Large decrease in the critical temperature of superconducting LaFeAsO0.85 compounds doped with 3% atomic weight of nonmagnetic Zn impurities

We observed a large decrease in T-c by no more than 3 at. % of Zn doped into the T-c-optimized superconductor LaFeAsO0.85 (T-c=26 K), confirmed by measurements of electrical resistivity, magnetic susceptibility, specific heat, Mossbauer spectroscopy, Hall coefficient, and an electron probe microanalysis. The rate similar to 9 K/% is remarkably higher than others achieved by nonmagnetic impurities. The Tc suppression is likely due to pair breaking caused by scattering associated with the highly localized electronic state of Zn in the Fe2As2 layer. If this is true, the present result well accords with the theoretical prediction that suggests a sign reversal s-wave pairing model for the Fe-pnictide superconductors.