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

TL;DR

This study demonstrates that improving edge quality in wide MoSi superconducting strips significantly enhances vortex flux flow speed and reduces quasiparticle relaxation times, advancing understanding of fluxon dynamics and superconductor performance.

Contribution

It shows that edge quality critically influences flux-flow instability parameters, revealing faster vortex velocities and shorter relaxation times in smoother-edged MoSi superconductors.

Findings

Smooth edges increase critical current by a factor of 3.

Maximal vortex velocity reaches 20 km/s with smooth edges.

Quasiparticle relaxation time shortens by a factor of 40 with improved edge quality.

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 $\tau_\epsilon$ 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 the current-voltage ($I$-$V$) curve measurements, a factor of 3 larger critical currents $I_\mathrm{c}$, a factor of 20 higher maximal vortex velocities of 20 km/s, and a factor of 40 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 $I_\mathrm{c}$ points to the dominating edge pinning of vortices.