Effect of Mn impurities on the superconductivity in Nax Co O2 • y H2 O

To shed light on the symmetry of the superconducting order parameter in Nax Co O2 • y H2 O, the Mn doping effects are studied. X-ray absorption spectroscopy verifies that the doped Mn impurities occupy the Co sites and are with a valance close to +4. Impurity scattering by Mn is in the unitary limit that, however, does not lead to strong Tc suppression. This absence of the strong impurity effects on Tc is inconsistent with the simple picture of a sign-changing order parameter. Coexistence of the s -wave superconducting order parameter and the nodal one is proposed to reconcile all the existing experiments and has been implied by the specific heat experiments.