Optical signatures of multifold fermions in the chiral topological semimetals RhSi and CoSi*

The chiral topological semimetals RhSi and CoSi exhibit band degeneracies near the Fermi level enforced by the crystal symmetries. The low-energy quasiparticles emerging near these band degeneracies, referred to as multifold fermions, have no counterpart as elementary fermionic particles. We calculate the linear optical conductivity of all chiral multifold fermions[1] and show that it provides an experimental fingerprint for each type of multifold fermion. We use a tight-binding model for space group 198, where RhSi and CoSi crystallize, and obtain the parameters for both materials from first-principles calculations. The results reveal that the location of the chemical potential is crucial to understand the optical response seen in experiments[2,3], determined at low energies by the threefold fermion at the Γ point in both materials, and providing signatures of the existence of a spin-3/2 fourfold fermion in CoSi.

[1] M.-Á S.-M. et al., Phys. Rev. B, 99 155145 (2019).
[2] B. Xu et al, PNAS Nov 2020, 117 (44) 27104-27110
[3] Z. Ni et al., arXiv:2005.13473 (2020).