Table of contents

Basic experimental data are presented for a new class of high-temperature superconductors — iron-based layered compounds of the types REOFeAs (RE = La, Ce, Nd, Pr, Sm, ...), AFe₂As₂ (A = Ba, Sr, ...), AFeAs (A = Li, ...), and FeSe(Te). The structure of electronic spectra in these compounds is discussed, including the correlation effects, as is the spectrum and role of collective excitations (phonons and spin waves). Basic models for describing various types of magnetic ordering and Cooper pairing are reviewed.

The discovery in February 2008 of superconductivity with the transition temperature about 26 K in fluorine-doped oxyarsenide LaO_1–xF_xFeAs stimulated numerous studies of superconducting and other physical properties of this and related materials, resulting in a new family of high-temperature (T_c ∼ 26–55 K) superconductors.The experimental and theoretical state of the art is reviewed concerning the synthesis, properties and simulation of the new family and related systems.

This is the first systematic review of a new class of high-T_c superconductors which includes iron-based layered compounds such as REOFeAs (RE is a rare-earth element), AFe₂As₂ (A= Ba, Sr, Ca), and LiFeAs, all of which are antiferromagnetic normal metals while being stoichiometric and becoming superconducting (with the current maximum T_c given by 55 K) when doped with an element of a different valence. The common structural element of all these compounds is layers formed by FeAs₄ complexes. Electron states near the Fermi level are formed by Fe 3d states. As was shown theoretically by LDA calculations and experimentally by ARPES, the electronic structure of all compounds of the FeAs class is similar; their Fermi surface is multi-sheeted, consisting of two hole pockets at the center and two electron pockets at the corners of the Brillouin zone. In this paper, the superconducting properties of such systems are reviewed in detail, including the dependence of T_c on the doping level, external pressure, superconducting critical field, and superconducting order parameter. The controversy over the order parameter symmetry determined from different measurements is discussed. The transport, magnetic, and superconducting properties of FeAs systems are analyzed in comparison with those of cuprates. Basic electronic models of FeAs compounds, with their electronic structure and the proximity of the state of doped compounds to the antiferromagnetic ordering taken into account, are described to explain the specific features of electron pairing in them. It is shown that unlike the cuprates, superconducting FeAs systems are weakly (or moderately) correlated materials that are far from the Mott – Hubbard transition. A conclusion is made that the physical properties of FeAs compounds have mainly been well understood, except for the symmetry of the superconducting order parameter.

In the late 1950s and early 1960s several American scientists recognized the importance of results appearing in Russian language journals. Their efforts, aided by 'Cold War' considerations, culminated in the launch of the AIP program of translations into English of the then Soviet Union's leading physics journals. The present brief review gives a personal perspective on the history of that development.

A special session of the Editorial Board of Physics – Uspekhi (its oral issue) celebrating the 90th anniversary of the journal and the 50th anniversary of its English version took place on November 19, 2008 in the conference hall of the P N Lebedev Physical Institute (FIAN) of the Russian Academy of Sciences. The following reports were presented at the session:

(1) Ginzburg V L (P N Lebedev Physical Institute, RAS, Moscow), Aksent'eva M S (Uspekhi Fizicheskikh Nauk, RAS, Moscow) "On the history of UFN (introductory talk)"; (2) Dremin I M (P N Lebedev Physical Institute, RAS, Moscow) "The physics of the Large Hadron Collider"; (3) Shirkov D V (Joint Institute for Nuclear Research, Dubna, Moscow region) "Pair correlations and spontaneous symmetry breaking"; (4) Smirnov B M (Institute for High Temperatures, RAS, Moscow) "Modeling of gas-discharge plasma"; (5) Sadovskii M V (Institute of Electrophysics, RAS Ural Branch, Ekaterinburg) "High-temperature superconductivity in iron-based layered compounds"; (6) Aleksandrov E B (All-Russian Research Center 'S I Vavilov State Optical Institute', St.-Petersburg) "Physical limits in the metrology of a magnetic field by atomic spectroscopy techniques"; (7) Maksimov E G (P N Lebedev Physical Institute, RAS, Moscow) "Microscopic studies of the nature of the ferroelectric transition"; (8) Fortov V E (Institute for High Energy Density, RAS, Moscow) "Extreme states of matter".

We plan to publish reports Nos 1 – 4 and 6 – 8 in one of the 2009 issues of Physics – Uspekhi, while report No. 5 is published in the present issue for reasons specified in the Editor-in-Chiefs foreword to this article.