Lee, K.K. Alexandrov, A.S. Liang, W.Y. Lorenz number in high Tc superconductors: evidence for bipolarons Strong electron-phonon interaction in the 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. superconductivity;strongly correlated electrons;Physical Sciences not elsewhere classified 2006-03-31
    https://repository.lboro.ac.uk/articles/preprint/Lorenz_number_in_high_Tc_superconductors_evidence_for_bipolarons/9411248