Equidistant resonance jumps in superconducting coplanar resonators driven by Abrikosov vortices
Superconducting coplanar resonators are key building blocks of cryogenic microwave circuits, yet their performance in perpendicular magnetic fields is ultimately limited by Abrikosov vortices. In this work we investigate the dependence of the transmission parameter $S_{21}$ of niobium quarter-wave coplanar resonators on perpendicular magnetic fields up to 40 Oe and at temperatures between 18 mK and 5 K. Beyond the reversible Meissner regime, the entire resonance peak exhibits abrupt, staircase-like jumps as a function of magnetic field. Upon reversal of the field sweep, these jumps form an almost equidistant series with spacing 1.7-1.8 Oe, which, in agreement with theoretical estimates, we interpret as signatures of multiple-vortex entry and exit events. Additionally, we observe the non-proportional responses of the resonant frequency and the internal quality factor that indicate a complex contribution of vortex and antivortex configurations. We believe that our results will stimulate further studies of large vortex-antivortex systems, explicitly accounting for their discrete nature.