Discovery of colossal Seebeck effect in metallic CuSe.

Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor Sσ is generally limited to less than 0.01 W m K. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, CuSe, shows two sign reversals and colossal values of S exceeding ±2 mV K in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm leads to a colossal value of Sσ = 2.3 W m K. The small thermal conductivity less than 2 W m K results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.