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

TL;DR

This study reports a giant anomalous Nernst effect in Co$_2$MnGa, linked to Weyl fermions and quantum critical scaling, with implications for thermoelectric applications.

Contribution

It reveals a significantly enhanced ANE in a ferromagnetic semimetal and connects it to quantum Lifshitz transition and Weyl fermions, providing new insights into topological thermoelectric phenomena.

Findings

Giant ANE reaching -6 μV/K at room temperature

Crossover in thermoelectric conductivity indicating Mott formula violation

Evidence of Weyl fermions through chiral anomaly and magnetoconductance

Abstract

In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE). Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co$_2$MnGa, reaching $S_{yx}\sim -6$ $\mu$V/K at room $T$, one order of magnitude larger than the maximum value reported for a magnetic conductor. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient $\alpha_{yx}$ shows a crossover between $T$-linear and $-T \log(T)$ behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions is responsible for the observed crossover properties and an enhanced $\alpha_{yx}$. The $T$ dependence of $\alpha_{yx}$ 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 chiral anomaly or an unsaturated positive longitudinal magnetoconductance also provide evidence for the existence of Weyl fermions in Co$_2$MnGa.