FeAs systems: a new class of high-temperature superconductors

This is the first systematic review of a new class of high-$T_c$ superconductors which includes iron-based layered compounds of the types $RE\mathrm{OFeAs}$ ($RE$: a rare earth element), $A\mathrm{Fe_2As_2}$ ($A$: Ba, Sr, Ca), LiFeAs, etc., all of which are antiferromagnetic metals when stoichiometric and become superconducting (with a $T_c$ maximum currently of 55 K) when doped with elements of different valence. The common structural element for all these compounds is layers of $\mathrm{FeAs_4}$ complexes. The electronic states near the Fermi level are formed by Fe 3d-states. As shown theoretically by LDA calculations and experimentally by ARPES, the electronic structure of a FeAs compound consists of a multisheet Fermi surface with 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 superconducting critical field, superconducting order parameter (OP), and $T_c$ as a function of the doping level. The fact that different measurements lead to different conclusions on the OP symmetry is discussed. The transport, magnetic and superconducting properties of FeAs systems are analyzed in comparison with those of the cuprates. The basic electronic models of the FeAs compounds are described, which use their electronic structure and take advantage of the antiferromagnetic ordering of doped compounds to explain the electron pairing. It is shown that unlike the cuprates the superconducting FeAs systems are weakly (or moderately) correlated materials far from the Mott–Hubbard transition. It is concluded that setting aside the symmetry of the superconducting order parameter, the physical properties of FeAs compounds are fairly well understood.