Flux trapping in NbTiN strips – Part1: Experiments

Understanding vortex behavior in superconducting structures is important for applications such as superconducting digital circuits (SDCs), high-Q resonators, and detectors. In finite-size structures like thin strips, the presence of edges alters vortex behavior compared to continuous films. We use scanning superconducting quantum interference device (SQUID) microscopy to image vortex configurations in strips fabricated from NbTiN, which is a promising material for realizing SDCs.

We study strips of varying widths (5 - 40 um) by cooling them through the superconducting transition under varying out-of-plane magnetic field. From magnetic images, we extract the threshold field for the first vortex in a strip, the number of vortices, and the vortex positions as a function of field. We identify pronounced pinning sites through repeated thermal cycles under the same field, which reveals statistically significant locations that trap vortices. Prior studies mainly focused on validating theoretical expressions for the threshold field. Our work extends this by studying the interplay of pinning, vortex-vortex interactions and finite size of the strip at fields beyond the threshold field, and by combining our measurements with detailed simulations. Future work aims at exploring the effect of the cooling rate, using an applied current to characterize the strength of pinning sites and ultimately establishing a characterization and simulation protocol with predictive power for flux trapping in complex superconducting structures.