Possible coexistence of $s$-wave and unconventional pairing in hydrated Na$_{x}$CoO$_{2}$: a new insight from impurity effects

Impurity effects on $T_{c}$ were known to provide indispensable information in many novel superconductors. To shed light on the symmetry of the superconducting order parameter in hydrated Na$_ {x}$CoO$_{2}$, the Mn doping effects have been studied. X-ray absorption spectroscopy reveals that the doped Mn impurities occupy the Co sites and have a valance close to 4+. Impurity scattering by Mn is in the unitary limit that, however, does not lead to strong $T_{c}$ suppression. This absence of the strong impurity effects on $T_{c}$ is not consistent with the simple picture of a sign-changing order parameter. For reconciliation among all existing experiments, the model of coexistence of $s$-wave and unconventional pairing is proposed.