Charge Dynamics in the Metallic and Superconducting States of the Electron-Doped 122-Type Iron Arsenides

Understanding charge dynamics and the origin of superconductivity in iron-based materials is one of the most important topics in condensed matter physics. Among different structures of iron-based materials, 122-type iron arsenides are of considerable interest due to their diverse phase diagrams, relatively high superconducting transition temperatures, and the availability of high quality single crystals. In this dissertation, we study temperature and frequency dependence of charge dynamics of the electron-doped 122-type iron arsenides in the metallic and superconducting states using broadband infrared spectroscopy at cryogenic temperatures. We have investigated the charge dynamics and the nature of many-body interactions in metallic La- and Pr- doped CaFe2As2. From the infrared part of the optical conductivity, we discover that the scattering rate of mobile carriers above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic transport. However, the dc resistivity continues to increase with temperature above 200 K due to the loss of Drude spectral weight. The loss of Drude spectral weight with increasing temperature is seen in a wide temperature range in the uncollapsed tetragonal phase, and this spectral weight is recovered at energy scales about one order of magnitude larger than the Fermi energy scale in these semimetals. The phenomena noted above have been observed previously in other correlated metals in which the dominant interactions are electronic in origin. Further evidence of significant electron-electron interactions is obtained from the presence of quadratic temperature and frequency-dependence scattering rate at low temperatures and frequencies in the uncollapsed tetragonal structures of La- and Pr-doped CaFe2As2. We also observe weakening of electronic correlations and a decrease of Drude spectral weight upon the transition to the collapsed tetragonal phase in Pr-doped CaFe2As2. We have measured infrared reflectance spectra of BaFe1.9Pt0.1As2 in the normal and superconducting states. We find that this superconductor has fully gapped Fermi surfaces. Importantly, we observe strong-coupling electron-boson interaction features in the infrared absorption spectra. By using two modeling methods which include strong-coupling effects via the Eliashberg function, we obtain a good quantitative description of the energy gaps and the temperature dependent strong-coupling features. Our experimental data and analysis provide compelling evidence that superconductivity in BaFe1.9Pt0.1As2 is induced by the coupling of electrons to a low energy bosonic mode