Electronic thermoelectric power factor and metal-insulator transition in FeSb2

We show that synthesis-induced Metal -Insulator transition (MIT) for electronic transport along the orthorombic c axis of FeSb${}_2$ single crystals has greatly enhanced electrical conductivity while keeping the thermopower at a relatively high level. By this means, the thermoelectric power factor is enhanced to a new record high S${}^2$$\sigma$ $\sim$ 8000 $\mu$WK${}^{-2}$cm${}^{-1}$ at 28 K. We find that the large thermopower in FeSb${}_2$ can be rationalized within the correlated electron model with two bands having large quasiparaticle disparity, whereas MIT is induced by subtle structural differences. The results in this work testify that correlated electrons can produce extreme power factor values.