Unified Picture for the Colossal Thermopower Compound ${\mathrm{FeSb}}_2$

We identify the driving mechanism of the gigantic Seebeck coefficient in ${\mathrm{FeSb}}_2$ as the phonon-drag effect associated with an in-gap density of states that we demonstrate to derive from excess iron. We accurately model electronic and thermoelectric transport coefficients and explain the so far ill-understood correlation of maxima and inflection points in different response functions. Our scenario has far-reaching consequences for attempts to harvest the spectacular power factor of ${\mathrm{FeSb}}_2$.

Figure 1

Modeled transport properties of ${\mathrm{FeSb}}_2$ with one in-gap state compared to experiment [14]. (a) Density of states (left) and chemical potential $\mu$ as a function of temperature (right). (b) Electrical conductivity $\sigma$ on a logarithmic scale, consisting of valence (VB), conduction (CB), and in-gap band (B1) contributions. (c) Seebeck coefficient, (d) magnetoresistance (MR), (e) Hall coefficient $R_H$, and (f) Nernst coefficient. The (gray) vertical line indicates the temperature at which the chemical potential passes the in-gap state $E_1$.

Figure 2

Same as Fig. 1 but for three in-gap states: $E_i=11.4$, 19.8, 24.3 meV. The (gray) vertical lines are guides to the eye indicating maxima in the MR. For details of the fitting procedure see the text and the Supplemental Material [18].

Figure 3

Band structure of ${\mathrm{Fe}}_{25}{\mathrm{Sb}}_{47}$, mimicking ${\mathrm{Fe}}_{1+x}{\mathrm{Sb}}_{2-x}$. Hues of green indicate orbital admixtures originating from the extra iron atom. Inset: density of states of pure (black) and Fe-rich (green) ${\mathrm{FeSb}}_2$.