range, which likely relates to the spin-momentum locking feature of the TI surface states. However, in DSM and WSM, it's more complicated because both topologically trivial and nontrivial 3D bands contribute to Fermi Surface (FS) and thus they are both involved in the origination of plasmons. In PdTe₂ (a type-II DSM) and MoTe₂ (a type-II WSM), we both observed possible surface and bulk plasmon modes. By carefully examining the dependence of the plasmon energy on the momentum, electron density and Fermi velocity, we provide possible explanations to these modes.