Vortex creep and critical current densities in superconducting (Ba,K)Fe$_{2}$As$_{2}$ single crystals

TL;DR

This study investigates vortex creep and critical current densities in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals, developing a new analysis method to understand flux relaxation and pinning mechanisms.

Contribution

A new straightforward data treatment technique for analyzing flux creep in iron-based superconductors, revealing the creep mechanism and exponent without relying on standard high-temperature superconductor methods.

Findings

Creep exponent $$ varies slightly with temperature and magnetic flux.

Effective activation barrier depends on temperature and magnetic field.

Measured current density approaches the zero-temperature critical current density at low temperatures.

Abstract

The surprisingly rapid relaxation of the sustainable current density in the critical state of single crystalline Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ is investigated for magnetic fields oriented parallel to the c-axis and to the $ab$--plane respectively. Due to the inadequacy of standard analysis procedures developed for flux creep in the high temperature superconducting cuprates, we develop a simple, straightforward data treatment technique that reveals the creep mechanism and the creep exponent $\mu$. At low magnetic fields, below the second magnetization peak, $\mu$ varies only slightly as function of temperature and magnetic flux density $B$. From the data, we determine the temperature- and field dependence of the effective activation barrier for creep. At low temperatures, the measured current density approaches the zero--temperature critical current density (in the absence of creep) to within a factor 2, thus lending credence to earlier conclusions drawn with respect to the pinning mechanism. The comparable values of the experimental screening current density and the zero-temperature critical current density reveals the limited usefulness of the widely used "interpolation formula".