59Co, 23Na, and 1H NMR Studies of Double-Layer Hydrated Superconductors NaxCoO2-yH2O
Abstract
We present our NMR studies of double-layer hydrated cobalt oxides NaxCoO2-yH2O (x ~ 0.35, y ~ 1.3) with various Tc = 0 - 4.8 K and magnetic transition temperatures. High-resolution 1H NMR spectrum served as an evidence for the existence of H3O+ oxonium ions. 23Na nuclear spin-lattice relaxation rates served to detect local field fluctuations sensitive to Tc. 59Co nuclear quadrupole resonance (NQR) spectra served to classify the various Tc samples. From the classification by 59Co NQR frequency, the double-layer hydrated compounds were found to have two superconducting phases closely located to a magnetic phase. In the normal state and at a magnetic field in the ab-plane, two 59Co NMR signals with different Knight shifts and different 59Co nuclear spin-lattice relaxation times 59T1 were observed. The two 59Co NMR signals suggest magnetic disproportionation of two Co sites or in-plane (XY) anisotropy of a single Co site. Non Korringa behavior and power law behavior in zero-field NQR 1/59T1 above and below Tc suggest non-Fermi liquid and unconventional superconductivity.