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(Fe0.88Co0.12)2As2. We found that the as-grown Sr(Fe0.88−yCo0.12Mny)2As2 single crystals exhibit a Tc suppression rate of ∼30 mK/μΩcm. 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 Tc suppression rate for magnetic impurities is often cited as an evidence for the nonvalidity of the s++-wave symmetry, which should have suppressed Tc 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 Tc suppression rate due to Mn impurities is revealed. We thus estimate the actual Tc suppression rate due to Mn alone to be ≥325 mK/μΩcm, and that due to the nonmagnetic crystallographic defects to be nearly 35 mK/μΩcm. 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 SrFe2As2. The crucial information that we provide here is that the magnetic pair breaking in these superconductors is not as weak as is generally believed.

Source-reported materials — not catalogue approval

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