Charge Dynamics in 122 Iron-Based Superconductors
Springer Science & Business Media, Sep 12, 2013 - Technology & Engineering - 130 pages
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.
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American Physical Society analysis antiferromagnetic antiferromagnetic phase ARPES Ba1−x Kx band BCS theory BKFA blue Brillouin zone charge carriers Charnukha Cooper pairs cuprate dielectric function Drude Drude model Eliashberg theory ellipsometry experimental far-infrared Fermi level Fermi surface figure with permission high-temperature high-temperature superconductivity in-plane interaction interband transitions iron arsenide iron pnictides iron-based materials iron-based superconductors itinerant Kx Fe2 As2 lattice layer low-energy measurements metallic microscopic multiband muon muon spin near-field normal observed obtained optical conductivity optical properties optical response orbital order parameter pairing paramagnetic permission from Ref phase diagram phase separation Photon Photon energy Physical Review Letters plasma frequency polarization quasiparticle s-SNOM sample surface scattering rate shown in Fig single crystals spectral range spectral weight spectroscopy spin fluctuations super supercon superconducting gap superconducting phase superconducting transition temperature superconductivity-induced symmetry techniques tetragonal theory of superconductivity unit cell ε1 ω