Scientists researching superconductors at the U.S. Department of Energy’s Argonne National Laboratory (ANL) have discovered a previously unknown phase in a class of superconductors called iron arsenides. The discovery offers new information on a debate over the interactions between atoms and electrons that are responsible for their unusual superconductivity.

Ray Osborn, an Argonne physicist and coauthor on the paper “Magnetically Driven Suppression of Nematic Order in an Iron-Based Superconductor,” published in Nature Communications said, “This new magnetic phase, which has never been observed before, could have significant implications for our understanding of unconventional superconductivity.”

Neutron Diffraction Image.  Click image for more info.

Neutron Diffraction Image. Click image for more info.  Click here for the ANL graphics page.

Scientists and engineers are dedicated to researching superconductors because they are capable of carrying electric current without any resistance that loses power to heat. Not having resistance is unique among all conductors: even the good ones like copper wiring used in most generators, motors and power lines and cords that lose energy along the way.

The goal is likely a superconductor discovery that can operate without cooling at least 50º C or 122º F, something over “room temperature of “ambient” with 80º or 90º a top of the line goal.

So far the biggest drawback is 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, the temperature at which they work could be raised and their power harnessed for a wide range of new technologies.

Here’s the background. The theory behind the 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. (Whereas in a plain old conductor such as copper or aluminum, these electrons bounce off the atoms, producing heat).

In “unconventional” superconductors, the electrons still form pairs, but its not understood what binds them together.

Also, superconductors are notoriously 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 sodium is added to the mix, the magnetism is suppressed and the materials eventually become superconducting below roughly -400º F.

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 the ANL team came up with a new 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.

“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 suggests 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. Thus, perhaps what binds the pairs of electrons together in iron arsenide superconductors is magnetism.

Osborn said, “Orbital theories do not predict a return to four-fold symmetry at this point, 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.”

Osborn points out the new information could also affect our understanding of superconductivity in other types of superconductors, such as the copper oxides, where nematic distortions have also been seen.

While seeming to be a rather obscure point for now the potential of superconductors used at temps nearing that of boiling water would have an immense impact from generating power, transporting electricity and to economy of use by consumers.

Useable superconductors would be an energy revolution across the entire electrical field.


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