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Spin singlet pairing in the superconducting state of NaxCoO2·1.3H2O: evidence from a 59Co Knight shift in a single crystal

Guo-qing Zheng, Kazuaki Matano, D. P. Chen, C. T. Lin

cond-mat.supr-conarXiv:cond-mat/0605212

Abstract

We report a 59Co Knight shift measurement in a single crystal of the cobalt oxide superconductor NaxCoO2·1.3H2O (Tc=4.25 K). We find that the shift due to the spin susceptibility, Ks, is substantially large and anisotropic, with the spin shift along the a-axis Ksa being two times that along the c-axis Ksc. The shift decreases with decreasing temperature (T) down to T100 K, then becomes a constant until superconductivity sets in. Both Ksa and Ksc decrease below Tc. Our results indicate unambiguously that the electron pairing in the superconducting state is in the spin singlet form.

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