Observation of Large Wiedemann-Franz Law Violation in Ferromagnetic, Heusler, Weyl Semimetal Co₂MnAl

The Wiedemann-Franz (WF) law states that in metals the ratio between the electronic component of thermal conductivity and the electrical conductivity is proportional to the temperature because both are mediated by charge carriers. This proportionality constant is called the Lorenz number. This relation has been found to hold for many metals over a wide temperature range, however a number of topological materials have been found to violate the WF law[1,2]. Co2MnAl is a Heusler, ferromagnetic, Weyl semimetal whose band structure is dependent on applied magnetic field. Under no applied field, Co2MnAl’s band structure has 4 Weyl nodal rings that form a Hopf-like chain protected by mirror symmetries. When a magnetic field is applied, these rings can be gapped out creating Weyl nodes and changing the Berry curvature[3,4]. In this talk, we report thermal conductivity and electrical resistivity measurements and analysis that demonstrates a substantial violation of the WF law in Co2MnAl. Further, we observe nonclassical temperature dependence of the Lorenz number. Comparison between our results and those of other WF law violating materials indicates that there is no one effect that can fully explain the observed violation and temperature dependence. Rather, our observations are likely due to a number of effects, related to Co2MnAl’s exotic band structure, working in tandem.