Wiedemann-Franz law and abrupt change in conductivity across the pseudogap critical point of a cuprate superconductor

TL;DR

This study measures thermal conductivity in a cuprate superconductor across the pseudogap critical point, revealing an abrupt change in carrier density and confirming the metallic nature of the pseudogap phase.

Contribution

It provides direct experimental evidence of a sudden drop in carrier density at the pseudogap critical point and confirms the validity of the Wiedemann-Franz law in this phase.

Findings

Abrupt decrease in $\kappa_0/T$ below $p^{\star}$ indicating reduced carrier density

Wiedemann-Franz law holds at all dopings including the critical point

Pseudogap phase is a conventional metal with fermionic excitations

Abstract

The thermal conductivity $\kappa$ 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 $\kappa_0/T$ as $T \to 0$ across the pseudogap critical point $p^{\star}= 0.23$. In the normal state, we observe an abrupt drop in $\kappa_0/T$ upon crossing below $p^{\star}$, 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 $\kappa_0/T$ 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, $\kappa_0/T=L_0/\rho(0)$, is obeyed at all dopings, including at the critical point where the electrical resistivity $\rho(T)$ is $T$-linear down to $T \to 0$. We conclude that the non-superconducting ground state of the pseudogap phase at $T=0$ is a metal whose fermionic excitations carry heat and charge as conventional electrons do.