Probing Trapped Vortices in Superconductors with Near-Field Magnetic Microwave Microscopy

Type-II superconductors host DC vortices when exposed to a DC magnetic field exceeding their first critical field. These vortices can be pinned by defects in the superconductor even after the DC field is removed. One possible approach to investigate residual trapped vortices is to shake them with a local and intense RF magnetic field and analyze the nonlinear response. In this study, we employ a near-field magnetic microwave microscope [1] to locally apply an intense RF magnetic field to stimulate residual trapped vortices under the probe and collect the resulting second harmonic response. Time-dependent Ginzburg-Landau (TDGL) simulations in COMSOL [2] predict that the wiggling of trapped vortices, induced by a locally-applied RF magnetic field, typically produces a strong second harmonic response. Our experimental results on a Nb sample with an anti-dot array confirm the predictions, demonstrating a strong second harmonic response. The dependence of this response on the strength of the cooldown field and the temperature is investigated. In particular, the response exhibits several discrete jumps with increasing temperature, which reveals configuration changes of trapped vortices as the temperature increases. The data show that these trapped vortices are stable at low temperatures and become more mobile near the critical temperature. The microscope should be useful in studying the vortex pinning force near the critical temperature.

References:

[1] C-Y Wang, C. Pereira, S. Leith, G. Rosaz, S. M. Anlage, “Microscopic Examination of SRF-quality Nb Films through Local Nonlinear Microwave Response,” arXiv:2305.07746 (https://arxiv.org/abs/2305.07746).

[2] B. Oripov and S. M. Anlage, “Time-dependent Ginzburg-Landau Treatment of RF Magnetic Vortices in Superconductors: Vortex Semiloops in a Spatially Nonuniform Magnetic Field,” Phys. Rev. E 101, 033306 (2020). (https://doi.org/10.1103/PhysRevE.101.033306).