The discovery of high temperature superconductivity in iron pnictides and chalcogenides has resulted in surprising new insights into high temperature superconductivity and its relationship with magnetism. Here we provide an overview of some of what is known about these materials and in particular about the interplay of magnetism and superconductivity in them. Similarities and contrasts with cuprate superconductors are emphasized and the superconducting pairing is discussed within the framework

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... f spin uctuation induced pairing. PACS: 74.70.Xa, 74.20.Rp, 74.20.Pq Background and introduction The discovery of superconductivity in a family of iron based compounds that are in proximity to magnetism [1, 2] has led to renewed interest in the interplay of these two phenomena in metals and many interesting discoveries. Actually, the interplay of magnetism and superconductivity has a long history, starting with the Meissner eect and subsequently the Abrikosov ux lattice and type-II superconductivity. The development of the BCS theory of electronphonon superconductivity led to understanding of the trends in the critical temperatures of the transition elements in terms of their electronic densities of states, N (E F ), and their Debye temperatures. Essentially, the Debye temperature sets the energy scale, and the electronphonon interaction was proportional to N (E F ). This understanding led to the recognition of certain elements that did not t the trends, most notably, Pd, which is not superconducting, but does have a high density of states. This non-superconducting behavior was explained by Berk and Schrieer in terms of the nearness of Pd to ferromagnetism [3]. The nearness of Pd to ferromagnetism comes also from its high density of states, as understood within the Stoner theory. Thus, the Fermi surfaces of the transition elements show two competing instabilities, one towards electronphonon superconductivity, and the other towards ferromagnetism, both of which become enhanced as N (E F ) increases. Thus, ferromagnetism and superconductivity are competing instabilities. Clearly, the occurrence of these two phases nearby is a necessary condition for supposing that the superconductivity could be related to magnetism. However, as is clear from this historical example, such a proximity is by no means a sucient condition, and in fact the superconductivity of transition metal elements is caused by the electronphonon interaction. As discussed below, this is opposite to the case for the iron pnictides. These Fe-based materials show both antiferromagnetism and superconductivity in their phase diagrams, with competition [4, 5] . This at rst sight may seem rather like the cuprates, but the magnetic phases are actually very dierent between the two classes of materials, as will be discussed below. One similarly is that in neither material can the superconductivity be explained within standard electronphonon theory. This became apparent very quickly both from the fact that the phonon spectrum of LaFeAsO did not show the high phonon frequencies that would be needed to explain a high T c [6], and from direct calculations that showed in addition that the electronphonon coupling is weak [7] .