Electronic collective excitations in topological semimetals

Plasmons are self-sustained collective excitations of electron liquid, which have received increasing attention since its proposal by David Pines at 1960s. For the great potential in applications, the researches on plasmons make great advances all the way from semiconductors, metals, semimetals, to monolayer graphene. With the fast development of the field of topological materials, the research of plasmons has been extended into topological insulators, generating many exciting discoveries related to the topologically protected surface states. Topological semimetals, exhibiting various fantastic properties different from topological insulators, have become another research focus in condensed matter. Recently the plasmons in topological semimetals, providing a new perspective to further understand and utilize the topological states, have been attracting more and more attention. In this article, we review the recent theoretical and experimental investigations on the plasmons of topological semimetals, including the Dirac, Weyl and nodal line semimetals. In theoretical aspects, main different behaviors between the plasmons of topological semimetals and traditional metals are reviewed, such as the quantum nature, unusual dependence on temperature and charge carrier density, and the properties related to the chiral anomaly and Fermi arcs. The experimental studies are less reported, and the review is mainly focused on the measurements of optical conductivity and electron energy loss spectra in several typical real materials. Finally, the prospects of the future of the plasmons in topological semimetals in theories and experiments are outlooked.