The discovery of vanishing electrical resistance in iron arsenides has stimulated a worldwide interest in this class of materials. Since some of the multinary arsenide phases exhibit surprisingly high transition temperatures of up to 55 K, these pnictides clearly belong to the group of high-temperature superconductors. Even more important for applications are the exceptionally high and rather isotropic critical magnetic fields. However, despite extensive investigations of this effect in other materials within the last two decades, the understanding of superconductivity is still incomplete so that the underlying mechanisms are still a topic of basic research.

As a characteristic feature, the crystal structures of these new materials comprise layers that are formed from iron and arsenic atoms. In a subset of these compounds, a symmetry breaking transition is observed at temperatures below typically 200 K. The distortion is associated with a special type of magnetic ordering – the so called columnar ordering – where the iron moments order in an antiparallel fashion along the a-axis and in a parallel fashion along the b-axis which becomes approximately one percent longer (see Figure 4). The microscopic origin of this special type of magnetic order is a subject of intense ongoing discussions.

Fig. 4: Projection of the crystal structure of superconducting SrFe2As2 along the crystallographic c axis. Shown is a slightly corrugated layer (decorated 44 net) formed by arsenic (blue) and iron atoms (orange). The orientation of the magnetic moments in the ordered phase is indicated schematically by arrows.


By changing the chemical composition, the onset of this ordering can be shifted in the direction of lower temperatures and can even be completely suppressed. In parallel to the disappearance of the magnetic order, an onset of superconductivity is observed with transition temperatures Tc up to 38 K. A disadvantage of chemical substitution is that the exchange interaction of the atoms is not only influenced by changing the interatomic distances, but also by differences in the electronic structure of the compounds due to a modified electron count. To study the effect of the atomic spatial separation in pure form, we performed angle-dispersive X-ray diffraction measurement at high pressures and low temperatures at beamline ID09A. By combining the results of this structural investigation with laboratory measurements of the electrical resistivity, we could characterise the interrelation between magnetic order and atomic arrangement in a rather extended part of the parameter field (see Figure 5).

Fig. 5: Stability fields of high (green) and broken symmetry (shaded grey) of superconducting SrFe2As2. The investigations show perfect agreement between results obtained by electrical resistivity measurements (T0 extracted from ) and the structural distortion (T0 obtained from X-ray diffraction experiments, XRD). Transition temperatures extracted from are indicated by brown crosses or black dots; open blue squares symbolise diffraction diagrams that are compatible with tetragonal symmetry, open red triangles stand for peak splitting, an indication of orthorhombic symmetry, and filled orange diamonds correspond to interpolated transition temperatures from XRD. Upper inset: Suppression of the lattice distortion as evidenced by the experimental observation of vanishing splitting of X-ray diffraction profiles at elevated pressures. Lower inset: Resistivity as a function of temperature at different pressures. The discontinuous changes indicate that the phase transition is shifted towards lower temperatures at higher pressures.


The onset of the magnetic ordering yields a pronounced change of the temperature dependence of the electrical resistivity. With increasing pressure, the transition shifts in the direction of lower temperatures. The related change of symmetry is directly observed by following the splitting of diffraction lines in the powder diffraction diagrams (see upper inset in Figure 5). In full agreement with the results of the resistivity measurements, the structural transition is shifted in the direction of higher pressures at lower temperatures (see lower inset of Figure 5). Density functional electronic structure calculations demonstrate that the structural and the magnetic phase transition in this class of materials are intimately linked. With increasing pressure, the interatomic overlap increases, the electronic bands broaden and, in consequence, the density of states at the Fermi level, responsible for the magnetic instability, decreases. Thus, the experimental results are in very good agreement with our theoretical prediction that the transition temperature decreases strongly with pressure.

The results clearly show that the new class of compounds differs fundamentally from the copper- and oxygen-containing high-temperature superconductors which have been studied intensely over the last twenty years. In these materials, the magnetic ordering temperature increases with pressure while the new materials show a decrease and are, thus, similar to classical superconducting intermetallic compounds. Compared to the classic intermetallic supercondutors, the significantly higher transition temperatures and the increased stability in high magnetic fields qualify the new materials as very promising candidates for scientific and technical applications.


Principal publication and authors

M. Kumar (a), M. Nicklas (a), A. Jesche (a), N. Caroca-Canales (a), M. Schmitt (a), M. Hanfland (b), D. Kasinathan (a), U. Schwarz (a), H. Rosner (a), C. Geibel (a), Physical Review B 78, 184516 (2008).
(a) MPI für Chemische Physik fester Stoffe, Dresden (Germany)
(b) ESRF