Determining the Wiedemann-Franz ratio from the thermal Hall conductivity: Application to Cu and YBa2Cu3O6.95

The Wiedemann-Franz (WF) ratio compares the thermal and electrical conducti vities in a metal. We describe anew way to determine its value, based on th e thermal Hall conductivity. The technique is applied to copper and to untw inned YBaCuO. In the latter, we uncover a T-linear dependence and suppressi on of the Hall-channel WF ratio. We discuss the implications of this suppre ssion. The general suppression of the WF ratio in systems with predominant electron-electron scattering is discussed.