1212.3240 (Can-Li Song et al.)

Electronic Inhomogeneity and Vortex Disorder in Superconducting
Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$    [PDF]

Can-Li Song, Yi Yin, Martin Zech, Tess Williams, Michael Yee, Gen-Fu Chen, Jian-Lin Luo, Nan-Lin Wang, Eric. W. Hudson, Jennifer. E. Hoffman

We use scanning tunneling microscopy to investigate the surface structure, superconducting and vortex properties in the hole-doped superconductor Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ ($T_{c}$=32 K). We resolve the long debate over the nature of the $A$Fe$_2$As$_2$ cleaved surface, finding a dominant Sr/K termination with $1 \times 2$ reconstruction and ubiquitous superconducting gap, with rarer patches of gapless, unreconstructed As termination. 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. At 9 T we observe isotropic vortices which provide a measure of the superconducting coherence length $\xi=2.8$ nm, and constrain the pairing symmetry. Furthermore, we observe a vitreous vortex phase and quantify its correlation length in comparison to other iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering and a new avenue to optimize vortex pinning for increased critical current.

View original: http://arxiv.org/abs/1212.3240