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Superconductivity

Superconductivity

Surprise return

01 Sep 2008

High-temperature superconductors are centre stage once again

Surprise return

Every so often a new scientific discovery is made that catches researchers by surprise. That is exactly what happened earlier this year when scientists in Japan reported a new high-temperature superconductor containing a layer of iron and arsenic sandwiched between layers of lanthanum and oxygen. The material, known as an “iron oxypnictide”, was found to carry electric current without resistance when cooled below a transition temperature (Tc) of about 26 K. By tweaking the new superconductor’s composition, other researchers had, within weeks, boosted the Tc of the oxypnictides to as high as 55 K (see “Rebirth of the hot”).

Over 100 papers have so far been written about these materials. One reason for this interest is that we now know that the cuprates, discovered in 1986, are no longer the only type of high-temperature superconductor. And if there are two classes of such materials, there may well be others. Moreover, the behaviour of the oxypnictides could shed much-needed light on why the cuprates superconduct — a riddle that has left theorists stumped.

Researchers are also fascinated because the doping, composition and structure of these new materials can be almost endlessly modified. With suitable alchemical tinkering, their Tc could possibly be boosted to above the all-important temperature of liquid nitrogen (77 K). That could allow such materials to be used in commercial applications such as lossless electricity transmission lines — provided they can be fashioned cheaply into wires, that is. More fundamentally, researchers are mystified as to why the presence of iron, which is magnetic, does not destroy the superconductivity of the oxypnictides; magnetic fields are usually the death knell for supercurrents.

Cynics will say that we have been here before. There was, after all, similar excitement after the discovery in 2001 that magnesium diboride (MgB2) could superconduct, yet interest faded fast after its Tc stalled at about 39 K. Moreover, the oxypnictides, which contain arsenic, can be dangerous. But iron oxypnictides are different from MgB2: they have a higher Tc, their behaviour is more mysterious, and their chemical structure can be varied. The challenge now is for experimentalists to grow good-quality single crystals of the oxypnictides, so that their physical properties can be measured more accurately, while theorists should try to explain why the materials superconduct. It is highly unlikely that the oxypnictides will ever superconduct at room temperature, but many more surprises are sure to lie in store.

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