Possible singlet-to-triplet pairing transition in NaxCoO2•yH2O -: art. no. 144516

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.