Pinning of spiral fluxons by giant screw dislocations in YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals: Josephson analog of the fishtail effect

By using a highly sensitive homemade AC magnetic susceptibility technique, the magnetic flux penetration has been measured in $\rm YBa_2Cu_3O_{7-\delta}$ single crystals with giant screw dislocations (having the structure of the Archimedean spirals) exhibiting $a=3$ spiral turnings, the pitch $b=18.7\,\mu$m and the step height $c=1.2\,$nm (the last parameter is responsible for creation of extended weak-link structure around the giant defects). The magnetic field applied parallel to the surface enters winding around the weak-link regions of the screw in the form of the so-called spiral Josephson fluxons characterized by the temperature dependent pitch $b_f(T)$. For a given temperature, a stabilization of the fluxon structure occurs when $b_f(T)$ matches $b$ (meaning an optimal pinning by the screw dislocations) and manifests itself as a pronounced low-field peak in the dependence of the susceptibility on magnetic field (applied normally to the surface) in the form resembling the high-field (Abrikosov) fishtail effect.