Possible spin triplet superconductivity inNaxCoO2·yH2O —59Co NMR studies

Abstract

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, theab-plane Knight shift in the normal state was estimated from the magnetic field dependence ofthe second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift showsa Curie–Weiss-like temperature dependence, similarly to the bulk magnetic susceptibilityχ. From the analysisof the so-called K–χ plot, the spin and the orbital components ofK and the positive hyperfine coupling constant were estimated. The onset temperature of thesuperconducting transition in the Knight shift does not change much in an appliedmagnetic field up to 7 T, which is consistent with the reported high upper critical fieldHc2. The Knight shift at 7 T shows an invariant behaviour belowTc. No coherencepeak just below Tc was observed in the temperature dependence of the nuclear spin–lattice relaxation rate1/T1 in either case (NMR, nuclear quadrupole resonance). Weconclude that the invariant behaviour of the Knight shift belowTc and unconventionalbehaviours of 1/T1 may indicate spin triplet superconductivity with p- or f-wave symmetry.