Vortex critical velocities in metallic-capped epitaxial NbN thin films

We report the impact of proximity effects induced by different capping layers on the vortex dynamics and Larkin-Ovchinnikov instability in epitaxial NbN thin films. Bilayers consisting of 5 nm and 10 nm thick NbN were combined with capping layers of magnetic 5 nm thick Fe_0.2Ni_0.8 and non-magnetic 5 nm thick Al and 10 nm thick Pt. The critical temperature (T_c), which is approximately 16 K in pure samples, decreases due to the influence of the capping layers. This reduction is more pronounced for Fe_0.2Ni_0.8, where T_c is fully suppressed for 5 nm thick NbN and decreases to 10.1 K for 10 nm thick films. Additionally, T_c is restored when an ultrathin AlN insulating layer is introduced, preventing electronic interaction. In contrast, Al and Pt capping layers also reduce T_c, but to a lesser extent compared to magnetic layers. By performing current-voltage curves, we determined the vortex critical velocities as a function of the magnetic field. Initially, the velocities start at approximately 6 km/s at low fields, subsequently decreasing to values between 1.8–2.8 km/s at fields around 1 T While a negligible impact is observed at low fields, the velocities at moderate and high fields tend to increase near T_c for non-magnetic capping layers. These increases (approximately 20 %) may be attributed to changes in the quasiparticle relaxation time, likely due to phonon escape contributions, as well as modifications in the electron diffusion coefficient and heat dissipation to local heating generated by inhomogeneities. Comparatively, the impact of capping layers is significantly smaller than previously reported for NbN and other disordered superconductors, where the initial vortex velocities in pure films were much lower than those observed in the films used in this study.