Abnormally enhanced Hall Lorenz number in the magnetic Weyl semimetal NdAlSi

Abstract In Landau’s celebrated Fermi liquid theory, electrons in a metal obey the Wiedemann–Franz law at the lowest temperatures. This law states that electron heat and charge transport are linked by a constant L 0, i.e., the Sommerfeld value of the Lorenz number (L). Such relation can be violated at elevated temperatures where the abundant inelastic scattering leads to a reduction of the Lorenz number (L < L 0). Here, we report a rare case of remarkably enhanced Lorenz number (L > L 0) discovered in the magnetic topological semimetal NdAlSi. Measurements of the transverse electrical and thermal transport coefficients reveal that the Hall Lorenz number L x y in NdAlSi starts to deviate from the canonical value far above its magnetic ordering temperature. Moreover, L x y displays strong nonmonotonic temperature and field dependence, reaching its maximum value close to 2L 0 in an intermediate parameter range. Further analysis excludes charge-neutral excitations as the origin of enhanced L x y . Alternatively, we attribute it to the Kondo-type elastic scattering off localized 4f electrons, which creates a peculiar energy distribution of the quasiparticle relaxation time. Our results provide insights into the perplexing transport phenomena caused by the interplay between charge and spin degrees of freedom.