Dynamical conductivity in the multiply degenerate point-nodal semimetal CoSi

We theoretically investigate the dynamical conductivity in the multiply degenerate point-nodal semimetal CoSi. The dynamical conductivity is calculated as a function of photon energy by using the first-principles band calculation and the linear response theory. In the nodal semimetal, the band structure holds point nodes at the F and R points in the Brillouin zone and more than three bands touch at the nodes. Around the nodes, electronic states are predicted to be described as the multifold chiral fermion, a class of fermion in condensed matter. We show that the dynamical conductivity exhibits a characteristic spectrum corresponding to the band structure and the chiral fermionic states. The chirality leads to the prohibition of transition between the lower and upper bands of threefold chiral fermion and thus the transition between the middle and lower bands is relevant to the dynamical conductivity. This transition property is different from the Dirac and Weyl semimetals, the other point-nodal semimetals, where the excitation between the upper and lower bands is relevant to the dynamical conductivity. We show the prohibition causes the reduction of dynamical conductivity in the low-photon energy region.