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

TL;DR

This study investigates how giant screw dislocations in YBa_2Cu_3O_7 single crystals pin spiral Josephson fluxons, creating a fishtail-like magnetic susceptibility peak, revealing a novel flux pinning mechanism.

Contribution

It demonstrates the pinning of spiral Josephson fluxons by giant screw dislocations with Archimedean spiral structure in YBa_2Cu_3O_7 crystals, a new analog of the fishtail effect.

Findings

Observation of spiral Josephson fluxons stabilized by screw dislocations.

Identification of a low-field susceptibility peak resembling the fishtail effect.

Correlation between fluxon structure matching and optimal pinning.

Abstract

By using a highly sensitive homemade AC magnetic susceptibility technique, the magnetic flux penetration has been measured in YBa_2Cu_3O_7 single crystals with giant screw dislocations (having the structure of the Archimedean spirals) exhibiting a=3 spiral turnings, the pitch b=18.7 microns and the step height c=1.2nm (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.