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 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.

7 pages, 5 figures, 1 table

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• Exploring of point-contact spectra of Ba1-xNaxFe2As2 in the normal and superconducting state
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• Vortex lattice and vortex bound states in CsFe${}_2$As${}_2$ investigated by scanning tunneling microscopy/spectroscopy
• Planar pinning induced, lowering of vortex dimensionality and low field melting in a single crystal of Ba0.6K0.4Fe2As2
• Surface symmetry breaking and disorder effects on superconductivity in perovskite BaBi3 epitaxial films
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