Charge Dynamics in 122 Iron-Based Superconductors, Softcover reprint of the original 1st ed. 2014 Springer Theses Series

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.

Introduction.- Iron-based superconductors.- Experimental and theoretical methods.- Results and discussion.- Summary.

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.