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(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 2L(0) 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.