Anomalous transverse response of Co_2MnGa and universality of the room-temperature α_(ij)~A/σ_(ij)~A ratio across topological magnets

摘要

The off-diagonal (electric, thermal, and thermoelectric) transport coefficients of a solid can acquire an anomalous component due to the nontrivial topology of the Bloch waves. We present a study of the anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and thermal Hall effect in the Heusler-Weyl ferromagnet Co_2MnGa. The anomalous Wiedemann-Franz law, linking electric and thermal responses, was found to be valid over the whole temperature window. This indicates that the AHE has an intrinsic origin and the Berry spectrum is smooth in the immediate vicinity of the Fermi level. We extract α_(ij)~A from our ANE data and find that the α_(ij)~A/σ_(ij)~A ratio approaches k_B/e at room temperature. Scrutinizing all topological magnets previously explored, we observe that this ratio is a sizable fraction of k_B/e at room temperature. We provide a rough explanation for this feature by arguing that the two anomalous transverse coefficients depend on universal constants, the Berry curvature averaged over a window set by either the Fermi wavelength (for Hall) or the de Broglie thermal length (for Nernst). The universal scaling indicates that the widths of the two windows approaches each other at room temperature.