Low-temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly doped cuprate a2Cu3O6:33. Samples were measured with doping concentrations on either side of the superconducting phase boundary. We report the observation of delocalized fermionic excitations at zero energy in the nonsuperconducting state, which shows that the ground state of underdoped cuprates is a thermal metal. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e., nodal. The insulating ground state observed in underdoped La2-xSrxCuO4 is attributed to the competing spin-density-wave order.

The in-plane resistivity ρ and thermal conductivity к of a single crystal of NaxCoO2 (x = 0.7) were measured down to 40 mK. Verification of the Wiedemann-Franz law, к/T = L0/ρ as T→0, and observation of a T2 dependence of ρ at low temperature, △ρ= AT2, establish the existence of a well-defined Fermi-liquid state. The measured value of coefficient A reveals enormous electronelectron scattering, characterized by the largest Kadowaki-Woods ratio A/γ 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.