Thermally activated vortex velocities in the field-independent Larkin-Ovchinnikov regime of disordered molybdenum nitride thin films

Abstract We report the influence of microstructural disorder, introduced by varying nitrogen content, on the vortex dissipation mechanisms of molybdenum nitride thin films grown by reactive sputtering at room temperature. The 22 nm-thick films were deposited at a total pressure of 0.66 Pa using (Ar+N 2 ) mixtures with nitrogen concentrations of 7.5%, 10%, 15%, and 20%. Under these conditions, the superconducting critical temperature ( T c ) gradually decreases from approximately 7.6 K to 6.4 K as the N 2 concentration in the reactive mixture increases. This reduction in T c is attributed to increased disorder induced by interstitial nitrogen and amorphous phases, affecting the microstructure of the films. Using current–voltage curves, we investigated vortex instability and the corresponding vortex velocities as functions of temperature and magnetic field. While inhomogeneities and pinning centers are expected to reduce vortex velocities due to localized heating in the bridge, we observed that the velocity plateau at moderate and high fields—typically indicative of a uniform quasiparticle distribution—depends on the microstructure of the samples (linked to T c ) and scales with the absolute temperature at which measurements were conducted. For comparison, nitrogen-doped W thin films with T c ≈ 4.6 K, and a plateau in the vortex velocities at T = 3 K, show a good match with the values observed in molybdenum nitride samples at higher temperatures. This finding suggests that, in addition to the disorder and intrinsic superconducting properties of the films, thermally activated phonons, potentially associated with the silicon substrate, contribute to the enhancement of vortex velocities within the analyzed temperature range.