Rising Speed Limits for Fluxons via Edge-Quality Improvement in Wide MoSi Thin Films

Ultrafast vortex motion has recently become a subject of extensive investigations, triggered by the fundamental question regarding the ultimate speed limits for magnetic flux quanta and enhancements of single-photon detectors. In this regard, the current-biased quench of a dynamic flux-flow regime-flux-flow instability (FFI) has turned into a widely used method for the extraction of information about the relaxation of quasiparticles (unpaired electrons) in a superconductor. However, the large relaxation times tau(epsilon) deduced from FFI for many superconductors are often inconsistent with the fast relaxation processes implied by their single-photon counting capability. Here, we investigate FFI in 15-nm-thick 182-mu m-wide MoSi strips with rough and smooth edges produced by laser etching and milling by a focused ion beam. For the strip with smooth edges we deduce, from current-voltage (I-V) curve measurements, a factor of 3 larger critical currents I-c, a factor of 20 higher maximal vortex velocities of 20 km/s, and a factor of 20 shorter tau(epsilon). We argue that for the deduction of the intrinsic tau(epsilon) of the material from the I-V curves, utmost care should be taken regarding the edge and sample quality and such a deduction is justified only if the field dependence of I-c points to the dominating edge pinning of vortices.