The Wiedemann Franz ratio, expressed as the Lorenz number ℒ= (e/kB)2κe/σT, 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 κe. To get around the phonon background, we have measured with high precision the thermal Hall conductivity κxy (which does not involve phonons), and formed the Hall Lorenz number ℒxy = (e/kB)2κxy/σxyT. The method was checked against high purity copper. We find that ℒxy 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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Energy Engineering and Power Technology
- Electrical and Electronic Engineering