Optical Evidence of Chiral Plasmon mode in Dirac and Weyl semimetals*

The first observation of chiral plasmon modes in 3D Dirac and Weyl materials Na₃Bi and CoSi is reported through optical spectroscopy. Reflection measurements on single-crystals, over the spectral range from 20 mev to 2.5 eV and temperature ranging from 5K to 300K, show absorption features locked to the strongly temperature-dependent plasma edges. The frequency shifts of chiral plasmon features from the plasma edge are consistent with the Berry curvature associated with a Dirac or Weyl pair. The Lifshitz gap associated with inter-Fermi arcs transitions in CoSi is also reported.