Dopant Clustering, Electronic Inhomogeneity, and Vortex Pinning in Iron-Based Superconductors

TL;DR

This study uses scanning tunneling microscopy to explore the surface structure, electronic inhomogeneity, and vortex behavior in a hole-doped iron-based superconductor, revealing dopant clustering's role in vortex pinning.

Contribution

It demonstrates the impact of dopant size mismatch on electronic inhomogeneity and vortex pinning in Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$, a novel insight into superconductor behavior.

Findings

Superconducting gap varies by 16% on a 3 nm scale.

Vortex core size indicates a coherence length of 2.3 nm.

Dopant size mismatch correlates with electronic inhomogeneity and vortex pinning.

Abstract

We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ with $T_c$=32 K. We find the low-$T$ cleaved surface is dominated by a half-Sr/K termination with $1\times 2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $\sigma/\bar{\Delta}$=16% on a $\sim$3 nm length scale, with average $2\bar{\Delta}/k_B T_c=3.6$ in the weak coupling limit. The vortex core size provides a measure of the superconducting coherence length $\xi$=2.3 nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.