Scientists discover new magnetic phase in iron-based superconductors

May 23, 2014 by Louise Lerner
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: Jared Allred / Argonne National Laboratory.

(Phys.org) —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.

Scientists and engineers are fascinated with 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 in other types of superconductors, such as the copper oxides, where nematic distortions have also been seen, Osborn said.

Explore further: Insights into the stages of high-temperature superconductivity

More information: "Magnetically driven suppression of nematic order in an iron-based superconductor." S. Avci, et al. Nature Communications 5, Article number: 3845 DOI: 10.1038/ncomms4845. Received 10 March 2013 Accepted 08 April 2014 Published 22 May 2014

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Macksb
5 / 5 (1) May 23, 2014
This is consistent with a theory that I have discussed in many prior Physorg posts--a theory based on Art Winfree's law of coupled oscillators, which he proposed circa 1967. All three of the diagrams at the top, each showing four iron atoms (blue) and four red arrows suggesting relationships, are four oscillator systems. They exhibit three different Winfree patterns of coupling or interconnection (mathematical patterns) that can arise in four oscillator systems.

The lowest diagram happens to have a pattern of interconnection that becomes precise, robust and universal in this particular material below a certain temperature. The "glue" is simply Winfree's mathematical law of coupled oscillators. Periodic oscillations include both orbit and spin. I agree that spins (which, when coupled, give rise to varied forms of magnetic order) are implicated here.

Please see my other Physorg posts for background on Winfree's law and many instances of how it can be applied to physics.
George_Rajna
May 24, 2014
This comment has been removed by a moderator.
Macksb
not rated yet May 25, 2014
You may ask: What is so special about the order in the lowest diagram of the four iron atoms? My answer is that the order in the lowest diagram obeys not just one but two of the Winfree patterns for a four oscillator system.

First, the phases of atoms 1 and 3, as to their magnetic moments, are exactly anti synchronous, as are the phases of 2 and 4. Half out of phase to the diagonal partner. Perfectly opposed. Zero, 180 degrees. (Think horse, trotting.)

Second, the phases of all 4 atoms, as to their magnetic moments, are one quarter out of phase with their two nearest neighbors. 90 degrees. (Think elephant, walking.)

Thus, the composite picture of the four iron atoms and their four magnetic moment oscillations, in Winfree terms, is a four part system the oscillations of which are perfectly and precisely coupled twice over. Robust and amazingly precise structure, even as to the arrangement of the iron atoms. Cooper pairs, superconductivity result.

Macksb
not rated yet May 25, 2014
The net result is perfect overall synchrony of all quantum oscillations of the electrons, and perfect spacing and other positioning of the four iron atoms (in the lowest of the three similar diagrams). Locked in by Winfree's law. No irregularities to disrupt the flow of current. No rocks in the stream. No friction.
mreda14
not rated yet May 26, 2014
iron arsenides is unusual compound. It is composed of Iron with two oxidation state ( +2 and +3) and Arsenic. Arsenic compounds resemble in some respects those of phosphorus which occupies the same group (column) of the periodic table. Arsenic is less commonly observed in the pentavalent state, however. The most common oxidation states for arsenic are: −3 in the arsenides, such as alloy-like intermetallic compounds; and +3 in the arsenites, arsenates(III), and most organoarsenic compounds. This compound require serious study. This compound is definitely a mixed valence.It shows a singularity ( similar to 1/x as x approaches zero) where x is the electronic resistance
Macksb
not rated yet May 27, 2014
The three diagrams above, of blue spheres and red "connections," prove the overarching thesis that I have asserted in perhaps 100 Physorg posts over 4 years: Art Winfree's law of coupled oscillators underlies all of physics.

Matter, their phases, and their phase transitions, from mundane ( e.g., solid to liquid) to exotic (old and new superconductivity, both types of superfluidity, "Bosenovas" and all others. The periodic table itself. All of these are driven by Winfree's law, because quantum physics is periodic oscillations. 47 years ago Winfree showed that periodic oscillations tend to self-organize, and when they do, certain patterns--only certain patterns-- emerge. And he specified the various patterns, including the patterns in the three diagrams above to which I refer. His law is mathematical and universal. He was not a physicist.