Pinning of spiral fluxons by giant screw dislocations in YBa_2Cu_3O_7 single crystals: Josephson analog of the fishtail effect

摘要

By using a highly sensitive homemade AC magnetic susceptibility technique,the magnetic flux penetration has been measured in YBa_2Cu_3O_7 single crystalswith giant screw dislocations (having the structure of the Archimedean spirals)exhibiting a=3 spiral turnings, the pitch b=18.7 microns and the step heightc=1.2nm (the last parameter is responsible for creation of extended weak-linkstructure around the giant defects). The magnetic field applied parallel to thesurface enters winding around the weak-link regions of the screw in the form ofthe so-called spiral Josephson fluxons characterized by the temperaturedependent pitch b_f(T). For a given temperature, a stabilization of the fluxonstructure occurs when b_f(T) matches b (meaning an optimal pinning by the screwdislocations) and manifests itself as a pronounced low-field peak in thedependence of the susceptibility on magnetic field (applied normally to thesurface) in the form resembling the high-field (Abrikosov) fishtail effect.