Anisotropic behavior of knight shift in superconducting state of NaxCoO2 center dot yH(2)O

The Co Knight shift was measured in an aligned powder sample of NaxCoO2 center dot yH(2)O, which shows superconductivity at T-c similar to 4.6 K. The Knight-shift components parallel (K-c) and perpendicular to the c-axis (along the ab plane K-ab) were measured in both the normal and superconducting (SC) states. The temperature dependences of K-ab and K-c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K-ab decreases below 5 K, whereas K-c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c-axis is realized in NaxCoO2 center dot yH(2)O.