Semimetals are a class of materials characterized by a slight overlap between their conduction and valence bands. Owing to the band structure, the same number of electrons and holes coexist in an ideal semimetal, even at the lowest temperature, giving rise to exotic transport phenomena such as nonsaturating large magnetoresistance. On the other hand, it has been considered that semimetals are disadvantageous for the thermoelectric effect, that is, the mutual conversion between heat and electricity via conducting electrons, because the thermoelectric voltages originating from an electron and hole compensate for each other. Is there a way to avoid such compensation?

 One method is to make the carrier mobilities of the electrons and holes asymmetric. Because mobility provides the velocity of a carrier accelerated by a certain electric field, if electrons and holes have completely different mobilities, compensation in the longitudinal thermoelectric effect (Seebeck effect) is expected to be suppressed. Another method involves the use of a magnetic field. Although electrons and holes diffuse along the same direction in the Seebeck effect, a magnetic field bends the diffusion in the opposite direction for electrons and holes; thus, the transverse thermoelectric effect (Nernst effect) can avoid compensation.

In this study, we investigate the thermoelectric effect of the high-mobility semimetal Ta₂PdSe₆. The holes in Ta₂PdSe₆ are known to have a significantly high mobility of approximately 10⁵ cm² V^-1 s^-1, and the value is two orders of magnitude higher than that of electrons. In other words, Ta₂PdSe₆ satisfies the uncompensated condition for the Seebeck effect, and falls in the category of uncompensated semimetals. Furthermore, we can also expect a large Nernst effect because it is roughly proportional to the carrier mobility.

Single crystals of Ta₂PdSe₆, investigated in this study,exhibited large Seebeck and Nernst effects as expected, with the Seebeck coefficient of 40 μV K^-1 at 20 K and the Nernst coefficient of 150 μV K^-1 at 20 K and 4 T. Each value is topclass among thermoelectric materials reported thus far in the cryogenic temperature range. This demonstrates that semimetals with the significant mismatch of charge carrier (electron and hole) mobilities exhibit superior thermoelectric properties. Furthermore, we qualitatively explained the origin of the large Nernst coefficient in Ta₂PdSe₆ using the single-band semi-classical transport theory. The insights gained from this study on the thermoelectric effects in uncompensated semimetal Ta₂PdSe₆ are expected to stimulate further exploration of many other thermoelectric semimetals with high carrier mobilities.

(Written by A. Nakano on behalf of all authors)

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