Possible singlet-to-triplet pairing transition in Na x CoO 2 · y H 2 O

We present precise measurements of the upper critical field $({H}_{c2})$ in the recently discovered cobalt oxide superconductor. We have found that the critical field has an unusual temperature dependence; namely, there is an abrupt change of the slope of ${H}_{c2}(T)$ in a weak-field regime. In order to explain this result we have derived and solved Gor'kov equations on a triangular lattice. Our experimental results may be interpreted in terms of the field-induced transition from singlet to triplet superconductivity.