Scientists at the U.S. Department of Energy’s Argonne National Laboratory have discovered a previously unknown phase in a class of superconductors called iron arsenides. This sheds 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.
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 (about -300°F), but four-fold symmetry is restored below 40 K (-387°F). 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. Credit: Image by Jared Allred / Argonne National Laboratory
Scientists and engineers are fascinated with superconductors because they are capable of carrying electric current without any resistance. This is unique among all conductors: even good ones, like the copper wires used in most power cords, lose energy along the way. Why don’t we use superconductors for every power line in the country, then? Their biggest drawback is that they 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. 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 a plain old conductor, these electrons bounce off the atoms, producing heat). In “unconventional” superconductors, the electrons still form pairs, but we don’t know what binds them together.
Superconductors are notably finicky; in order to get to the superconducting phase—where electricity flows freely—they need a lot of coddling. The iron arsenides the researchers studied are normally magnetic, but as you add sodium to the mix, the magnetism is suppressed and the materials eventually become superconducting below roughly -400 degrees Fahrenheit.
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.” It was thought that this nematic order persists until the material becomes superconducting—until this result.
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).
“It is visible using neutron powder diffraction, which is exquisitely sensitive, but which you can only perform at this resolution in a very few places in the world,” Osborn said. Neutron powder diffraction reveals both the locations of the atoms and the directions of their microscopic magnetic moments.
The reason why the discovery of the new phase is interesting is that it may help to resolve a long-standing debate about the origin of nematic order. Theorists have been arguing whether it is caused by magnetism or by orbital ordering.
The orbital explanation posits that electrons like to sit in particular d orbitals, driving the lattice into the nematic phase. Magnetic models, on the other hand (developed by study co-authors Ilya Eremin and Andrey Chubukov at the Institut für Theoretische Physik in Germany and the University of Wisconsin-Madison, respectively) 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,” he said, “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, Osborn said.
The paper, titled “Magnetically driven suppression of nematic order in an iron-based superconductor,” was published today in Nature Communications.
Other coauthors on the paper were Argonne scientists Sevda Avci (now at Afyon Kocatepe University in Turkey), Omar Chmaissem (a joint appointment with Northern Illinois University), Jared M. Allred, Stephan Rosenkranz, Daniel Bugaris, Duck Young Chung, John-Paul Castellan, John Schlueter, Helmut Claus and Mercouri Kanatzidis (a joint appointment with Northwestern University); and Dmitry Khalyavin, Pascal Manuel and Aziz Daoud-Aladine at the ISIS Pulsed Neutron and Muon Source at the Rutherford Appleton Laboratory in Oxfordshire, U.K.
Funding for the research was provided by the U.S. Department of Energy’s Office of Science. The neutron powder diffraction was performed at ISIS.
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
Jared Sagoff | Eurek Alert!
Another Milestone in Hybrid Artificial Photosynthesis
31.08.2015 | Lawrence Berkeley National Laboratory
New high energy density automotive battery system from Fraunhofer IISB and international partners
25.08.2015 | Fraunhofer-Gesellschaft
China's Loess Plateau was formed by wind alternately depositing dust or removing dust over the last 2.6 million years, according to a new report from University of Arizona geoscientists. The study is the first to explain how the steep-fronted plateau formed.
China's Loess Plateau was formed by wind alternately depositing dust or removing dust over the last 2.6 million years, according to a new report from...
The leaves of the lotus flower, and other natural surfaces that repel water and dirt, have been the model for many types of engineered liquid-repelling surfaces. As slippery as these surfaces are, however, tiny water droplets still stick to them. Now, Penn State researchers have developed nano/micro-textured, highly slippery surfaces able to outperform these naturally inspired coatings, particularly when the water is a vapor or tiny droplets.
Enhancing the mobility of liquid droplets on rough surfaces could improve condensation heat transfer for power-plant heat exchangers, create more efficient...
Longer, more severe, and hotter droughts and a myriad of other threats, including diseases and more extensive and severe wildfires, are threatening to transform some of the world's temperate forests, a new study published in Science has found. Without informed management, some forests could convert to shrublands or grasslands within the coming decades.
"While we have been trying to manage for resilience of 20th century conditions, we realize now that we must prepare for transformations and attempt to ease...
A University of Oklahoma astrophysicist and his Chinese collaborator have found two supermassive black holes in Markarian 231, the nearest quasar to Earth, using observations from NASA's Hubble Space Telescope.
The discovery of two supermassive black holes--one larger one and a second, smaller one--are evidence of a binary black hole and suggests that supermassive...
A team of European researchers have developed a model to simulate the impact of tsunamis generated by earthquakes and applied it to the Eastern Mediterranean. The results show how tsunami waves could hit and inundate coastal areas in southern Italy and Greece. The study is published today (27 August) in Ocean Science, an open access journal of the European Geosciences Union (EGU).
Though not as frequent as in the Pacific and Indian oceans, tsunamis also occur in the Mediterranean, mainly due to earthquakes generated when the African...
20.08.2015 | Event News
20.08.2015 | Event News
19.08.2015 | Event News
02.09.2015 | Physics and Astronomy
02.09.2015 | Life Sciences
02.09.2015 | Awards Funding