Corrugated flat band as an origin of large thermopower in hole doped PtSb2

The origin of the recently discovered large thermopower in hole-doped PtSb2 is theoretically analyzed based on a model constructed from first principles band calculation. It is found that the valence band dispersion has an overall flatness combined with some local ups and downs, which gives small Fermi surfaces scattered over the entire Brillouin zone. The Seebeck coefficient is calculated using this model, which gives good agreement with the experiment. We conclude that the good thermoelectric property originates from this “corrugated flat band”, where the coexistence of large Seebeck coefficient and large electric conductivity is generally expected. Good thermoelectric materials are those materials that can transform heat into electricity with high efficiency. The efficiency of thermoelectric materials is characterized by the dimensionless figure of merit, ZT, where T is the temperature, and Z = S2σ/κ with S, σ and κ being the Seebeck coefficient, electric conductivity, and the thermal conductivity, respectively.1 In particular, the product P = S2σ is called the power factor, and a material with large P requires a large S and σ. It is known, however, that materials with large Seebeck coefficient usually have small conductivity, so that good thermoelectric materials are often found in semiconducting materials with rather small amount of carriers. In this context, the discovery of good thermoelectric properties in the sodium cobaltate NaxCoO22 has been of special interest in that a large power factor is observed in a material with large amount of doped holes and thus metallic conductivity. There have been various theoretical studies on this material,3-6 and in particular, one of the present authors along with Arita proposed that a peculiar band shape named the “pudding mold type” (Fig. 1(b)) is the origin of this coexistence of good conductivity and large thermopower.6 Namely in a system having a band with flat portion at the top (or bottom), connecting into a dispersive portion, the Fermi level is kept close to the band edge upon doping, and this gives rise to a large group velocity difference between electrons and holes, resulting in a large Seebeck coefficient. At the same time, the large group velocity of holes due to the dispersive portion of the band gives a large electric conductivity, and this combination results in a large power factor.