Breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon system, application to the cuprates

TL;DR

This paper investigates the breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon systems, specifically in cuprates, using a model that accounts for bipolarons and polarons, and fits magnetotransport data.

Contribution

It introduces a model that decouples electrons and phonons in the strong-coupling limit, explaining pseudogaps and thermal transport in cuprates, and validates a charged Bose gas approach.

Findings

Wiedemann-Franz law breaks down due to polaron and bipolaron interference.

Model fits magnetotransport data in cuprates.

Extracted phonon thermal conductivity component from experimental data.

Abstract

With the superconducting cuprates in mind, a set of unitary transformations was used to decouple electrons and phonons in the strong-coupling limit. While phonons remain almost unrenormalised, electrons are transformed into itinerent singlet and triplet bipolarons and thermally excited polarons. The triplet/singlet exchange energy and the binding energy of the bipolarons are thought to account for the spin and charge pseudogaps in the cuprates, respectively. We calculated the Hall Lorenz number of the system to show that the Wiedemann-Franz law breaks down due to the interference of the polaron and bipolaron contributions to heat flow. The model provides a quantitative fit to magnetotransport data in the cuprates. Furthermore we are able to extract the phonon component of the thermal conductivity with the use of experimental data and the model. Our results further validate the use of a charged Bose gas model to describe normal and superconducting properties of unconventional superconductors.