Linking Superconductivity and Structure

Superconductivity is a rare physical state in which matter is able to conduct electricity - maintain a flow of electrons - without any resistance. It can only be found in certain materials, and even then it can only be achieved at very low temperatures and often only at high pressure. New research from Carnegie’s Elissaios Stavrou, Alexander Goncharov, and Xiao-Jia Chen, along with scientists from SUNY Stony Brook and The University of Science and Technology of China, hones in on the structural changes underlying superconductivity in iron arsenide compounds.

Although superconductivity has many practical applications for electronics (including scientific research instruments), medical engineering (MRI machines), and potential future applications including high-performance power transmission and storage, and very fast train travel, the difficulty of creating superconducting materials prevents it from being used to its full potential. Any newly discovered superconducting is therefore of great interest to both scientists and engineers.

Iron arsenides are relatively recently discovered superconductors. The nature of superconductivity in these particular materials remains a challenge for modern solid state physics. If the complex links between superconductivity, structure, and magnetism in these materials are unlocked, then iron arsenides could potentially be used to reveal superconductivity at much higher temperatures than previously seen, which would vastly increase the ease of practical applications.  It is currently know that, when iron arsenide (FeAs) is combined with a metal, such as in the sodium-containing NaFe2As2 studied here, the resulting  compound crystallizes in a tetragonal structure (Fig.1 ). But until now, a detailed knowledge of the atomic positions involved and how they change under pressure had not been determined.

The layering of arsenic and iron (As-Fe-As) in this structure is believed to be the key to the compound’s superconductivity. Under pressure, however, this structure is thought to transform to the so-called “collapsed” tetragonal lattice, which is no longer capable of superconductivity, or has a diminished superconducting ability.

Stavrou and co-workters combined experimental evidence with computational modeling to document the structural changes at the atomic level during the tetragonal to collapsed tetragonal transformation. This is just the first step toward definitively determining the link between structure and superconductivity, which could potentially make higher-temperature superconductivity a real possibility.

The results show that adt about 4 GPa, NaFe2As2 takes on the collapsed tetragonal structure. This changes the angles in the arsenic-iron-arsenic layers and is coincident with the loss in superconductivity.  Moreover, it is found that this transition is accompanied by a major change in bonding coordination in the formation of the interlayer arsenic-arsenic bonds. A direct consequence of this new coordination is that the system loses its two-dimensionality, and with it, superconductivity.

These findings are an important step in identifying the hypothesized connection between structure and superconductivity in iron-containing compounds. Understanding the loss of superconductivity on an atomic level could enhance the ease of manufacturing such compounds for practical applications, as well as improving understanding of the phenomenon of superconductivity generally [E. Stavrou et al.Sci. Reports 5, 9868 (2015)].