  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.

Strongly Correlated Electrons