Regensburg 2007 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 13: Symposium “Superconductivity and Magnetism in Lamellar Cobaltates”

TT 13.3: Talk

Tuesday, March 27, 2007, 15:00–15:25, H18

Cooper-pair symmetry and spin correlations in the cobaltate superconductors Na_xCoO₂ * 1.3 H₂O — •Guo-qing Zheng — Okayama University, Okayama 700-8530, Japan

We will present NMR/NQR results on the symmetry of the Cooper pairs in the superconducting state and the spin correlations in the normal state of Na_xCoO₂ * 1.3 H₂O. We find that below T_c the spin-lattice relaxation rate (1/T₁), decreases in proportion to T³ and down to very low temperature, strongly suggesting line nodes in the gap function [1,2]. The spin susceptibility measured via the Knight shift decreases below T_c in all directions [3]. These results indicate anisotropic, spin-singlet, (e.g., d-wave), pairing. In the normal state, the data point to antiferromagnetic spin correlations in the CoO₂ plane. We will also discuss the Na-content and pressure dependencies of the spin correlations and the superdconducting properties. This work was done in collaboration with K. Matano (Okayama U), R.L. Meng, J. Cmaidalka, C.W. Chu (Houston U), D.P. Chen and C.T. Lin (Max Plank Institute, Stuttgart).

[1] T. Fujimoto et al, Phys. Rev. Lett. 92, 047004 (2004). [2] G. - q. Zheng et al, J. Phys.: Condens. Matter 18, L63 (2006). [3] G. - q. Zheng et al, Phys. Rev. B73, 180503 (R) (2006).