The Lorenz number in CeCoIn₅ inferred from the thermal and charge Hall currents

The thermal Hall conductivity κ_xy and Hall conductivity σ_xy in CeCoIn₅ are used to determine the Lorenz number ℒ_H at low temperature T. This enables the separation of the observed thermal conductivity into its electronic and non-electronic parts. We uncover evidence for a charge-neutral, field-dependent thermal conductivity, which we identify with spin excitations. At low T, these excitations dominate the scattering of charge carriers. We show that suppression of the spin excitations in high fields leads to a steep enhancement of the electron mean-free-path, which leads to an interesting scaling relation between the magnetoresistance, thermal conductivity and σ_xy.