Highly Dispersive Electron Relaxation and Colossal Thermoelectricity in the Correlated Semiconductor FeSb${}_2$

We show that the colossal thermoelectric power, $S(T)$, observed in the correlated semiconductor FeSb${}_2$ below 30,K is accompanied by a huge Nernst coefficient $\nu (T)$ and magnetoresistance MR$(T)$. Markedly, the latter two quantities are enhanced in a strikingly similar manner. While in the same temperature range, $S(T)$ of the reference compound FeAs${}_2$, which has a seven-times larger energy gap, amounts to nearly half of that of FeSb${}_2$, its $\nu (T)$ and MR$(T)$ are intrinsically different to FeSb${}_2$: they are smaller by two orders of magnitude and have no common features. With the charge transport of FeAs${}_2$ successfully captured by the density functional theory, we emphasize a significantly dispersive electron-relaxation time $\tau ({ϵ}_k)$ due to electron-electron correlations to be at the heart of the peculiar thermoelectricity and magnetoresistance of FeSb${}_2$.