  Optical conductivity of a moderately disordered superconducting NbN film was
investigated by terahertz time-domain spectroscopy in external magnetic field
applied along the film plane. The film thickness of about 5 nm was comparable
with the coherence length, so vortices should not form. This was confirmed by
the fact that no marked difference between the spectra with terahertz electric
field set perpendicular and parallel to the external magnetic field was
observed. Simultaneous use of Maxwell-Garnett effective medium theory and the
model of optical conductivity by Herman and Hlubina proved to correctly
reproduce the terahertz spectra obtained experimentally in a magnetic field of
up to 7 T. This let us conclude that the magnetic field tends to suppress the
superconductivity, resulting in an inhomogeneous state where superconducting
domains are enclosed within a normal-state matrix. The scattering rate due to
pair-breaking effects was found to linearly increase with magnetic field.
