Wiedemann-Franz Law and Abrupt Change in Conductivity across the Pseudogap Critical Point of a Cuprate Superconductor

The thermal conductivity κ of the cuprate superconductor La_{1.6-x}Nd_{0.4}Sr_{x}CuO_{4} was measured down to 50 mK in seven crystals with doping from p=0.12 to p=0.24, both in the superconducting state and in the magnetic field-induced normal state. We obtain the electronic residual linear term κ_{0}/T as T→0 across the pseudogap critical point p^{⋆}=0.23. In the normal state, we observe an abrupt drop in κ_{0}/T upon crossing below p^{⋆}, consistent with a drop in carrier density n from 1+p to p, the signature of the pseudogap phase inferred from the Hall coefficient. A similar drop in κ_{0}/T is observed at H=0, showing that the pseudogap critical point and its signatures are unaffected by the magnetic field. In the normal state, the Wiedemann-Franz law, κ_{0}/T=L_{0}/ρ(0), is obeyed at all dopings, including at the critical point where the electrical resistivity ρ(T) is T linear down to T→0. We conclude that the nonsuperconducting ground state of the pseudogap phase at T=0 is a metal whose fermionic excitations carry heat and charge as conventional electrons do