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.
Nominated as an outstanding Ph.D. thesis by the Max Planck Institute for Solid State Research, GermanyApplies multiple independent experimental probes to reveal several common trends in the charge dynamics of 122 iron-based superconductorsProvides a universal description of the optical conductivity of 122 iron pnictides in the framework of the Eliashberg theory of superconductivityPresents the discovery of the nanoscale layering of superconducting and antiferromagnetic phases in 245 iron selenides