Flat Optical Conductivity in ZrSiS due to Two-Dimensional Dirac Bands

ZrSiS exhibits a frequency-independent interband conductivity sigma(omega) = constd (omega) equivalent to sigma(flat) in a broad range from 250 to 2500 cm(-1) (30-300 meV). This makes ZrSiS similar to (quasi-) two-dimensional Dirac electron systems, such as graphite and graphene. We assign the flat optical conductivity to the transitions between quasi-two-dimensional Dirac bands near the Fermi level. In contrast to graphene, sigma(flat) is not universal but related to the length of the nodal line in the reciprocal space, k(0). Because of spin-orbit coupling, the discussed Dirac bands in ZrSiS possess a small gap Delta, for which we determine an upper bound max(Delta) = 30 meV from our optical measurements. At low temperatures the momentum-relaxation rate collapses, and the characteristic length scale of momentum relaxation is of the order of microns below 50 K.