  The magneto-optical properties of (001)-oriented NbAs single crystals have
been studied in the spectral range from 5 to 150 meV and in magnetic fields of
up to 13 T. A rich spectrum of inter-Landau-level transitions is revealed by
these measurements. The transitions follow a square-root-like dependence with
magnetic field, but the simple linear-band approximation is unable to
accurately reproduce the observed behavior of the transitions in applied
fields. We argue that the detected magneto-optical spectra should be related to
crossing hyperbolic bands, which form the W1 cones. We propose a model
Hamiltonian, which describes coupled hyperbolic bands and reproduces the shape
of the relevant bands in NbAs. The magneto-optical spectra computed from this
Hamiltonian nicely reproduce our observations. We conclude that the
hyperbolic-band approach is a minimal model to adequately describe the
magneto-optical response of NbAs and that the chiral (conical) bands do not
explicitly manifest themselves in the spectra.
