Charge Dynamics in 122 Iron-Based Superconductors

This thesis combines highly accurate optical spectroscopy data on the recently discovered iron-based high-temperature superconductors with an incisive theoretical analysis. Three outstanding results are reported: (1) The superconductivity-induced modification of the far-infrared conductivity of an iron arsenide with minimal chemical disorder is quantitatively described by means of a strong-coupling theory for spin fluctuation mediated Cooper pairing. The formalism developed in this thesis also describes prior spectroscopic data on more disordered compounds. (2) The same materials exhibit a sharp superconductivity-induced anomaly for photon energies around 2.5 eV, two orders of magnitude larger than the superconducting energy gap. The author provides a qualitative interpretation of this unprecedented observation, which is based on the multiband nature of the superconducting state. (3) The thesis also develops a comprehensive description of a superconducting, yet optically transparent iron chalcogenide compound. The author shows that this highly unusual behavior can be explained as a result of the nanoscopic coexistence of insulating and superconducting phases, and he uses a combination of two complementary experimental methods - scanning near-field optical microscopy and low-energy muon spin rotation - to directly image the phase coexistence and quantitatively determine the phase composition. These data have important implications for the interpretation of data from other experimental probes.

The author obtained his diploma in theoretical physics from the Belarusian State University in 2007 with highest honors. He then attended a Master's course in experimental physics at the University of Stuttgart, specializing in Bose-Einstein condensation in dilute gases. Upon successful completion of this course of study in May 2009 he commenced his doctoral research in the solid-state spectroscopy group of Bernhard Keimer at the Max Planck Institute for Solid-State Research in Stuttgart. In December 2012 he graduated from the University of Stuttgart with highest honors.