Superconductivity in Iron Compounds G. R. Stewart Department of Physics, University of Florida, Gainesville, FL 32611-8440 Abstract: Kamihara and coworkers' report of superconductivity at 26 K in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled 'conventional' superconductors. Clearly superconductivity and magnetism/magnetic fluctuations are intimately related in the FePn/Ch - and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With Tc values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review attempts to provide an overview, using numerous references, with a focus on the materials and their superconductivity. The discovery of superconductivity at 26 K in LaFeAsO doped with F in 2008 was not the first discovery of an iron-containing superconductor, nor even the first reported superconducting iron pnictide (LaFePO, Tc ≈ 5 K, Kamihara et al., 2006). Although iron has been considered deleterious to superconductivity due to its strong local magnetic moment, a number of superconducting compounds containing iron in which the iron is non-magnetic have long been known. However, the discovery of Kamihara et al. is ground breaking for a number of reasons. One is that – just like the discovery of superconductivity at 35 K in Ba-doped La2CuO4 (Bednorz and Müller 1986) – it led to the almost immediate further discovery of even higher Tc materials, with the current record Tc ≈56 K observed in Gd0.8Th0.2FeAsO (C. Wang et al., 2008), Sr0.5Sm0.5FeAsF (G. Wu et al., 2009) and in Ca0.4Nd0.6FeAsF (Cheng et al., 2009). The path to this higher transition temperature was also similar to that in the high Tc cuprates, where pressure experiments (Chu etSuperconductivity in Iron Compounds G. R. Stewart Department of Physics, University of Florida, Gainesville, FL 32611-8440 Abstract: Kamihara and coworkers' report of superconductivity at 26 K in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled 'conventional' superconductors. Clearly superconductivity and magnetism/magnetic fluctuations are intimately related in the FePn/Ch - and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With Tc values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review attempts to provide an overview, using numerous references, with a focus on the materials and their superconductivity. The discovery of superconductivity at 26 K in LaFeAsO doped with F in 2008 was not the first discovery of an iron-containing superconductor, nor even the first reported superconducting iron pnictide (LaFePO, Tc ≈ 5 K, Kamihara et al., 2006). Although iron has been considered deleterious to superconductivity due to its strong local magnetic moment, a number of superconducting compounds containing iron in which the iron is non-magnetic have long been known. However, the discovery of Kamihara et al. is ground breaking for a number of reasons. One is that – just like the discovery of superconductivity at 35 K in Ba-doped La2CuO4 (Bednorz and Müller 1986) – it led to the almost immediate further discovery of even higher Tc materials, with the current record Tc ≈56 K observed in Gd0.8Th0.2FeAsO (C. Wang et al., 2008), Sr0.5Sm0.5FeAsF (G. Wu et al., 2009) and in Ca0.4Nd0.6FeAsF (Cheng et al., 2009). The path to this higher transition temperature was also similar to that in the high Tc cuprates, where pressure experiments (Chu et