New high-temperature iron-based superconductors (or iron-pnictides), principally comprising iron-arsenide, iron-phosphor, and iron-selenide alloys, were discovered in 2008. In a move to understand the origins of their behavior, an international group of researchers from the Paul Scherrer Institute (PSI), Harwell Science and Innovation Campus, Chinese Academy of Sciences, the University of Tennessee, and the Leibniz Institute for Solid State and Materials Research has now found that magnetic interactions are of fundamental importance in their high-temperature superconductivity.

As reported in the February 12 issue of Nature Communications (DOI: 10.1038/ncomms2428), the researchers compared a sample of a superconducting material with a sample of the parent material, which is non-superconducting. The base material—in this case a barium-iron-arsenide compound—becomes superconducting when it is doped with a defined quantity of potassium atoms.

The research team was particularly interested in the dynamic magnetic properties of the base materials and superconductors. In order to investigate these properties, they excited magnetic fluctuations in the material samples, where these are accompanied by a reorientation of the neighboring electron spins, which extends in a wavelike manner through the sample. In the base material, spin waves are clearly detectable.

The researchers wanted to know if this was also true for the doped material samples. At first sight, one might suspect that the holes would constitute obstacles, breaking the long-range magnetic order of spins and thus strongly attenuating the spin waves. However, the research team found that the spin waves experienced little attentuation in the superconductor, and that they exhibited almost the same intensity as in the base material.

In the experiment, the dynamic magnetic properties of the base material and the superconductor were examined using resonant inelastic x-ray scattering. In this spectroscopic method, the material being investigated is irradiated with x-rays, which excite spin waves in the sample—and thereby lose energy. “By comparing the energies of the incident and the outgoing light, one can deduce information on the properties of the spin waves, ” said PSI postdoc Kejin Zhou, who conducted these measurements.

It is generally accepted that superconductivity arises through “Cooper Pairs” of two electrons “glued” together. In a high-temperature superconductor, magnetic interactions could potentially be responsible for binding the electron pairs. “Spin waves are the hottest candidates for this,” said Thorsten Schmitt, leader of PSI’s Spectroscopy of Novel Materials Group.