Specificities of Current–Voltage Characteristics of NbN Superconducting Films in a Magnetic Field

The current–voltage characteristics (CVCs) of wide superconducting NbN films in magnetic fields up to 8 T are studied in this work. Linear portions that do not change the tilt angle in magnetic fields below the superconducting transition temperature have been found in the CVCs of such films. A model of viscous magnetic flux under conditions of rigid fixation of a part of the vortex system is proposed to explain these linear portions. The magnetic field dependences of the depinning current and the vortex motion velocity are found within the model of collective pinning of the vortex lattice. The value of the disorder parameter is obtained, and the depairing current density is estimated. From the Larkin–Ovchinnikov theory of vortex lattice instability, the dependence of the energy relaxation time of quasiparticles on the magnetic field is determined.