Lorenz number in high Tc superconductors: evidence for bipolarons

TL;DR

This paper demonstrates that strong electron-phonon interactions in cuprates lead to bipolaron formation, causing a breakdown of the Wiedemann-Franz law, with numerical fits supporting bipolarons as evidence in high Tc superconductors.

Contribution

The study provides a theoretical calculation of the Lorenz number showing the Wiedemann-Franz law breakdown due to bipolaron and polaron interference, supporting the bipolaron model in cuprates.

Findings

Breakdown of Wiedemann-Franz law due to bipolaron-polaron interference

Numerical fit of the Hall Lorenz number with experimental data

Evidence supporting bipolarons in high Tc superconductors

Abstract

Strong electron-phonon interaction in cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalised, electrons are transformed into itinerent bipolarons and thermally excited polarons when the electron-phonon interaction is strong. We calculate the Lorenz number of the system to show that the Wiedemann-Franz law breaks down because of the interference of polaron and bipolaron contributions in the heat flow. The model fits numerically the experimental Hall Lorenz number, which provides a direct evidence for bipolarons in the cuprates.