LAUSR

The Wiedemann–Franz law establishes a link between heat and charge transport due to electrons in solids. The extent of its validity in the presence of inelastic scattering is a question raised in different contexts. Here we report on a study of the electrical, σ, and thermal, κ, conductivities in WP2 single crystals. The Wiedemann-Franz law holds at 2 K, but a downward deviation rapidly emerges upon warming. At 13 K, there is an exceptionally large mismatch between the Lorenz number and the Sommerfeld value. We show that this is driven by a fivefold discrepancy between the T-square prefactors of electrical and thermal resistivities, both caused by electron–electron scattering. This implies the existence of abundant small-scattering-angle collisions between electrons, due to strong screening. By quantifying the relative frequency of collisions conserving momentum flux, but degrading heat flux, we identify a narrow temperature window where the hierarchy of scattering times may correspond to the hydrodynamic regime.Topological materials| Breakdown of the Wiedemann-Franz law at finite temperaturesThe temperature dependence of the ratio between electrical and thermal conductivity in a type-II Weyl semimetal highlights deviations from the Wiedemann-Franz law. In a metal, if phonons carry negligible heat and the electrons do not undergo inelastic scattering, the Wiedemann-Franz law links the electrical and thermal conductivity so that their ratio divided by temperature is a universal constant. Now, experiments reported by Alexandre Jaoui, Kamran Behnia and colleagues reveal that in WP2 the Wiedemann-Franz law is obeyed at very low temperature, but substantial deviations appear at higher temperatures owing to electron-electron scattering. Because the relevant collisions are characterized by a small scattering angle and this type of collision conserves the electric current but not the heat current, the authors could identify a temperature window in which the conditions for the hydrodynamic regime are realized.