High-temperature superconductors could revolutionize the use of electricity.
Superconducting materials, compounds that conduct electricity without resistance, were discovered almost a century ago and are today used in applications ranging from medical equipment to power cables. Scientists are still unsure exactly how the most useful examples of these high-temperature superconductors work. If they can ferret out the details of high-temperature superconductivity, the thinking goes, they should be able to design new kinds of superconducting materials. Since superconductivity could provide major gains in energy efficiency, being able to tailor the new materials for ease of use could be a gamechanging technological achievement.
If perfected, high-temperature superconductors could revolutionize the generation, storage, distribution and use of electricity. Currently, a substantial part of all electricity generated is lost to resistance, either in the power grid before getting to users or in the powering of machinery. The potential energy savings from generators, transformers, power cables and motors equipped with superconducting components would make a substantial contribution to addressing America's energy challenge.
The instruments at the Spallation Neutron Source provide a powerful boost to superconductivity research. Their value resides in the capability to reveal the precise position and motions of the atoms inside materials, as well as the magnetic moments associated with these atoms. The Wide-Angular Range Chopper Spectrometer (ARCS) is especially valuable for investigating exotic compounds such as superconductors because SNS's high neutron flux enables the use of small samples, says ORNL researcher Andy Christianson, one of several scientists using ARCS to probe iron-based superconductors. The instrument's huge bank of more than 900 detectors enables coverage of a huge range of neutron scattering angles, from –25° to 133° horizontally and from –28° to 27° vertically. Christianson characterizes the ARCS as providing "a broad view of a wide range of momentum and energy transfer for the excitation spectrum."
Christianson, Mark Lumsden and Takeshi Egami are members of a team that recently used ARCS to obtain the first-ever neutron scattering measurements of magnetic excitations in single crystals of an iron-based superconductor. The ARCS data showed a large spike in magnetic excitations, or spin fluctuations, in a sample of barium-iron-cobalt-arsenic (FBCA) just as the temperature reached 22K and the material became superconducting.
The ARCS results advance the understanding of "unconventional" superconductivity, the type researchers consider to have the most technological potential. In "conventional" superconductors discovered in 1911, vibrations within the atomic lattices of materials compel electrons to form pairs that move through the lattice without resistance. The materials typically acquire superconductivity only at temperatures close to absolute zero, thus limiting their potential uses. In the 1980s, physicists discovered superconductivity in copper oxide alloys, or cuprates, which lose their resistance to electricity at more easily attainable temperatures as high as 138 K. Two decades of research suggest that something other than lattice vibrations causes superconductivity in cuprates, leading to their label as "unconventional superconductors." New revelations emerged in 2008 when Japanese scientists announced the observation of unconventional superconductivity in a class of iron compounds.
The ARCS findings, along with followon data from other scattering experiments by Christianson, Lumsden and their colleagues, bolster the opinion of many researchers that spin fluctuations play an important role in unconventional superconductivity. The neutron scattering results provide additional evidence that magnetic excitations are key to superconducting behavior in both cuprates and iron-based superconductors.
Christianson says the precise role of magnetism in superconductivity has been an interesting question. "We don't think lattice vibrations by themselves are sufficient to explain electron pairing. Some other mechanism must also be involved. An obvious question is how much magnetism is influencing superconductivity."
The FBCA sample used in the experiment, synthesized in Oak Ridge, comprised three single crystals, each about 7 mm long and 1 mm thick. Because the material is so difficult to synthesize, three crystals were required to form a sample large enough for inelastic neutron scattering investigation, about 1.8 grams. Without the high neutron intensity available at SNS, such a tiny sample could not have been measured. For the experiments, researchers placed the crystals inside a sample canister that could be cooled with liquid helium to within a few degrees of absolute zero.
To produce the sample, a parent compound, barium-iron-arsenic, was doped with cobalt. At low temperatures, the parent compound has long-range or static magnetic order, that is, a regular pattern of spins throughout a sample. When the parent compound is doped with cobalt, the static magnetic order disappears. The change appears to occur when the cobalt "squeezes" the atomic lattice, causing the spins of the electrons to fluctuate. The same sort of effect is produced by applying pressure on the parent compound.
Superconductivity and long-range magnetic order appear to be mutually exclusive—as one appears, the other subsides. Thus doping, by destroying the magnetic order, opens the way for superconductivity to surface when the FBCA sample falls to the critical temperature of 22 K.
Christianson believes the precise relationship between static magnetic order and superconductivity remains an open question. "One explanation could be competing interactions. When pressure is applied,one is tuned over the other, enhancing the reactions that give rise to superconductivity." He concedes that researchers do not yet know if destroying the long-range magnetic order makes superconductivity happen or enables it to happen.
The magnetic interactions involving superconducting FBCA are two-dimensional, taking place only within each plane, unlike electrons in the parent compound that interact both within and between planes. Two-dimensionality is also observed in the cuprates. The energy of the magnetic excitation measured with ARCS, 8.6 meV, is related to the superconducting transition temperature in a similar way as observed in the cuprates, providing further evidence that the physics underlying superconductivity in the two classes of materials is related.
Theorists will now try to calculate what the experiments have measured, and the ARCS results will allow them to build more reliable models, Christianson notes. "Any theory proposing that spin excitations are the mechanism behind superconductivity must, at a minimum, get the spin excitations right. Theorists have not fully characterized the excitations, but we now know about them in a limited but substantial way.
As researchers pore over the ARCS data, their interpretations may lead in different theoretical directions. Egami notes that for unconventional superconductivity, "there are as many theories as there are physicists. Everyone involved knows the research on iron-based superconductors is tremendously significant because it involves not just superconductivity but also the beginning of 21st century physics, which deals with many-body physics and electron correlations," he adds. "The field resembles quantum mechanics at the beginning of the 20th century."
Egami's take from the ARCS results is that just as lattice vibrations are not sufficient to explain unconventional superconductivity, neither are spin fluctuations. "I think both spins and phonon vibrations are involved," he says. "There is an inherent coupling between spin and lattice. Change the lattice and the spin disappears. That effect in this system is profound."
A key advantage of studying the ironbased compounds is that they provide a simpler model for unconventional superconductivity than the cuprates, Christianson says. "We hope that by observing superconductivity in a simpler model we may gain greater understanding."
The hope is that in time researchers will understand the superconductivity mechanism in the iron-based materials, an understanding that will translate back to the cuprates. The goal is the ability to design superconducting materials that operate at higher temperatures.
Egami says the ARCS results lead to the hypothesis that the same mechanism causes superconductivity in both the cuprates and the iron-based materials. "That simplifies the search for the mechanism—we're not chasing dozens of different possibilities. There are signs everywhere, but no one has yet put them all together. The progress using the ironbased compounds is much faster than with the cuprates—probably 10 times faster.
"It's a wonderful coincidence that this compound came along just as SNS was available to study it," Egami notes. "Everyone thought it was impossible for iron to be superconducting. Now that superconductivity has been found in iron compounds, it could be in anything."
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