${\text{FeSb}}_2$: Prototype of huge electron-diffusion thermoelectricity

Huge negative values of both the thermopower, $S=-10\text{mV}/\text{K}$ at 10 K, and the Nernst coefficient, $\nu =-550\mu \text{V}/\text{K}\text{T}$ at 7.5 K, are observed for ${\text{FeSb}}_2$, a narrow-gap semiconductor showing strong indications of correlation effects. A semiquantitative interpretation based on electron diffusion is presented, which suggests that the dominant term in $S\left(T\right)$, although electronic in origin, is enhanced by a factor larger than 10. This is ascribed to the presence of electron-electron correlations in ${\text{FeSb}}_2$ and excludes phonon drag as the dominating mechanism for the large thermopower. The correlation-enhanced thermoelectricity is of great interest for theoretical exploration as well as potential application in cooling devices at cryogenic temperatures.

Figure 1

(Color online) Magnetic susceptibility $\chi \left(T\right)$ and the thermoelectric power factor $\text{PF}\left(T\right)$ (inset) of ${\text{FeSb}}_2$ and ${\text{RuSb}}_2$ (Refs. 7, 8).

Figure 2

(Color online) Arrhenius plots of $\rho$ and $\left|R_H\right|$ for ${\text{FeSb}}_2$. Fitting $\rho \left(T\right)$ in the linear regimes, 7–20 and 50–200 K, to the thermal activation function $\rho ={\rho }_0\text{exp}\left(E_g/2T\right)$, one obtains two energy gaps $E_{g1}=52\left(\pm 8\right)\text{K}$ and $E_{g2}=350\left(\pm 30\right)\text{K}$, respectively. In the range of 10–20 K, $\left|R_H\left(T\right)\right|$ is also thermally activated, showing a similarly small gap of 54 K. Upper inset displays $\rho$ vs $\text{log}T$. Lower inset shows $T$-dependent Hall mobility ${\mu }_H$.

Figure 3

(Color online) Absolute values of (a) the thermopower $S\left(T\right)$ and (b) the Nernst coefficient $\nu \left(T\right)$. $\left|S\left(T\right)\right|$ is qualitatively described by curves (1) and (2) based on Eqs. (1, 2) with $m^{\ast }=m_0$ (see text). By applying a factor of 30 (respectively, 15) to these curves, the measured data are well described. Inset shows a plot of $\left|S\right|$ vs $1/T$. $\left|\nu \left(T\right)\right|$ is measured in $B=0.5$ and 2 T, and is compared to the calculated curve $S\text{tan}{\theta }_H/B$ (see text).

Figure 4

(Color online) Thermal conductivity $\kappa$ and mean-free path of phonons, $l_p$, and electrons, $l_e$. $l_p$ was estimated by the kinetic formula ${\kappa }_L=\frac{1}{3}{C}_L{\upsilon }_s{l}_p$ with sound velocity ${\upsilon }_s=3116{\text{ms}}^{-1}$ (Ref. 7), where ${\kappa }_L$ is the lattice contribution to thermal conductivity, practically identical to $\kappa$ as measured (see text), and $C_L$ is the lattice contribution to the specific heat (Ref. 2). $l_e$ was determined within the Drude theory, $l_e=m^{\ast }{\upsilon }_e/\rho n{e}^2$, with ${\upsilon }_e=\sqrt{3k_{B}T/m^{\ast }}$ being the thermal velocity of the conduction electrons and $m^{\ast }=m_0$.