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 ${}^1$H NMR spectrum served as an evidence for the existence of H${}_3$O${}^{+}$ oxonium ions. ${}^{23}$Na nuclear spin-lattice relaxation rates served to detect local field fluctuations sensitive to Tc. ${}^{59}$Co nuclear quadrupole resonance (NQR) spectra served to classify the various Tc samples. From the classification by ${}^{59}$Co 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 ${}^{59}$Co NMR signals with different Knight shifts and different ${}^{59}$Co nuclear spin-lattice relaxation times ${}^{59}{T}_1$ were observed. The two ${}^{59}$Co 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/${}^{59}{T}_1$ above and below Tc suggest non-Fermi liquid and unconventional superconductivity.

12 pages, 7 figures, Submitted to DPG spring meeting at Regensburg, March 2007