Optical Characterization of Nonreciprocal Plasmons in a Magnetic Weyl Semimetal*

The discovery of topological semimetals with nontrivial electronic surface states has caused an explosion of interest in the development of optical techniques to sample such states. In particular, the recent discovery of magnetic Weyl semimetals, which support non-closed topological Fermi arcs at their surfaces, has attracted attention to the time-reversal symmetry breaking properties of the resultant fermions, which are present even in the absence of an external magnetic field. Here we measure the full dielectric tensor of Co­₃Sn₂S₂, a Co-based half-metallic ferromagnet that supports Weyl points. Through identification of an epsilon-near-zero point at ~0.1 eV, we identify the MIR regime supporting nonreciprocal plasmons. We verify that the crystal is in a Voigt configuration with respect to the optical plane of incidence and its intrinsic magnetic moment through X-ray crystallographic studies of the exposed (110) facet. We also theoretically model the microscopic subsurface optical fields and compare them to the experimental characterization of the dielectric function and the angle-resolved reflection characteristic. Our results demonstrate the utility of magnetic Weyl semimetals in magneto-optical device configurations.