Meissner holes and turbulent structures in superconductors in unidirectional and rotating fields

Abstract

Flux distributions on different faces of YBCO crystal plates remagnetized by unidirectional and rotating fields are studied using advanced magneto-optical techniques. Bending of vortex lines is shown to be crucial for the remagnetization process even in plates with large width to thickness ratios in in-plane fields. Closed vortex loops smaller than a critical radius collapse, forming a flux-free cylinder: the Meissner hole. This results in the collection of superconducting currents into cords along the Meissner holes and the formation of turbulent flux patterns. Similar structures and unusual patterns are found when the field is rotated in the basal or end face plane. Values of the currents in the structures are retrieved from fits of measured flux profiles by model current distributions. The remagnetization patterns revealed in rotating fields are important for understanding the behavior of superconducting parts in motors and levitating devices.