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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., Comment: 23 pages, 5 figures

The antiferromagnetic kagome lattice compound FeGe has been revealed to host an emergent charge-density-wave (CDW) state which manifests complex interplay between the spin, charge and lattice degrees of freedom. Here, we present a comprehensive study of the de Haas-van Alphen effect by measuring torque magnetometry under magnetic fields up to 45.2 T to map Fermi surfaces in this unusual CDW state. For field along the $c$ direction, we resolve four cyclotron orbits; the largest one roughly corresponding to the area of the 2$\times$2 folded Brillouin zone. Three smaller orbits are characterized by light effective cyclotron masses range from (0.18-0.30) $m_e$. Angle-resolved measurements identify one Fermi surface segment with weak anisotropy. Combined with band structure calculations, our results suggest that features of unfolded Fermi surfaces are robust against CDW reconstruction, corroborating the novel effect of a short-ranged CDW on the electronic structure., Comment: 7 pages, 4 figures, to be published in Phys. Rev. Research

The coupling between band structure and magnetism can lead to intricate Fermi surface modifications. Here we report on the comprehensive study of the Shubnikov-de Haas (SdH) effect in two rare-earth-based magnetic Weyl semimetals, NdAlSi and CeAlSi$_{0.8}$Ge$_{0.2}$. The results show that the temperature evolution of topologically nontrivial Fermi surfaces strongly depends on magnetic configurations. In NdAlSi, the SdH frequencies vary with temperature in both the paramagnetic state and the magnetically ordered state with a chiral spin texture, but become temperature independent in the high-field fully polarized state. In CeAlSi$_{0.8}$Ge$_{0.2}$, SdH frequencies are temperature-dependent only in the ferromagnetic state with magnetic fields applied along the $c$ axis. First-principles calculations suggest that the notable temperature and magnetic-configuration dependence of Fermi surface morphology can be attributed to strong exchange coupling between the conduction electrons and local magnetic moments., Comment: 8 pages, 5 figures