Enhancement of the transverse thermoelectric conductivity originating from stationary points in nodal lines

Motivated by the recent discovery of a large anomalous Nernst effect in Co2MnGa, Fe3X (X=Al, Ga) and Co3Sn2S2, we performed a first-principles study to clarify the origin of the enhancement of the transverse thermoelectric conductivity αij in these ferromagnets. The intrinsic contribution to αij can be understood in terms of the Berry curvature ω around the Fermi level, and ω 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 αij is most dramatically enhanced. We also find that αij/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.