A critical assessment of the superconducting pairing symmetry in NaxCoO2•yH2O

Na(x)CoO(2)center dot yH(2)O must be hydrated to superconduct(1), and its triangular CoO2 layers provide an intriguing contrast with the square CuO2 layers of the high-temperature superconductors. Its superconductivity is often assumed to be unconventional in that the Cooper pairs are not in a spin singlet state with s-wave symmetry, as with conventional superconductors. According to the Pauli exclusion principle, pairs with a singlet (triplet) spin part have a corresponding even (odd) spatial part, designated as s-, p, d- or f-wave pairing in accordance with the pair angular momentum. However, in Na(x)CoO(2)center dot yH(2)O, experimental reports are often contradictory(2-6) and solid evidence for any particular pairing state remains lacking. This has led to an unprecedented number of proposals(2,3,5,7-13) for the pairing symmetry (perhaps the greatest number ever for a single compound), each in agreement with some subset of the available data. Here we test each of the 25 symmetry-allowed pairing states(14) against firmly established properties of the compound. Surprisingly, this eliminates most possible pairings. The two remaining states both have f -wave symmetry, suggesting that Na(x)CoO(2)center dot yH(2)O may be the most exotic superconductor discovered so far. We discuss expected features of these states and suggest experiments to distinguish between them.