Abstract
In cobaltate superconductor Nax(H3O)zCoO2·yH2O with bilayered hydrate (BLH) structure, Co nuclear magnetic resonance (NMR) measurement was performed to investigate the low-temperature magnetic excitations in magnetic field. The nuclear spin-lattice relaxation rate (1/T1) of Co was measured in the field range of 1.5–14T parallel to the CoO2 plane. In the normal state, 1/T1T follows the temperature dependence expressed as 1/T1T∝1/T-θ in all the field range. At the field lower than 10T, 1/T1T continues to increase down to Tc and decrease due to superconductivity below Tc. T1T=Const. behavior is not observed at all above Tc. When the field is higher than 10T, 1/T1T does not show clear decrease, but stop increasing below T∗. Using Tc and T∗ we determine, a H–T phase diagram is developed. The field dependence of Tc at low fields is as small as dTc/dH∼−0.06K/T. A novel phase characterized by a small kink of 1/T1T seems to be present at high-field region above 10T.
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