Optical conductivity of a moderately disordered superconducting NbN film was investigated by terahertz time-domain spectroscopy in an external magnetic field applied along the film plane. The film thickness of $\sim 5$ 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 a 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 [Phys. Rev. B 96, 014509 (2017)] 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 the magnetic field.

• Received 31 August 2021
• Revised 8 November 2021
• Accepted 22 December 2021

DOI:https://doi.org/10.1103/PhysRevB.105.014506