Vortex counting and velocimetry for slitted superconducting thin strips

TL;DR

This paper introduces a method to accurately determine the number of vortices and their maximum speed in superconducting strips by analyzing kinks in current-voltage curves, advancing understanding of flux dynamics in superconductors.

Contribution

The authors develop a quantitative approach to measure vortex count and velocity in superconducting strips using kink analysis in I-V curves, supported by experimental and numerical results.

Findings

Identified kinks in I-V curves indicating fluxon dynamics transition.

Deduced vortex velocity of approximately 12 km/s.

Observed transition from vortex chain to vortex river with increasing vortex number.

Abstract

The maximal speed $v^\ast$ for magnetic flux quanta is determined by the energy relaxation of unpaired electrons and is thus essential for superconducting microstrip single-photon detectors (SMSPDs). However, the deduction of $v^\ast$ from the current-voltage ($I$-$V$) curves at zero magnetic field is hindered by the unknown number of vortices, $n_\mathrm{v}$, as a small number of fast-moving vortices can induce the same voltage as a large number of slow-moving ones. Here, we introduce an approach for the quantitative determination of $n_\mathrm{v}$ and $v^\ast$. The idea is based on the Aslamazov and Larkin prediction of kinks in the $I$-$V$ curves of wide and short superconducting constrictions when the number of fluxons crossing the constriction is increased by one. We realize such conditions in wide MoSi thin strips with slits milled by a focused ion beam and reveal quantum effects in a macroscopic system. By observing kinks in the $I$-$V$ curves with increase of the transport current, we evidence a crossover from a single- to multi-fluxon dynamics and deduce $v^\ast\simeq12\,$km/s. Our experimental observations are augmented with numerical modeling results which reveal a transition from a vortex chain over a vortex jet to a vortex river with increase of $n_\mathrm{v}$ and the vortex velocity. Our findings are essential for the development of 1D and 2D few-fluxon devices and provide a demanded approach for the deduction of $v^\ast$ at the SMSPD operation conditions.