Possible unconventional superconductivity in Na x CoO 2 · y H 2 O probed by muon spin rotation and relaxation

The superconducting property of sodium cobalt oxyhydrate, ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}1.3{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0.2em}{0ex}}({T}_{c}=4.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, has been studied by means of muon spin rotation/relaxation down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. It was found that the zero-field muon spin relaxation rate is independent of the temperature, indicating that no static magnetism appears in this compound, at least above $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The result also provides evidence of no breaking of time-reversal symmetry for the superconductivity. Meanwhile, the muon Knight shift at $60\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, which is proportional to the local spin susceptibility, shows no obvious reduction below ${T}_{c}$. Considering these observations, possible unconventional superconductivity of ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}∙1.3{\mathrm{H}}_{2}\mathrm{O}$ is discussed.