Magneto-optical probe of the fully gapped Dirac band in ZrSiS

We present a far-infrared magneto-optical study of the gapped nodal-line semimetal ZrSiS in magnetic fields B up to 7 T. The observed field-dependent features, which represent intra- (cyclotron resonance) and interband transitions, develop as sqrt[B] in increasing field and can be consistently explained within a simple two-dimensional Dirac band model with a gap of 26 meV and an averaged Fermi velocity of 3×10^{5} m/s. This indicates a rather narrow distribution of these parameters along the in-plane portions of the nodal line in the Brillouin zone. A field-induced feature with an energy position that does not depend on B is also detected in the spectra. Possible origins of this feature are discussed.