In our preceding study, a turbulent vortex structure at the remagnetization boundary of a superconductor, called the Meissner hole, was reported in a single crystalline '122-type' iron-based superconductor (IBS) (Ba0.67Rb0.33)Fe2As2 . The Meissner hole was observed in a pristine crystal and the one with artificial defects, accompanied by an excess local current at the remagnetization boundary.
The Meissner hole, we trust, should be a ubiquitous vortex phenomenon that is not bound to one or few dedicated types of superconductors. Recently we observed Meissner hole-like vortex behavior (see attached figure) in the '11-type' IBS Fe(Te0.6Se0.4). This material is currently under intense investigation for its potential to realize a topological non-trivial surface state, where the Majorana Fermion is assumed to be hosted. Taking this opportunity, we decide to extend the same magneto-optical observation to its parent material, FeSe. Unlike other iron-pnictide parent compounds like LaFeAsO or BaFe2As2, FeSe undergoes a structural transition at ~90 K without being accompanied by antiferromagnetism. The magnetic transition, however, is seemingly cooperative with its superconductivity since it emerges along with the superconducting transition temperature under pressures . More knowledge of this mysterious phase transition requires the resolution of the vortex state below Tc (~8K), which is easily covered by our low-temperature limit for observation (3 K).
 T. Ren, S. Pyon, and T. Tamegai, J. Phys. Conf. Ser. 1975, 012013 (2021).
 T. Terashima et al., J. Phys. Soc. Jpn. 84, 063701 (2015).
Keywords: Vortex, Magneto-optical imaging, Meissner hole