Giant Electron-Electron Scattering in the Fermi-Liquid State of Na_0.7CoO₂

The in-plane resistivity ρ and thermal conductivity κ of single crystal Na_0.7CoO₂ were measured down to 40 mK. Verification of the Wiedemann-Franz law, κ/T=L₀/ρ as T→0, and observation of a T² dependence of ρ 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/γ² 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.