Rising speed limits for fluxons via edge quality improvement in wide MoSi thin films

Abstract

Ultra-fast 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 the superconductor. However, the large relaxation times τ ε 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 µm-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 the 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 40 shorter τ ε . We argue that for the deduction of the intrinsic τ ε 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.