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

Abstract

We study magnetic pair breaking due to Mn impurities in the optimally electron-doped superconductor Sr ( Fe 0.88 Co 0.12 ) 2 As 2 . We found that the as-grown Sr ( Fe 0.88 − y Co 0.12 Mn y ) 2 As 2 single crystals exhibit a T c suppression rate of ∼ 30 mK/ μ Ω cm . This rate is slow but in good agreement with the previous reports on various magnetic/nonmagnetic impurities doped in other structurally analogous iron-based superconductors. The slow T c suppression rate for magnetic impurities is often cited as an evidence for the nonvalidity of the s + + -wave symmetry, which should have suppressed T c in accordance with the Abrikosov-Gor'kov theory. Here, we show that the crystallographic defects are the main source of pair breaking in the as-grown crystals. Once these defects are healed by a low-temperature annealing, the true T c suppression rate due to Mn impurities is revealed. We thus estimate the actual T c suppression rate due to Mn alone to be ≥ 325 mK / μ Ω cm , and that due to the nonmagnetic crystallographic defects to be nearly 35 mK/ μ Ω cm . These findings can be reconciled with the fully gapped s + − -wave symmetry provided the interband scattering is rather weak. On the other hand, the s + + -wave symmetry, which is resilient to the nonmagnetic defects and fragile against the magnetic impurities, can be a possible pairing symmetry in the optimally electron-doped SrFe 2 As 2 . The crucial information that we provide here is that the magnetic pair breaking in these superconductors is not as weak as is generally believed.