Unchanged thermopower enhancement at the semiconductor-metal transition in correlated FeSb${}_{2-x}$Te${}_x$

Substitution of Sb in FeSb${}_2$ by less than 0.5% of Te induces a transition from a correlated semiconductor to an unconventional metal with large effective charge carrier mass $m^{\ast }$. Spanning the entire range of the semiconductor-metal crossover, we observed an almost constant enhancement of the measured thermopower compared to that estimated by the classical theory of electron diffusion. Using the latter for a quantitative description one has to employ an enhancement factor of 10-30. Our observations point to the importance of electron-electron correlations in the thermal transport of FeSb${}_2$, and suggest a route to design thermoelectric materials for cryogenic applications.