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

TL;DR

This study uses nuclear quadrupolar resonance to investigate the superconducting properties of Na$_{0.35}$CoO$_{2}$$ullet$ yH$_{2}$O, revealing antiferromagnetic correlations and evidence for unconventional, non s-wave superconductivity with line nodes.

Contribution

First precise NQR measurements on this superconductor revealing magnetic correlations and unconventional pairing symmetry.

Findings

Normal state shows Curie-Weiss spin fluctuations.

Superconducting state lacks coherence peak and shows line nodes.

Superconductivity is likely non s-wave with gap nodes.

Abstract

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 superconductor from $T$=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, $1/T_1T=C/(T-\theta)$, with $\theta$=-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\simeq$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.