Published online 28 January 2009 | Nature | doi:10.1038/news.2009.64

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Superconducting in 3-D

Newly discovered superconductor family begins to reveal its charms.

Levitating neodymium–iron–boron magnet and a YBa2Cu3O7 superconductor.Superconductors have come a long way since 1986.Nature

About a year ago, Japanese researchers discovered a new family of 'high-temperature' superconductors1 — the first in more than 20 years — and excitement quickly led to hope: could the iron arsenide compounds, known as pnictides, be the breakthrough that had eluded the superconducting community? Theoretical explanations for high-temperature superconductivity have been complicated by recent discoveries2. But in practical terms, the pnictides are looking better by the day.

By tweaking the recipe for the pnictides, researchers have boosted the critical temperature below which they superconduct, or conduct electricity without resistance, to 56 kelvin3 — high enough to qualify as 'high-temperature', but well short of the holy grail of a room temperature superconductor. Yet temperature was never the strong suit of pnictides. Research published in Nature4,5 shows that they have other, distinct advantages over the previous high-temperature family to be discovered, the copper-based superconductors known as cuprates.

The pnictides, unlike the cuprates, seem to superconduct with little regard for their crystal orientation, and they also seem to have the capacity to carry more current and generate bigger magnetic fields. These characteristics could make them more suitable for building the magnets used in accelerators at the frontiers of high-energy physics, and in nuclear magnetic resonance techniques that molecular biologists use to characterize proteins.

"I'm not saying we have a next generation of magnetic wire on hand," says Paul Canfield, a physicist at the Ames National Laboratory in Iowa of the recent work on pnictides. "But it bodes very well for the types of [magnetic] fields you may be able to create with these types of materials."

Any which way

After high-temperature superconductor cuprates were discovered in 1986, a decade of work pushed their critical temperature up to 138 kelvin. But theorists remained mystified by the mechanism for the behaviour, which was different from that of conventional metallic superconductors with much lower critical temperatures. Many thought it had to do with the cuprates' two-dimensional structure. At first, the pnictides looked similar, and so the assumption was "that there must be some very similar physics going on in there", says John Singleton, of Los Alamos National Laboratory in New Mexico and an author of one of the new papers4.

The research — showing that the pnictides can superconduct almost equally well in any direction — suggests that they have none of the flat, two-dimensional limitations of the cuprates. The exact mechanism is still unknown, but experimentalists in search of new high-temperature superconductors no longer need to limit themselves to layered structures.

"It is broadening the search," says Canfield, who published similar research6 in 2008. The omni-directional behaviour also has a practical implication. Great pains had been taken, in fashioning wires and tapes from the cuprates, to align grains along the preferred superconducting direction. With the pnictides, there would be no such hassle. "It makes it much easier to put into a manufacturable form," says Harold Weinstock, a programme manager for superconducting research at the Air Force Office of Scientific Research in Arlington, Virginia.

Magnetic charm

Although superconductors are judged mainly by their critical temperature, this parameter has become less important and less costly as methods of cryogenic cooling have improved. Two other fundamental superconductor parameters — the ability to carry vast amounts of electrical current, and to generate big magnetic fields — can be more important, especially if the superconductor is to be used for making very strong magnets. And the pnictides could outperform the cuprates here, too. "It really looks as if they are better," says David Larbalestier, director of the Applied Superconductivity Center at the National High Magnetic Field Laboratory at Florida State University in Tallahassee.

One possible use would be in nuclear magnetic resonance (NMR) machines — the same technology that, using lower strength magnets, produces detailed medical images of the human body. State-of-the-art NMR machines, used by molecular biologists to characterize proteins, use magnets with field strengths of about 20 tesla — but higher strength magnets would offer increased sensitivity and resolution.

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A second arena is in high-energy physics, where superconducting magnets are used to accelerate particles. A muon collider, in particular, could use the high-strength magnets promised by the pnictides, because the short-lived particles would have to be accelerated quickly within a compact ring — rather than more slowly over larger rings, as for the longer-lived protons at the Large Hadron Collider at CERN, the European high-energy physics laboratory near Geneva, Switzerland, which uses magnets that will operate at around 8 tesla.

Although he finds the pnictides to be both "exciting" and "hopeful", Weinstock isn't convinced that they will suddenly find their way into all these applications. It has taken 20 years for materials scientists to engineer tapes and wires from the cuprates, and even now, the products are expensive, and difficult to wind into magnets, he says. "I don't envision that the pnictides will suddenly catch up to the [cuprates] in the next year or two." 

  • References

    1. Kamihara, Y., Watanabe, T., Hirano, M. & Hosono, H. J. Am. Chem. Soc. 130, 3296–3297 (2008). | Article | PubMed | ChemPort |
    2. Zaanen, J. Nature 457, 546–547 (2009). | Article |
    3. Grant, P. M. Nature 453, 1000–1001 (2008). | Article | PubMed | ChemPort |
    4. Yuan, H. Q. et al. Nature 457, 565–568 (2009). | Article |
    5. Zabolotnyy, V. B. et al. Nature 457, 569–572 (2009). | Article |
    6. Altarawneh, M. M. et al. Phys. Rev. B. 78, 220505 (2008). | Article | ChemPort |
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