Unusual electronic and vibrational properties in the colossal thermopower material FeSb$_2$

TL;DR

FeSb$_2$ exhibits unique electronic and vibrational properties, including anisotropic optical conductivity, one-dimensional semiconducting behavior, and narrow phonon modes, which collectively contribute to its exceptional thermoelectric performance at low temperatures.

Contribution

This study provides new insights into the electronic and vibrational mechanisms behind FeSb$_2$'s high thermopower, highlighting the roles of anisotropic optical responses and phonon dynamics.

Findings

Anisotropic optical conductivity indicating a metal-insulator transition.

Observation of one-dimensional semiconducting behavior along the b axis.

Presence of narrow infrared-active phonon modes and additional lattice modes.

Abstract

The iron antimonide FeSb$_2$ possesses an extraordinarily high thermoelectric power factor at low temperature, making it a leading candidate for cryogenic thermoelectric cooling devices. However, the origin of this unusual behavior is controversial, having been variously attributed to electronic correlations as well as the phonon-drag effect. The optical properties of a material provide information on both the electronic and vibrational properties. The optical conductivity reveals an anisotropic response at room temperature; the low-frequency optical conductivity decreases rapidly with temperature, signalling a metal-insulator transition. One-dimensional semiconducting behavior is observed along the $b$ axis at low temperature, in agreement with first-principle calculations. The infrared-active lattice vibrations are also symmetric and extremely narrow, indicating long phonon relaxation times and a lack of electron-phonon coupling. Surprisingly, there are more lattice modes along the $a$ axis than are predicted from group theory; several of these modes undergo significant changes below about 100 K, hinting at a weak structural distortion or phase transition. While the extremely narrow phonon line shapes favor the phonon-drag effect, the one-dimensional behavior of this system at low temperature may also contribute to the extraordinarily high thermopower observed in this material.