# Linear optical conductivity of chiral multifold fermions

## TL;DR

This paper calculates the linear optical conductivity of chiral multifold fermions, revealing characteristic features that can serve as experimental signatures and aiding in the detection of these quasiparticles in specific chiral crystals.

## Contribution

It provides the first comprehensive calculation of optical conductivity for all chiral multifold fermions, including realistic predictions for materials like RhSi, CoSi, and AlPt.

## Key findings

- Optical conductivity is enhanced compared to Weyl fermions with same Fermi velocity.
- Characteristic activation frequencies serve as fingerprints for different multifold fermion classes.
- Quantitative predictions for materials like RhSi, CoSi, and AlPt are provided.

## Abstract

Chiral multifold fermions are quasiparticles described by higher spin generalizations of the Weyl equation, and are realized as low energy excitations near symmetry protected band crossings in certain chiral crystals. In this work we calculate the linear optical conductivity of all chiral multifold fermions. We show that it is enhanced with respect to that of Weyl fermions with the same Fermi velocity, and features characteristic activation frequencies for each multifold fermion class, providing an experimental fingerprint to detect them. We calculate the conductivity for realistic chiral multifold semimetals by using lattice tight-binding Hamiltonians that match the effective models of multifold fermions at low energies, for space groups 199 and 198. The latter includes RhSi, for which we give quantitative predictions, and also CoSi and AlPt. Our predictions can be tested in absorption or penetration depth measurements, and are necessary to extract the recently proposed quantized photocurrents from experiments.