Transition from columnar to point pinning in coated conductors: critical currents, that are independent of magnetic field direction

TL;DR

This paper reports a phase transition in high-temperature superconductor vortex systems, where the dominant pinning mechanism shifts from nano-columnar to point-like, affecting the critical current's dependence on magnetic field orientation.

Contribution

It identifies a temperature-dependent phase transition in vortex pinning, revealing a switch from columnar to point pinning mechanisms in coated conductors.

Findings

Critical current density changes with magnetic field orientation.

A phase transition occurs at a specific field $B_X(T)$.

Maximum and minimum $J_C$ observed depending on field alignment.

Abstract

We identify a phase transition in the vortex system of a high-temperature superconductor with nano-columnar stacks of precipitates as strong vortex pinning centers. Above a particular, temperature-dependent field $B_X(T)$ the vortex response is no longer determined by the nano-columns, and is instead determined by point-like pinning. This phase transition leads to the change in the measured critical current density as a function of angle between the applied magnetic field and the nano-columns. Below the unbinding, there is a strong maximum in $J_C$ when field is aligned parallel to the columns. Above the unbinding, there is a minimum for this orientation.