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.
This is a pre-print. The definitive version: LEE, ALEXANDROV and LIANG, 2003. Lorenz number in high Tc superconductors: evidence for bipolarons. Physical Review Letters, 90(21), is available at: http://prl.aps.org/.