Spatial Inhomogeneities in the Superconducting Gap of SrFe1.6Co0.4As2 Single Crystals

Abstract

We report spatial inhomogeneity of the superconducting gap of optimally doped SrFe1.6Co0.4As2 single crystals using scanning tunneling microscopy and spectroscopy studies from 20 to 5 K. This composition becomes superconductor at TC$\sim $ 18 K and 60 % volume of the bulk becomes superconductor at 5 K. Local tunnel spectra at 5 K show coherence peaks with a depression near the Fermi energy. These spectra are asymmetric and amplitude of the coherence peaks vary with position. Gaps magnitude (Δ) is also not constant over the surface; we found a distribution of gap between 11.5 and 17.5 meV energy range along a line with an average value of $\bar {\Delta } =$ 13.7 meV and standard deviation σ = 1.4 meV. This leads to a fractional variation $\sigma /\bar {\Delta }~=~$ 10 %. The calculated reduced gap 2$\bar {\Delta }$/kBTC at 5 K shows a very high value of 17.68 as compared to the s/d-wave superconductor indicating a strong pairing strength for the SC order parameter. Above TC, spectra does not show any gap and the observed inhomogeneity is also less. By comparing our data with underdoped composition, we claim that spatial inhomogeneity in the SC gap is an intrinsic property of the Fe-As-based superconductors and phase boundary plays much important role for such inhomogeneity rather than the disorder or dopant.