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.

This volume presents an in-depth review of experimental and theoretical studies on the newly discovered Fe-based superconductors. Following the Introduction, which places iron-based superconductors in the context of other unconventional superconductors, the book is divided into three sections covering sample growth, experimental characterization, and theoretical understanding. To understand the complex structure-property relationships of these materials, results from a wide range of experimental techniques and theoretical approaches are described that probe the electronic and magnetic properties and offer insight into either itinerant or localized electronic states. The extensive reference lists provide a bridge to further reading. Iron-Based Superconductivity is essential reading for advanced undergraduate and graduate students as well as researchers active in the fields of condensed matter physics and materials science in general, particularly those with an interest in correlated metals, frustrated spin systems, superconductivity, and competing orders.

This NATO Advanced Research Workshop was held at a time when there was little consensus as to the mechanism for high temperature superconductivity, in the context of a world undergoing major changes in its political alignments and sense of the possibility for the future. It was characterized by generosity in the sharing of our uncertainties and speculations, as was appropriate for both the subject matter and the context. The workshop was organized, of necessity around the experimental work, as is this volume. Where the theoretical work is directly relevant to particular experiments, it is included in the appropriate sections with them. Most of the participants felt strongly that magnetic fluctuations played an important role in the mechanism for high T c, although with the exception of the IlS R work reported by Luke showing results inconsistent with the anyon picture, and the work on flux phases by Lederer, the mechanism remained an issue in the background. A major focus was the phenomenological interpretation of the NMR data.

From fundamental physics point of view, iron-based superconductors have properties that are more amenable to band structural calculations. This book reviews the progress made in this fascinating field. With contributions from leading experts, the book provides a guide to understanding materials, physical properties, and superconductivity mechanism aspects, and is important for students and beginners to have an overall view of the recent progress in this active field.

Presented within are neutron scattering studies detailing the spin dynamics of BaNi[subscript x]Fe2[subscript x]As2 for x = 0 (parent), 0.04 (underdoped), and 0.1 (optimal) dopings, and FeSe[subscript x]Te1[subscript x] for x = 0 (parent), 0.3 (underdoped), and 0.4 (optimal) dopings. These recently discovered Fe-based superconducting compounds are strikingly similar, in many respects, to the cuprate class of unconventional superconductors and share qualitatively similar phase diagrams consisting of a long range ordered magnetic ground state in the parents which, upon doping, is supplanted in favor of superconductivity. The dopings discussed herein allow us to tune through the phase diagram, beginning with long range ordered parents and ending with optimally doped superconductors with short range magnetic correlations. For BaFe2As2, the excitations in the ordered state are strongly damped and persist up to 300meV. Low energies excitations are centered around Q[subscript AMF] and disperse towards the zone boundary with increasing energy. Only scattering above 100meV is effected when warming above T[subscript N]. In underdoped x = 0.04 BaNi[subscript x]Fe2−[subscript x]As2, we find an order of magnitude reduction in the coupling between layers and a corresponding crossover from 3D to 2D magnetism. In coauthor work on optimal doped x = 0.1 BaNi[subscript x]Fe2−[subscript x]As2 we establish the existence of a 3D resonance mode in the superconducting state. Excitations at optimal doping above the resonance are very similar to the paramagnetic scattering observed in the parent and consists of diffuse scattering below 100meV while above this threshold the signal has similar dispersion, linewidths, and intensity as the ordered state. For FeTe, I discuss our existing efforts and data collection aimed at addressing issues associated with calculating the effective moment from Q, E-integrated data. Tuning through the phase diagram to the x = 0.3 underdoped FeSe[subscript x]Te1−[subscript x] system we find filamentary superconductivity with magnetic spectral weight sitting at both the AFM and nesting vector. Upon reaching x = 0.4 optimal doping, the scattering completely transfers over to the nesting vector and a 2D resonance mode appears below T[subscript c].