When the applied field is reduced to zero from a sufficiently large value in superconductors with a small demagnetization factor (thin slab with parallel field), a full critical state is formed. A further reduction of the applied field toward negative values, called remagnetization, simply introduces antivortices with a well-defined boundary between pre-existing vortices. However, the situation becomes complicated in superconductors with a demagnetization factor close to unity (thin sample with perpendicular field). Near the boundary separating two regions with opposite polarities of vortices, vortices naturally form closed loops due to the presence of in-plane components of the field. Vortex loops have minimum diameters surrounding vortex free region, called “Meissner hole”. Experiments by Vlasko-Vlasov showed the presence of such structures in YBa₂Cu₃O₇ [1]. What is more, at the skin layer of the Meissner hole, there flows a local excess current which is twice as large as the critical current in the same area. The Meissner hole is believed to affect the low-field vortex dynamics of superconductors since its unstable turbulent structure blurs the interface between vortices and anti-vortices.

In this work, we report high-resolution images of Meissner holes captured by magneto-optical (MO) imaging in 122-type iron-based superconductor (Ba_0.67Rb_0.33)Fe₂As₂ single crystal (T_c = 37.5 K) as shown in Fig. 1(a). This is the third material in which the Meissner hole is confirmed experimentally after YBa--₂Cu₃O₇ and Ba(Fe_1-xCo_x)_-2As₂ [2]. The local current is extracted from the MO image using inverse Biot-Savart’s law (Fig. 1(b)). To investigate how the artificial defects could influence the evolution of the Meissner hole, we analyzed and compared the vortex dynamics in both pristine and 3 MeV proton irradiated crystals. We also tracked the motion of the Meissner hole, and compare it with the relaxation of magnetization. By further tracking the local magnetization with Hall probes, we might reveal how the existence of the Meissner hole affects the relaxation of magnetization in superconductors at low-field states.

Fig. 1 (a) MO image of a remagnetized (Ba_0.67Rb_0.33)Fe₂As₂ single crystal at 15 K. The boundary between the bright and dark areas indicates the contour of the Meissner hole. (b) Modulus of current density (|J|) map converted from (a). (c) Normalized |J| profile along the red dotted line in (b). Broken lines in (c) show edges of the crystal.

References
[1] V. Vlasko-Vlasov et al., Phys. Rev. B 56, 5622 (1997).
[2] S. Mohan et al., Phys. Rev. B 84, 180504 (2011).

Keywords: iron-based superconductor, Meissner hole, vortex dynamics