Pairing symmetry of the hydrated cobaltate superconductor

We report NMR/NQR measurements on the hydrated cobaltate superconductor Na$_{x}$CoO$_{2}$*1.3H$_{2}$O at elevated pressures. The spin-lattice relaxation rate (1/$T_{1})$ decreases below $T_{c}$ with no coherence peak [1], and is in proportion to $T^{3 }$down to $T\sim T_{c}$/10, which provides compelling evidence for the existence of line nodes in the gap function [2,3]. The spin susceptibility obtained from the Knight shift measurement in a single crystal decreases below $T_{c}$ along all crystal-axis directions [4]. These results indicate anisotropic, spin-singlet pairing, and are most consistent with a $d$-wave gap. The electron correlations in the normal state are antiferromagnetic-like, which increases with decreasing Na-content [1,2]. The phase diagrams of $T_{c}$ and various physical properties as functions of Na-content [2], and pressure [3] will be presented, and the inter-relation between the superconductivity and the spin correlations will be discussed. \newline \newline References: \newline [1] T. Fujimoto, G. - q. Zheng, Y. Kitaoka, R.L. Meng, J. Cmaidalka, and C.W. Chu, Phys. Rev. Lett. \textbf{92}, 047004 (2004). \newline [2] G. - q. Zheng, K. Matano, R.L. Meng, J. Cmaidalka, and C.W. Chu, J. Phys.: Condens. Matter \textbf{18}, L63 (2006). \newline [3] E. Kusano, S. Kawasaki, K. Matano, G. - q. Zheng, R.L. Meng, J. Cmaidalka, and C.W. Chu, Phys. Rev. B \textbf{76}, 100506 (R) (2007). \newline [4] G. - q. Zheng, K. Matano, D.P. Chen and C.T. Lin, Phys. Rev. \textbf{B73}, 180503 (R) (2006).