Possible spin triplet superconductivity in NaxCoO2·yH2O —59Co NMR studies

We report 59Co NMR studies on magnetically oriented powder samples of Co oxide superconductors, NaxCoO2·yH2O, with Tc∼4.7 K. From the two-dimensional powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal state was estimated from the magnetic field dependence of the second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift shows a Curie–Weiss-like temperature dependence, similarly to the bulk magnetic susceptibility χ. From the analysis of the so-called K–χ plot, the spin and the orbital components of K and the positive hyperfine coupling constant were estimated. The onset temperature of the superconducting transition in the Knight shift does not change much in an applied magnetic field up to 7 T, which is consistent with the reported high upper critical field Hc2. The Knight shift at 7 T shows an invariant behaviour below Tc. No coherence peak just below Tc was observed in the temperature dependence of the nuclear spin–lattice relaxation rate 1/T1 in either case (NMR, nuclear quadrupole resonance). We conclude that the invariant behaviour of the Knight shift below Tc and unconventional behaviours of 1/T1 may indicate spin triplet superconductivity with p- or f-wave symmetry.