Magnetic disorder and gap symmetry in the optimally electron-doped Sr(Fe,Co)2As2 superconductor

Abstract

We investigate the magnetic pair-breaking due to Mn impurities in the optimally electron doped Sr(Fe0.88Co0.12)2As2 superconductor to deduce the symmetry of the superconducting order parameter. Experiments on the as-grown crystals reveled a Tc suppression rate of ∼30 mK/µΩcm, which is in close agreement with similarly slower values of Tc suppression rates reported previously for various transition metal impurities, both, magnetic and non-magnetic, in several structurally analogous iron-based superconductors. However, careful annealing of these crystals at low temperature for longer durations reveals new information crucial to the determination of the pairing symmetry. We found that the crystallographic defects are a significant source of pair-breaking in the as-grown crystals. We first establish that these defects are point-like by showing that their sole effect on electrical transport is to add a temperature independent scattering term that shifts the whole ρ vs. T curves rigidly up. The Tc suppression rate due to these point-like defects is slow, ≤ 35 mK/µΩcm. On the other hand, Tc suppression rate due to magnetic pair-breaking is estimated to be faster than 325 mK/µΩcm. A slower pair-breaking rate (measured in mK/µΩcm) than expected due to non-magnetic crystallographic defects, together with a faster pair-breaking rate due to magnetic impurities disfavors a sign-changing s+−-wave and argues in the favor of a non-sign-changing s++-wave state in the optimally electron doped SrFe2As2 superconductor.

Magnetotransport studies of optimally doped Sr(Fe1−x Co x )2As2

“…T C is assigned to the midpoint of the transition (≈14.5 K), which is approximately the average of onset and offset temperatures which are shown by the arrows. Similar values of optimal T C were reported previously [18,45,46]. Interestingly, maximum T C in the as grown crystals corresponding to the optimally doped composition (x ∼ 0.07) in the Ba(Fe 1−x Co x ) 2 As 2 series is ∼25 K [19,47,48].…”

“…The lattice parameters (a = 3.930 Å, c = 12.264 Å, c/a = 3.120) obtained from x-ray powder diffraction are in good agreement with existing literature [18]. Additional details of single crystal growth and characterization can be found elsewhere [45]. Magnetotransport measurements were carried out in a 9 T Quantum Design PPMS.…”

“…Here, x = 0.12 is actual composition of the crystals determined by Energy Dispersive X-Ray Spectroscopy (EDX). Details of single crystal growth and characterization can be found elsewhere [44].…”

Evolution of the magnetic order of Fe and Eu sublattices in Eu1−x Ca x Fe2As2 (0 ⩽ x ⩽ 1) single crystals

“…In order to quanti fy the influence of strain and crystallographic defects on T 0 and T N of these samples, a few representative single crystals with different Eu compositions, namely, Eu1.00, Eu0.48, Eu0.31 and Eu0.00 were annealed at low temperatures (350 °C) for 10 d under vacuum, similar to conditions described in [33,47,50]. The EDX analysis of these specimens showed no change in stoichiometry due to annealing.…”