Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal

In conducting ferromagnets, an anomalous Nernst effect—the generation of an electric voltage perpendicular to both the magnetization and an applied temperature gradient—can be driven by the nontrivial geometric structure, or Berry curvature, of the wavefunction of the electrons ^1,2 . Here, we report the observation of a giant anomalous Nernst effect at room temperature in the full-Heusler ferromagnet Co ₂ MnGa, an order of magnitude larger than the previous maximum value reported for a magnetic conductor ^3,4 . Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions ^5–7 is responsible for the observed –Tlog(T) behaviour, with T denoting the temperature, and the enhanced value of the transverse thermoelectric conductivity. The temperature dependence of the thermoelectric response in experiments and numerical calculations can be understood in terms of a quantum critical-scaling function predicted by the low-energy effective theory over more than a decade of temperatures. Moreover, the observation of an unsaturated positive longitudinal magnetoconductance, or chiral anomaly ^8–10 , also provides evidence for the existence of Weyl fermions ^11,12 in Co ₂ MnGa.