${}^{17}$O NMR Measurements on Superconducting Na${}_{0.35}$CoO${}_2\cdot y$H${}_2$O

Abstract

An ${}^{17}$O NMR measurement was performed on nonoriented polycrystalline Na${}_{0.35}$CoO${}_2\cdot y$H${}_2$O with superconducting (SC) transition temperature $T_{\text{c}}=4.6$ K. A weak temperature dependence was observed in the Knight shift at the O site (${}^{17}K$). The spin part of ${}^{17}K$ (${}^{17}{K}_{\mathrm{spin}}$) is estimated from the plot of ${}^{17}K$ against bulk susceptibility $\chi$. The ${}^{17}{K}_{\mathrm{spin}}$ decreases in the SC state, indicative of the decrease in the in-plane component of the spin susceptibility. The nuclear spin-lattice relaxation rate $1/T_1$ at the O site ${}^{17}(1/T_1)$ shows a good scaling with $1/T_1$ at the Co site ${}^{59}(1/T_1)$. This indicates that the spin fluctuations at the O site originate from the Co spin dynamics. The relationships between ${}^{17}(1/T_{1}T)$ and ${}^{17}{K}_{\mathrm{spin}}$ and between ${}^{17}(1/T_{1}T)$ and ${}^{59}(1/T_{1}T)$ show the development of incommensurate fluctuations at {\bf q $\sim$ 0} other than {\bf q $=$ 0} below 30 K. A clear indication of ferromagnetic correlations at {\bf q $=$ 0} was not observed from the present ${}^{17}$O-NMR studies.

5 pages, 5 figures, to be published in Journal of physical Society of Japan vol.74, No.8