Collective plasmonic modes in the chiral multifold fermionic material CoSi

Plasmonics in topological semimetals offer exciting opportunities for fundamental physics exploration as well technological applications. Here, we investigate plasmons the exemplar chiral crystal CoSi, which hosts a variety of multifold fermionic excitations. We show that CoSi two distinct plasmon modes infrared regime at 0.1 and 1.1 eV long-wavelength limit. The is found to be highly dispersive, it originates from intraband collective oscillations associated with double spin-1 excitation, while dispersionless involves interband correlations. Both lie outside particle-hole continuum possess long lifetime. Our study indicates class materials will provide an interesting platform exploring aspects plasmonics.