New Fe - based Superconductors The Iron Age of High - Temperature Superconductivity

Root of a Carbon nanotube on the sub-strate with elongated catalyst particle Rolled up AlGaAs/Cr/Pt microtube Corrosion pit generated on a shot-peened Zr-based bulk metallic glass Lattice structure of a typical edge-shared chain cuprate: Li 2 CuO 2 In February 2008, a paper appeared showing superconductivity in a layered iron arse-nide material with a transition temperature (T c) of 26K[1]. This discovery has triggered an enormous amount of research activities all over the world. Meanwhile there are more than 500 papers dealing with this new class of high temperature superconductors. At first glance the story looks very similar to that of the cuprates: There are non-superconduct-ing antiferromagnetic parent compounds, as e.g. LaOFeAs, the parent compound of the so-called '1111' structure. The systems become superconducting upon doping, for example by replacing some of the oxygen in LaOFeAs by fluorine. Also the crystal structures of the new superconductors are somehow reminiscent of the cuprates, with layers of FeAs separated by spacer layers, such as LaO, where the dopants are introduced (Fig.). However , looking in more detail on the physical properties reveals pronounced differences between pnictides and cuprates. For example, the parent compounds of the pnictides are not Mott insulators but 'bad metals' with a spin density wave state [2,3]. Moreover, comparing experimental data to DFT calculations suggests that electron correlations are less important in the pnictides. On the other hand, there are good arguments, that the superconductivity in the pnictides can not be explained within the frame of a conventional electron phonon scenario. Meanwhile there is a whole family of pnictide superconductors. The highest critical temperatures are obtained by substituting La with smaller rare earths, at present the maximum T c amounts to about 54 K in the Sm based 1111 compound [4]. A simpler class of materials based on the BaFe 2 As 2 parent compound (the '122' structure, Fig. right) that does not have the LaO spacer layers also superconducts with a comparable T c [5]. More recently, superconductivity has been discovered in even simpler LiFeAs '111' and FeSe materials [6,7]. The race is therefore on, just as for the cuprates, to discover materials with increasingly higher T c. At the IFW, research on the pnictides superconductors started in April 2008 and meanwhile there are more than 20 papers on this topic dealing with (i) the synthesis of bulk materials and thin films, (ii) experimental studies of superconductivity and …