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Saturday, January 10, 2009

scientists prove unconventional superconductivity in new iron arsenide compounds


A new superconductor could iron out theoretical wrinkles in high-temperature superconductivity, which here produces magnetic levitation with liquid nitrogen.

Materials engineer Duck Young Chung examines a sample of superconducting crystals before characterizing them in a X-ray diffractometer. The crystals signify the next generation of experiments utilizing neutron scattering to determine unconventional superconductivity.
Scientists at U.S. Department of Energy's Argonne National Laboratory used inelastic neutron scattering to show that superconductivity in a new family of iron arsenide superconductors cannot be explained by conventional theories.

"The normal techniques for revealing unconventional superconductivity don't work with these compounds," physicist Ray Osborn said. "Inelastic neutron scattering is so far the only technique that does."

Conventional superconductivity can be explained by a theory developed by Bardeen, Cooper and Schrieffer (BCS) in 1957. In BCS theory, electrons in a superconductor combine to form pairs, called Cooper pairs, which are able to move through the crystal lattice without resistance when an electric voltage is applied. Even when the voltage is removed, the current continues to flow indefinitely, the most remarkable property of superconductivity, and one that explains the keen interest in their technological potential.

Normally, electrons repel each other because of their similar charge, but, in superconductors, they coordinate with vibrations of the crystal lattice to overcome this repulsion. But scientists don't believe the vibrational mechanism in the iron arsenides is strong enough to make them superconducting. This has led theorists to propose that this superconductivity has an unconventional mechanism, perhaps like high-temperature copper-oxide superconductors. Some iron arsenides are antiferromagnetic, rather than superconducting, so magnetism rather than atomic vibrations might provide the electron glue.

In BCS superconductors, the energy gap between the superconducting and normal electronic states is constant, but in unconventional superconductors the gap varies with the direction the electrons are moving. In some directions, the gap may be zero. The puzzle is that the gap does not seem to vary with direction in the iron arsenides. Theorists have argued that, while the size of the gap shows no directional dependence in these new compounds, the sign of the gap is opposite for different electronic states. The standard techniques to measure the gap, such as photoemission, are not sensitive to this change in sign.

But inelastic neutron scattering is sensitive. Osborn, along with Argonne physicist Stephan Rosenkranz, led an international collaboration to perform neutron experiments using samples of the new compounds made in Argonne's Materials Science Division, and discovered a magnetic excitation in the superconducting state that can only exist if the energy gap changes sign from one electron orbital to another.

"Our results suggest that the mechanism that makes electrons pair together could be provided by antiferromagnetic fluctuations rather than lattice vibrations," Rosenkranz said. "It certainly gives direct evidence that the superconductivity is unconventional."

Inelastic neutron scattering continues to be an important tool in identifying unconventional superconductivity, not only in the iron arsenides, but also in new families of superconductors that may be discovered in the future.

A paper ("Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering") on Osborn's and Rosenkranz's work has been published in volume 456, pages 930-932, of Nature. Funding for this research was provided by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

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Superconductivity is key to conserving energy
Dr. Fatih Dogan, a professor of materials science and engineering at the University of Missouri-Rolla, is working with superconducting materials that might eventually revolutionize the way energy is conserved.
Dogan is an author of a new article about the possible mechanisms of superconductivity at high temperatures. The paper was published this week by Nature Physics.

Superconductivity is a phenomenon that occurs in some materials at temperatures hundreds of degrees below zero. The phenomenon is characterized by exactly zero electrical resistance. In ordinary conductors the amount of resistance never reaches zero.

Normal conductors like copper generate heat, causing a certain amount of the energy transported through copper wires to be lost. For the same reason, a lot of energy is wasted in the processes of burning coal and oil. Superconducting materials don’t produce heat and are therefore much more energy efficient.

Fifty years ago, Nobel Prize-winning scientists explained the superconductivity of materials at low temperatures. But for the materials to be useful in the transportation of electricity, for example, they would have to be superconductive at much higher temperatures.

“Ideally, we’re talking room temperatures or higher,” Dogan says. “If we understand the mechanisms of high-temperature superconductivity, we could discover new materials that could be superconducting. Computers would work extremely fast without heating up and power lines could transport electricity on thin lines without losing energy.”

Dogan is working with a mixture containing versions of four elements: yttrium, barium, copper and oxygen. In a UMR lab, high-quality crystals of the mixture are grown. The crystals are used by physicists around the world for neutron scattering measurements.

“The periodic table has billions of possibilities,” Dogan says. “You have to have a good idea about what might work before you start.”

Dogan says physicists and other scientists around the world have been working on the superconductivity problem for a long time. Some of them have turned to Dogan, because he has developed a reputation for being able to grow large crystals of the complex elemental mixture that is believed to have unique qualities conducive to superconductivity at high temperatures.

Powder from the four elements is heated, melted, and then allowed to cool in a disc shape about the size of a silver dollar. The trick to getting the material in the disc to form as a single high-quality crystal, according to Dogan, is to place a seed crystal that melts at higher temperatures in the center of the mixture. Under precisely controlled conditions during the cooling process, the seed crystal colonizes the surrounding material.

Conventional superconductivity can be explained by a theory developed by Bardeen, Cooper, and Schrieffer (BCS) in 1957. In BCS theory, electrons in a superconductor combine to form pairs, called Cooper pairs, which are able to move through the crystal lattice without resistance when an electric voltage is applied. Even when the voltage is removed, the current continues to flow indefinitely, the most remarkable property of superconductivity, and one that explains the keen interest in their technological potential.

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