Collective modes of Dirac and Weyl semimetals in strong magnetic fields

Dirac and Weyl semimetals provide a new example of three-dimensional electron gases which are sensitive to strong magnetic fields. In this paper we address their collective excitations in the extreme quantum limit in which the Hamiltonian can be projected to the $N=0$ Landau level. We show that the energy of the plasmon collective mode is not shifted by interactions, even though the quasiparticle bands are strongly renormalized, and that both optical and acoustic magnetoplasmons emerge from the particle-hole continuum when the chemical potential is not at the band-crossing point. The rare presence of a sharp acoustic magnetoplasmon excitation can be understood as a consequence of the unusual particle-hole excitation phase space of these pseudo-1D systems, and could prove valuable as a sample characterization tool. We comment on the relationship between the collective mode dispersion and the non-Fermi-liquid character of these interacting electron systems.