Temperature and magnetic field dependences of the critical current in superconducting films of niobium nitride

The current-voltage characteristics (C–V characteristics) of superconducting films of niobium nitride (NbN) were studied at temperatures below the transition temperature to the superconducting state $(T_{\mathrm{c}})$ in a constant magnetic field. The temperature and magnetic field dependences of the critical film depinning current density $(j_\mathrm{d})$ are determined. Within the framework of models of collective pinning and rigid pinning of vortices, functional dependences of $j_\mathrm{d}$ were found in the temperature range from 00.7$T_{\mathrm{c}}$ to 0.95$T_{\mathrm{c}}$ for magnetic fields with strengths up to 80 kOe. From the magnetic field dependences of $j_\mathrm{d}$, confirmation of the presence of a region of constant critical current for the films under study in fields of 11–13 kOe was obtained, which is explained by the existence of a regular structure of pinning centers in the samples. The distances between pinning centers and their sizes are estimated. The characteristic values of the vortex pinning force and the viscosity coefficient during their motion are determined.