FeAs systems: a new class of high-temperature superconductors

Abstract

This is the first systematic review of a new class of high-Tc 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 Tc 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 Tc 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.