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

Abstract

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 ${\ensuremath{\tau}}_{ϵ}$ 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 $182$-$\ensuremath{\mu}\mathrm{m}$-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 ${\ensuremath{\tau}}_{ϵ}$. We argue that for the deduction of the intrinsic ${\ensuremath{\tau}}_{ϵ}$ 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.