Wiedemann-Franz law in the underdoped cuprate superconductor YBa2Cu3Oy

TL;DR

This study tests the Wiedemann-Franz law in underdoped YBa2Cu3Oy at high magnetic fields, providing evidence against a vortex liquid state and constraining the normal state properties of underdoped cuprates.

Contribution

High-field measurements of electrical and thermal Hall conductivities in YBa2Cu3Oy demonstrate the validity of the Wiedemann-Franz law, challenging previous vortex liquid interpretations.

Findings

Wiedemann-Franz law holds above the vortex-melting field

Vortex liquid state is ruled out at T=0 in underdoped cuprates

Normal state constraints are tightened by these measurements

Abstract

The recent detection of charge-density modulations in YBa2Cu3Oy and other cuprate superconductors raises new questions about the normal state of underdoped cuprates. In one class of theories, the modulations are intertwined with pairing in a dual state, expected to persist up to high magnetic fields as a vortex liquid. In support of such a state, specific heat and magnetisation data on YBa2Cu3Oy have been interpreted in terms of a vortex liquid persisting above the vortex-melting field Hvs at T = 0. Here we report high-field measurements of the electrical and thermal Hall conductivities in YBa2Cu3O6.54 that allow us to probe the Wiedemann-Franz law, a sensitive test of the presence of superconductivity in a metal. In the T = 0 limit, we find that the law is satisfied for fields immediately above Hvs. This rules out the existence of a vortex liquid and it places strict constraints on the nature of the normal state in underdoped cuprates.