  We report the discovery of a very large thermoelectric power over -400 microV
K-1 in the whisker crystals of a one-dimensional telluride Ta4SiTe4, while
maintaining a low electrical resistivity of rho = 2 mohm cm, yielding a very
large power factor of P = 80 microW cm-1 K-2 at an optimum temperature of 130
K. This temperature is widely controlled from the cryogenic temperature of 50 K
to room temperature by chemical doping, resulting in the largest P of 170
microW cm-1 K-2 at 220-280 K. These P values far exceed those of the
Bi2Te3-Sb2Te3 alloys at around room temperature, offering an avenue for
realizing the practical-level thermoelectric cooling at low temperatures. The
coexistence of a one-dimensional electronic structure and a very small band gap
appearing in the vicinity of the Dirac semimetals probably causes the very
large power factors in Ta4SiTe4, indicating that the "one-dimensional Dirac
semimetal" is a promising way to find high-performance thermoelectric materials
for the low temperature applications.
