Quantum critical behavior in superconductingNax(H3O)zCoO2∙yH2Oobserved in a high-field Co NMR experiment

Abstract

Co nuclear magnetic resonance (NMR) measurements are performed on superconducting ${\mathrm{Na}}_{x}{({\mathrm{H}}_{3}\mathrm{O})}_{z}\mathrm{Co}{\mathrm{O}}_{2}∙y{\mathrm{H}}_{2}\mathrm{O}$ with a bilayered hydrate structure. External fields are applied exactly parallel to the $\mathrm{Co}{\mathrm{O}}_{2}$ plane on a high-quality powdered sample, which is aligned in a strong magnetic field. When superconductivity is suppressed by external magnetic fields, the nuclear spin-lattice relaxation rate divided by temperature $(1∕{T}_{1}T)$ continues to increase down to ${T}_{c}(H)$, which is the superconducting transition temperature under a magnetic field $H$. From the analyses of the temperature variation, it was found that a sample with ${T}_{c}\ensuremath{\sim}4.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is the maximum ${T}_{c}$, possesses magnetic fluctuations with a quantum critical character. This suggests the intimate relationship between superconductivity and quantum critical fluctuations in the hydrate cobaltate. We determined ${T}_{c}(H)$ and constructed the ${H}_{c2}\ensuremath{-}{T}_{c}$ phase diagram from the present NMR measurements. The field dependence of ${T}_{c}(H)$ is discussed.