Abnormally enhanced Hall Lorenz number in the magnetic Weyl semimetal NdAlSi

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 L₀, 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 < L₀). Here, we report a rare case of remarkably enhanced Lorenz number (L > L₀) discovered in the magnetic topological semimetal NdAlSi. Measurements of the transverse electrical and thermal transport coefficients reveal that the Hall Lorenz number L_xy in NdAlSi starts to deviate from the canonical value far above its magnetic ordering temperature. Moreover, L_xy displays strong nonmonotonic temperature and field dependence, reaching its maximum value close to 2L₀ in an intermediate parameter range. Further analysis excludes charge-neutral excitations as the origin of enhanced L_xy. 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.