3D Dirac Electrons and Large Thermoelectric Properties in CoGe

Abstract

We have synthesized B20-type Co$_{1-x}$Fe$_x$Ge and Co$_{1-y}$Ni$_y$Ge by a high-pressure method and investigated the band-filling dependence of thermoelectric properties. Non-doped CoGe shows a large figure of merit ($ZT\approx 0.11$) due to its low resistivity and large negative Seebeck coefficient ($S=-82\,\mathrm {\mu V/K}$) at 300 K. S shows a sign change with increasing Fe concentration, yielding its maximum value ($S=28\,\mathrm {\mu V/K}$ at 300 K) at $x=0.2$. The Boltzmann transport model can explain semi-quantitatively the experimental results, from which we conclude that the large positive or negative S originates from the asymmetric band structure composed of the Dirac cone and the flat band with sharp bends around the Fermi energy ($E_F$).