Zn doping and gap symmetry in optimally electron doped Sr(Fe,Co)2As2 superconductors

Abstract

The surprising discovery of Iron based superconductors with TC reaching as high as 56 K has attracted huge interest from scientific community in recent years. Despite significant progress, a central question about the pairing symmetry of the Cooper pair in such superconductors has remains unanswered. This problem has been already studied by means of penetration depth studies, NMR, ARPES and other techniques. While most studies support the fully gapped s-wave pairing in the superconducting state, the exact nature of pairing symmetry is still debatable between s++ and s+-. Introducing impurities and studying their effect on the suppression rate of the superconducting TC is a powerful method of probing the pairing symmetry of the superconducting wavefunction. According to the Abrikosov-Gorkov law, TC will vary differently for magnetic and non-magnetic impurities in the for s++ and s+- cases. The present study is continuation of a recently published paper our lab [1], where Mn, which is a magnetic impurity, was introduced in an optimally electron doped Sr(Fe,Co)2As2 superconductor, and based on variation of TC with impurity concentration in as-grown and annealed crystals, it was concluded that these superconductors should have the s++ pairing symmetry. In this study, we have taken optimally Co doped SrFe2-xCoxAs2 as parent compound, whose TC is around 13K and doped with the non-magnetic Zn at the Fe site and studied the variation of TC with respect to varying concentration of Zn for the as-grown (with structural defects) and annealed (healed crystal defects) specimens. We show that Zn-doping increases the TC for the as-grown specimens, but TC remains unchanged after annealing. These observations strongly support the s++ symmetry in agreement with Ref. [1].