  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.
