Strong pinning and vortex energy distributions in single crystalline Ba(Fe1-xCox)2As2

TL;DR

This study investigates flux pinning and vortex energy distributions in Ba(Fe1-xCox)2As2 single crystals, revealing strong pinning effects caused by spatial fluctuations in superconducting properties and their impact on vortex lattice disorder.

Contribution

It provides new insights into the microscopic origins of vortex pinning in iron-based superconductors by correlating vortex structures with local superconducting heterogeneity.

Findings

Vortex structures are highly disordered in studied crystals.

Disorder is linked to strong pinning from Tc and superfluid density fluctuations.

Disorder length scale is approximately 40-60 nm.

Abstract

The interrelation between heterogeneity and flux pinning is studied in Ba(Fe1-xCox)2As2 single crystals with widely varying Co-content x. Magnetic Bitter decoration of the superconducting vortex ensemble in crystals with x = 0:075 and x = 0:1 reveal highly disordered vortex structures. The width of the Meissner belt observed at the edges of the crystals, and above the surface steps formed by cleaving, as well as the width of the intervortex distance distribution, indicate that the observed vortex ensemble is established at a temperature just below the critical temperature Tc. The vortex interaction energy and pinning force distributions extracted from the images strongly suggest that the vortex lattice disorder is attributable to strong pinning due to spatial fluctuations of Tc and of the superfluid density. Correlating the results with the critical current density yields a typical length scale of the relevant disorder of 40 - 60 nm.