Characterization of topological band structures away from the Fermi level by anomalous Nernst measurements

TL;DR

This paper proposes using anomalous Nernst effect measurements to detect topological features in band structures, especially when traditional methods are limited by small signals or features far from the Fermi level.

Contribution

It introduces anomalous Nernst measurements as a novel approach to characterize topological band structures away from the Fermi level, demonstrated on Heusler compounds.

Findings

Anomalous Nernst effect reveals signatures of topological features elusive in Hall measurements.

Effective for room temperature and cryogenic quantum transport regimes.

Applicable to materials with topological nodal lines above the Fermi level.

Abstract

Resolving the structure of energy bands in transport experiments is a major challenge in condensed matter physics and material science. Sometimes, however, traditional electrical conductance or resistance measurements only provide very small signals, and thus limit the ability to obtain direct band structure information. In this case, it has been proven beneficial to employ thermoelectric measurements which are sensitive to the first derivative of the density of states with respect to energy, rather than to its value itself. Due to the large interest in topological effects these days, it is important to identify a similar concept for detecting the Berry curvature in a band structure. Nowadays, the common way to access the Berry curvature directly via measurements is the anomalous Hall effect, but the corresponding signal can be too small to be detected when the topological features of the band structure lie too far off the Fermi level. In this work we propose to investigate topological band structure features utilizing the anomalous Nernst effect which is tied to the derivative of the anomalous Hall effect with respect to energy. Thereby, also signatures become resolvable, which are elusive in anomalous Hall measurements. We demonstrate the underlying mechanisms for a minimal effective four-band model and exemplary for the real Heusler compounds Co$_2$Fe$X$ ($X$=Ge,Sn), which host topological nodal lines about 100 meV above the Fermi level. This work demonstrates that anomalous Nernst measurements can be an effective tool for the characterization of topological band structures, both at room temperature and in the quantum transport regime at cryogenic temperatures.