Giant Electron-Electron Scattering in the Fermi-Liquid State ofNa0.7CoO2

The in-plane resistivity $\rho$ and thermal conductivity $\kappa$ of single crystal ${\mathrm{N}\mathrm{a}}_{0.7}{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ were measured down to 40 mK. Verification of the Wiedemann-Franz law, $\kappa /T=L_0/\rho$ as $T\to 0$, and observation of a $T^2$ dependence of $\rho$ at low temperature establish the existence of a well-defined Fermi-liquid state. The measured value of coefficient $A$ reveals enormous electron-electron scattering, characterized by the largest Kadowaki-Woods ratio $A/{\gamma }^2$ encountered in any material. The rapid suppression of $A$ with magnetic field suggests a possible proximity to a magnetic quantum critical point. We also speculate on the possible role of magnetic frustration and proximity to a Mott insulator.