Unconventional critical state in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films with a vortex-pin lattice fabricated by masked He$^+$ ion beam irradiation

TL;DR

This study investigates a vortex-pin lattice in YBa₂Cu₃O₇−δ thin films created by He+ ion irradiation, revealing a nonuniform critical state with hysteretic magnetoresistance and angle-dependent vortex pinning effects.

Contribution

It demonstrates the fabrication of a vortex-pin lattice with specific defect structures and characterizes the resulting unconventional critical state and pinning behavior in the films.

Findings

Critical current peaks indicate vortex trapping in domain regions.

The critical state is nonuniform and terrace-like, with vortices occupying fixed numbers per pin.

Magnetoresistance minima align with critical current peaks and depend on magnetic field orientation.

Abstract

Thin superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films are patterned with a vortex-pin lattice consisting of columnar defect regions (CDs) with 180 nm diameter and 300 nm spacing. They are fabricated by irradiation with 75 keV He$^+$ ions through a stencil mask. Peaks of the critical current reveal the commensurate trapping of vortices in domains near the edges of the sample. Upon ramping an external magnetic field, the positions of the critical current peaks are shifted from their equilibrium values to lower magnetic fields in virgin and to higher fields in field-saturated down-sweep curves, respectively. Based on previous theoretical predictions, this irreversibility is interpreted as a nonuniform, terrace-like critical state, in which individual domains are occupied by a constant number of vortices per pinning site. The magnetoresistance, probed at low current densities, is hysteretic and angle dependent and exhibits minima that correspond to the peaks of the critical current. The minima's positions scale with the component of the magnetic field parallel to the axes of the CDs, as long as the tilted vortices can be accommodated within the CDs. This behavior, different from unirradiated films, confirms that the CDs dominate the pinning.