Exploring instabilities of bad metals with optical spectroscopy

This thesis is focused on the optical investigation of materials which exhibit novel electronic and magnetic properties, such as the iron-pnictide high-TC superconductor Co-doped BaFe2As2 and the proposed topological Kondo insulator SmB6. The (low temperature) groundstate of these two materials is completely different, but at high temperature they are in some sense similar: both materials feature a strongly temperature and frequency dependent optical response that has become known as a “bad metal”. In “clean” metals one would expect a so-called Drude response for the optical conductivity at low energy. The Drude response can be characterized by a frequency and temperature independent scattering rate; an average time between collisions that does not depend on temperature, indicating that the electrons behave as a gas of freely moving particles. Instead, bad metals feature a strong temperature and frequency dependent scattering rate indicative of residual interactions between the electrons. In such materials, a careful analysis of the optical response not only provides fingerprints of the interactions responsible for the bad metal behavior, but also of the key driving forces behind the instabilities that lead to the ordered electronic states at low temperature