Ingredients for enhanced thermoelectric power at cryo-temperatures in the correlated CoSbS semiconductor revealed by its optical response
We address the optical response of the novel thermoelectric material CoSbS at low temperatures. The semiconducting CoSbS is well known for its thermoelectric performance at high temperatures but it also attracts great interest because of its colossal low-temperature thermopower. It provides an arena for the study of the additional interplay between electronic correlations, which cannot be neglected at cryogenic temperatures, and the thermopower factor as well as the thermal conductivity.
Our work presents measurements of the optical reflectivity of CoSbS, collected from the far-infrared to the ultraviolet at nearly normal incidence as a function of temperature. This is the prerequisite in order to perform reliable Kramers-Kronig transformation of the measured quantity, giving access to all optical functions (Fig. 1.18). High-resolution scanning transmission electron microscopy investigations complement our experimental study.
The optical properties of CoSbS at low T signal the important role of coherent phonons and heavy carriers, both being an asset for the enhancement of the thermopower. This mixture of ingredients from lattice dynamics and electronic band structure turns out to be essential in order to enhance the thermopower at cryo-temperatures. The binding aspect for such a mixture in the correlated CoSbS semiconductor is given by the presence of in-gap impurity states, originating from interstitial Sb, which seem to play quite a decisive role (Fig. 1.19). Our data therefore proposes the selective tuning of impurity states as an affordable strategy towards the enhancement of the thermoelectric power.
