We present a review of basic experimental facts on the new class of high -temperature superconductors -iron based layered compounds like REOFeAs (RE=La,Ce,Nd,Pr,Sm...), AFe2As2 (A=Ba,Sr...), AFeAs (A=Li,...) and FeSe(Te). We discuss electronic structure, including the role of correlations, spectrum and role of collective excitations (phonons, spin waves), as well as the main models, describing possible types of magnetic ordering and Cooper pairing in these compounds 1 . ContentsConclusion: end of cuprate monopoly 38 What is in common between iron based and cuprate superconductors?38 What is different between iron based and cuprate superconductors? 39References 39• There exists a relatively well defined (at least in the part of the Brillouin zone) Fermi surface, in this sense these systems are metals.• However, appropriate stoichiometric compounds are antiferromagnetic insulators -superconductivity is realized close to the Mott metal -insulator phase transition (controlled by composition) induced by strong electronic correlations.• The strong anisotropy of all electronic properties is observed -conductivity (and superconductivity) is realized mainly within CuO 2 layers (quasi two-dimensionality!).At the same time, many things are still not understood. Accordingly, we may list What do we not really know about cuprates:• Mechanism of Cooper pairing (a "glue" leading to formation of Cooper pairs).Possible variants:1. Electron -phonon mechanism.2 Under pressure Tc of HgBa 2 Ca 2 Cu 3 O 8+δ reaches ∼ 150K. BASIC EXPERIMENTAL FACTS ON NEW SUPERCONDUCTORS Electrical properties and superconductivity REOF eAs (RE = La, Ce, P r, N d, Sm, ...) systemDiscovery of superconductivity with T c = 26K in LaO 1−x F x F eAs (x = 0.05 − 0.12) [15] was preceded by the studies of electrical properties of a number of oxypnictides like LaOM P n (M = M n, F e, Co, N i and P n = P, As) highlighted by discovery of superconductivity in LaOF eP with T c ∼ 5K [24] LaON iP T c ∼ 3K [25], which has not attracted much attention from HTSC community. This situation has changed sharply after Ref.[15] has appeared and shortly afterwards a lot of papers followed (see e.g. [26,27,28,29,30,31,32,33,34,35]), where this discovery was confirmed and substitution of lanthanum by a number of other rare -earths, according to a simple chemical formula (RE) +3 O −2 Fe +2 As −3 , has lead to more than doubling of T c up to the values of order of 55K in systems based upon N dOF eAs and SmOF eAs, with electron doping via addition of fluorine or creating oxygen deficit, or hole doping achieved by partial substitution of the rare -earth (e.g. La by Sr) [36]. Note also Ref. [37], where the record values of T c ∼ 55K were achieved by partial substitution of Gd in GdOF eAs by Th, which, according to the authors, also corresponds to electron doping. In these early works different measurements of electrical and thermodynamic properties were performed on polycrystalline samples.In Fig. 1 (a), taken from Ref.[33], we show typical temperature dependences of electric resistivi...

We generalize the dynamical-mean field (DMFT) approximation by including into the DMFT equations some length scale ξ via a momentum dependent "external" self-energy Σ k . This external self-energy describes non-local dynamical correlations induced by short-ranged collective SDW-

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We present a review of theoretical and experimental works on the problem of mutual interplay of Anderson localization and superconductivity in strongly disordered systems. Superconductivity exists close to the metalinsulator transition in some disordered systems such as amorphous metals, superconducting compounds disordered by fast neutron irradiation etc. Hightemperature superconductors are especially interesting from this point of view. Only bulk systems are considered in this review. The superconductor-insulator transition in purely two-dimensional disordered systems is not discussed.We start with brief discussion of modern aspects of localization theory including the basic concept of scaling, self-consistent theory and interaction effects. After that we analyze disorder effects on Cooper pairing and superconducting transition temperature as well as Ginzburg-Landau equations for superconductors which are close to the Anderson transition. A necessary generalization of usual theory of "dirty" superconductors is formulated which allows to analyze anomalies of the main superconducting properties close to disorder-induced metal-insulator transition. Under very rigid conditions superconductivity may persist even in the localized phase (Anderson insulator).Strong disordering leads to considerable reduction of superconducting transition temperature T c and to important anomalies in the behavior of the upper critical field H c2 . Fluctuation effects are also discussed. In the vicinity of Anderson transition inhomogeneous superconductivity appears due to statistical fluctuations of the local density of states.We briefly discuss a number of experiments demonstrating superconductivity close to the Anderson transition both in traditional and high-T c superconductors. In traditional systems superconductivity is in most cases destroyed before metal-insulator transition. In case of high-T c superconductors a number of anomalies show that superconductivity is apparently conserved in the localized phase before it is suppressed by strong enough disorder. The concept of electron localization 1 is basic for the understanding of electron properties of disordered systems 2,3 . In recent years a number of review papers had appeared, extensively discussing this problem [4][5][6][7] . According to this concept introduction of sufficiently strong disorder into a metallic system leads to spatial localization of electronic states near the Fermi level and thus to a transition to dielectric state (Anderson transition). After this transition dc conductivity (at zero temperature, T = 0) vanishes, despite the finite value of electronic density of states at the Fermi level (at least in one-electron approximation).At the same time it is well-known that even the smallest attraction of electrons close to the Fermi level leads to formation of Cooper pairs and the system becomes superconducting at sufficiently low temperatures 8,9 . It is known that the introduction of disorder which does not break the time-reversal invariance (normal, nonm...

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