Excitation of vortex-antivortex pairs in thin superconducting films and superlattices

Abstract

Direct imaging of the accumulation of magnetic flux and antiflux resulted from the excitation of vortex-antivortex pairs inside thin superconducting films is reported. Thin-film superconductors, like YBa2Cu3O7/PrBa2Cu3O7 superlattices or NbN films grown by pulsed laser deposition, were used in experiments. The superlattices provide enhanced pinning for vortices, facilitating imaging of accumulated flux, and feature nanoscale fractures ideal for excitation of vortex-antivortex pairs. The idea of the experiment is to record images using a specific magneto-optical mode that allows distinguishing between positive and negative magnetic field in the sample. Two types of flux-antiflux patterns are observed. In one type, flux and antiflux are entering on permanent defects, like nano-fractures formed in the process of film deposition. In another type, flux and antiflux patterns are formed by application of a strong localized magnetic field and not are linked to permanent defects. In the first type, the amount of flux and antiflux entering superconductor from the defect is strongly affected by an external magnetic field. In the second type, the permanently frozen pattern is hardly influenced by the field. An unusual and dramatic effect of the excitation of flux-antiflux dendritic avalanches is also reported.