New magnetic phase discovered in iron-based superconductors

A neutron diffraction image giving evidence for the new magnetic phase

A neutron diffraction image giving evidence for the new magnetic phase in iron-based superconductors discovered by Argonne scientists. It shows the scattering results from a sample of barium iron arsenide with sodium ions added to 24% of the barium sites. Nematic order sets in below 90 K but four-fold symmetry is restored below 40 K. The resulting atomic and magnetic structures are illustrated in the figure on the right, in which the blue spheres represent iron atoms and the red arrows show the direction of their magnetic moments. Image by Jared Allred.
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Scientists from the U.S. Department of Energy’s Argonne National Laboratory have discovered a previously unknown phase in a class of superconductors called iron arsenides using ISIS. Their findings shed light on a debate over the interactions between atoms and electrons that are responsible for their unusual superconductivity.

“This new magnetic phase, which has never been observed before, could have significant implications for our understanding of unconventional superconductivity,” said Ray Osborn, an Argonne physicist and coauthor on the paper.

Understanding superconductors is important because they are capable of carrying electric current without any resistance. This is unique among all conductors: even good ones, like the copper used in domestic appliances, lose energy from resistive heating. However current superconductors must be cooled to very, very cold temperatures to work.

Also, we do not fully understand how the newest types, called unconventional superconductors, work. Researchers hope that by figuring out the theory behind these superconductors, we could raise the temperature at which they work and harness their power for a wide range of new technologies.

The theory behind older, “conventional” superconductors is fairly well understood. However newer “unconventional” superconductors which have the potential to work at higher temperatures are less well understood. In conventional superconductors pairs of electrons, which normally repel each other, instead bind together by distorting the atoms around them and help each other travel through the metal. In “unconventional” superconductors, the electrons still form pairs, but it isn’t clear what binds them together.

The iron arsenides the researchers studied are normally magnetically ordered, but as you add sodium to the mix, the magnetism is suppressed and the materials eventually become superconducting below roughly 33K. Magnetic order also affects the atomic structure. At room temperature, the iron atoms sit on a square lattice, which has four-fold symmetry, but when cooled below the magnetic transition temperature, they distort to form a rectangular lattice, with only two-fold symmetry. This is sometimes called “nematic order”, a term borrowed from liquid crystals, whose molecules line up parallel to each other. It was thought that this nematic order persists until the material becomes superconducting, but this result may change that.

The Argonne team discovered a phase where the material returns to four-fold symmetry, rather than two-fold, close to the onset of superconductivity. (See diagram).

Pascal Manuel is the instrument scientist on WISH at ISIS, where some of the data was collected, and took part in the study. He says, “Neutron powder diffraction is an extremely sensitive tool for determining both nuclear and magnetic structures. This experiment was performed by using a combination of WISH and HRPD at ISIS, one of the few places in the world where this type of work is possible. The higher resolution of HRPD enabled us to clearly see tiny structural distortions going from four-fold to two-fold and then back to four-fold symmetry on cooling the sample and the high flux at long wavelengths on WISH to observe the associated changes in the magnetic structure, which are small because the magnetic moments are small.”

The orbital explanation posits that electrons preferentially sit in particular d orbitals, driving the lattice into the nematic phase. Magnetic models, on the other hand, developed by co-authors Ilya Eremin and Andrey Chubukov, suggest that magnetic interactions are what drive the two-fold symmetry—and that they are the key to the superconductivity itself. Perhaps what binds the pairs of electrons together in iron arsenide superconductors is magnetism.

“Orbital theories do not predict a return to four-fold symmetry at this point,” Osborn said, “but magnetic models do. So far, this effect has only been observed experimentally in these sodium-doped compounds, but we believe it provides evidence for a magnetic explanation of nematic order in the iron arsenides in general. It could also affect our understanding of superconductivity in other types of superconductors, such as the copper oxides, where nematic distortions have also been seen.”

The paper, titled “Magnetically driven suppression of nematic order in an iron-based superconductor,” has just been published in Nature Communications.

Sara Fletcher

Research date: May 2014

Further Information

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

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