Imaging of Super-fast Dynamics and Flow Instabilities of Superconducting Vortices
Superconducting vortices, being critical determinants of electromagnetic properties in superconductors, have been extensively studied in contexts of slow dynamics. However, the dynamics of vortices under ultrafast current conditions remain largely unexplored. This paper expands on the paper of vortices by demonstrating direct imaging of rapidly moving superconducting vortices in a lead (Pb) film, using a nanoscale scanning superconducting quantum interference device (SQUID). This experimental setup captures vortex penetration occurring at frequencies up to tens of GHz and velocities reaching tens of kilometers per second, dramatically exceeding both the speed of sound and the pair-breaking velocity of superconductors.
Key results include the formation of mesoscopic vortex channels, with complex bifurcation patterns observed as current and magnetic field intensify. These findings were supported by numerical simulations forecasting a transition from Abrikosov to mixed Abrikosov-Josephson vortices at elevated velocities, highlighting the transformation of vortex structures under extreme dynamic conditions. These insights into the dynamics of superconductors under such conditions have significant implications for enhancing the current-carrying capacity of superconducting materials, with potential applications in high-field magnets, superconducting digital circuits, and THz radiation sources.
The research presents crucial measurements that indicate vortex velocities up to 40 km/s, facilitated by the novel SQUID-on-tip (SOT) technology, capable of sub-nanometer displacement sensitivity, allowing unprecedented insight into the dynamics of these fast-moving vortices. Transport measurements combined with SOT imaging have enabled the correlation of vortex penetration frequency and trajectories with single-vortex resolution, critical for advancing the understanding of dissipation mechanisms.
This paper also addresses several vital questions pertinent to the limits of vortex velocity: whether moving vortices maintain their identity as topological defects under extreme velocities, the maximum velocity limits for vortices, and the underlying mechanisms dictating these limits. By achieving velocities an order of magnitude greater than previously recorded, this work underscores the potential for advancements in nonequilibrium superconductivity research, prompting further investigations into non-equilibrium instabilities and possibly enabling technologies operating in the THz frequency range.
Future research directions emerging from this paper include deepening the theoretical understanding of vortex physics under equivalent conditions, potentially leading to the identification and development of superconducting technologies that operate under extreme electromagnetic conditions. Additionally, enhancing the heat dissipation capabilities of experimental setups could allow access to even higher dynamic regimes in superconductors, broadening the scope for innovative electronic applications leveraging superconductivity at ultrafast scales.