Abnormally enhanced Hall Lorenz number in the magnetic Weyl semimetal NdAlSi

TL;DR

This study reports an unusual enhancement of the Hall Lorenz number in NdAlSi, a magnetic Weyl semimetal, revealing complex charge and spin interactions that challenge conventional Fermi liquid theory at elevated temperatures.

Contribution

It demonstrates the first observation of a significantly enhanced Hall Lorenz number in a magnetic topological semimetal, linked to Kondo-like scattering mechanisms.

Findings

Hall Lorenz number exceeds L0 at high temperatures

Lxy shows nonmonotonic temperature and field dependence

Enhanced Lxy attributed to Kondo-type elastic scattering

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 2$L_0$ 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 4$f$ 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.