title: Lorenz number in high T(c) superconductors: evidence for bipolarons.
abstract: The strong electron-phonon interaction in cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalized, electrons are transformed into itinerant 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 direct evidence for bipolarons in the cuprates.
authors.affiliation: IRC in Superconductivity, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom.
authors.forename: K K
authors.initials: KK
authors.lastname: Lee
authors.forename: A S
authors.initials: AS
authors.lastname: Alexandrov
authors.forename: W Y
authors.initials: WY
authors.lastname: Liang