The Wiedemann-Franz ratio in YBa2Cu3O7−δ determined from the thermal hall conductivity

The Wiedemann Franz ratio, expressed as the Lorenz number L = (e/k(B))(2)kappa (e)/sigmaT, provides important information on the charge and entropy carriers. In the normal state of the cuprates, L is not known reliably because the phonon conductivity dominates the electronic thermal conductivity kappa (e). To get around the phonon background, we have measured with high precision the thermal Hall conductivity k(xy) (which does not involve phonons), and formed the Hall Lorenz number L-xy = (e/k(B))(2) kappa (xy)/sigma T-xy. The method was checked against high purity copper. We find that L-xy in YBCO is linear in T and strongly suppressed from the Sommerfeld value, in conflict with previous claims. Implications of the finding will be discussed.