Dopant clustering, electronic inhomogeneity, and vortex pinning in iron-based superconductors

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 that the low-$T$ cleaved surface is dominated by a half Sr/K termination with $1\ifmmode\times\else\texttimes\fi{}2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $\ensuremath{\sigma}/\overline{\ensuremath{\Delta}}=16%$ on a $\ensuremath{\sim}$3 nm length scale, with average $2\overline{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.6$ in the weak-coupling limit. The vortex core size provides a measure of the superconducting coherence length $\ensuremath{\xi}=2.3$ 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.