Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na$_{0.35}$CoO$_{2}$$\cdot y$H$_{2}$O

We report a precise $^{59}$ Co nuclear quadrupolar resonance (NQR) measurement on the recently discovered cobalt oxyhydrate Na $_{0.35}$ CoO $_{2}$ $\cdot y $H$ _{2} $O s u p e r c o n d u c t o r f r o m$ T$=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate $1 / T_{1}$ follows a Curie-Weiss type temperature ( $T$ ) variation, $1 / T_{1} T = C / (T - θ)$ , with $θ$ =-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below $T_{c}$ =3.9 K, $1 / T_{1}$ decreases with no coherence peak and follows a $T^{n}$ dependence with $n ≃$ 2.2 down to $\sim$ 2.0 K but crosses over to a $1 / T_{1} \propto T$ variation below $T$ =1.4 K, which suggests non s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.

Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na $_{0.35}$ CoO $_{2}$ $\cdot y$ H $_{2}$ O

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Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na $_{0.35}$ CoO $_{2}$ $\cdot y$ H $_{2}$ O

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Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na0.350.35​CoO22​⋅y⋅yH22​O

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Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na0.350.35​CoO22​⋅y⋅yH22​O

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Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na $_{0.35}$ CoO $_{2}$ $\cdot y$ H $_{2}$ O

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Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na $_{0.35}$ CoO $_{2}$ $\cdot y$ H $_{2}$ O

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