Giant electron-electron scattering in the Fermi-liquid state of Na_0.7CoO_2

TL;DR

This study confirms a Fermi-liquid state in Na_0.7CoO_2 with giant electron-electron scattering, evidenced by resistivity and thermal conductivity measurements, and suggests proximity to a magnetic quantum critical point.

Contribution

It provides the first detailed measurements of resistivity and thermal conductivity in Na_0.7CoO_2 down to 40 mK, revealing unprecedented electron-electron scattering and potential quantum criticality.

Findings

Fermi-liquid behavior confirmed by Wiedemann-Franz law and T^2 resistivity dependence

Giant electron-electron scattering indicated by the largest Kadowaki-Woods ratio

Suppression of scattering with magnetic field suggests proximity to a quantum critical point

Abstract

The in-plane resistivity, rho, and thermal conductivity, kappa, of a single crystal of Na_0.7CoO_2 were measured down to 40 mK. Verification of the Wiedemann-Franz law, kappa/T = L_0/rho as T -> 0, and observation of a T^2 dependence of rho at low temperature, rho = rho_0 + AT^2, 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.