Enhancement of transverse thermoelectric conductivity originating from stationary points in nodal line

TL;DR

This study reveals that stationary points in nodal lines significantly enhance transverse thermoelectric conductivity in certain ferromagnets, providing insights for designing materials with large anomalous Nernst effects.

Contribution

It identifies stationary points in nodal lines as key factors in enhancing thermoelectric response, expanding understanding beyond Weyl points.

Findings

Stationary points cause sharp peaks in density of states.

$oldsymbol{ ext{alpha}}_{ij}/T$ breaks the Mott relation.

Enhanced $oldsymbol{ ext{alpha}}_{ij}$ at specific Fermi energies.

Abstract

Motivated by the recent discovery of a large anomalous Nernst effect in Co$_2$MnGa, Fe$_3X$ ($X$=Al, Ga) and Co$_3$Sn$_2$S$_2$, we performed a first-principles study to clarify the origin of the enhancement of the transverse thermoelectric conductivity ($\alpha_{ij}$) in these ferromagnets. The intrinsic contribution to $\alpha_{ij}$ can be understood in terms of the Berry curvature ($\Omega$) around the Fermi level, and $\Omega$ is singularly large along nodal lines (which are gapless in the absence of the spin-orbit coupling) in the Brillouin zone. We find that not only the Weyl points but also stationary points in the energy dispersion of the nodal lines play a crucial role. The stationary points make sharp peaks in the density of states projected onto the nodal line, clearly identifying the characteristic Fermi energies at which $\alpha_{ij}$ is most dramatically enhanced. We also find that $\alpha_{ij}/T$ breaks the Mott relation and show a peculiar temperature dependence at these energies. The present results suggest that the stationary points will give us a useful guiding principle to design magnets showing a large anomalous Nernst effect.