Unusual 'collapsing' iron superconductor sets record for its class
13 Feb 2012
A team from the University of Maryland and the National Institute of Standards and Technology (NIST) has found an iron-based superconductor that operates at the highest known temperature for a material in its class. The discovery inches iron-based superconductors closer to being useful in many practical applications.
Iron-based superconductors, discovered only about four years ago, are a hot research topic, in part because they are more amenable to commercial applications than copper-based superconductors, which are more difficult to make and are frequently brittle.
Of the four broad classes of iron-based superconductors, the 1:2:2 class-so named because their crystals are built around a hub of one atom of calcium, two of iron and two of arsenic-is particularly promising because these superconductors' properties can be custom-tailored by substituting other atoms for these basic elements.
Magnets made with low-temperature superconductors have already found use in hospital MRI machines, but less expensive MRI machines and other applications, such as superconducting cables for resistance-free power transmission over long distances, become closer to reality when manufacturers have more choices among superconductors.
Working at University of Maryland's Center for Nanophysics and Advanced Materials and the NIST Center for Neutron Research (NCNR), the team, led by University of Maryland physics professor Johnpierre Paglione, found that a particular type of 1:2:2 superconductor possesses some unexpected properties. Of perhaps greatest value to manufacturers is that its threshold temperature of superconductivity is 47 degrees Kelvin, the highest yet for the 1:2:2 class, whose previous record was 38K.
But the crystal also has a highly curious property: it can superconduct at this record temperature when a smaller atom is substituted for the crystal's original calcium in some of its hubs, adding extra electrons that the researchers believe is the key to stabilising superconductivity.
"It's amazing that these crystals become superconductors when we replace only a small fraction of the calcium with rare earth atoms." says Paglione. "We had a hunch this might happen, but not at such high temperatures as compared to other materials."
When this substitution is performed with the right elements, another intriguing transition occurs: the overall crystal instantaneously shrinks by about 10 per cent, a dramatic size change that is literally called a "structural collapse."
UMD researcher Shanta Saha, the lead scientist on the project, says that such a large change in crystal structure would normally be catastrophic for technological applications.
But the group's research has determined how to make the substitution while eluding the collapsed state altogether, resulting in another record - this time in the material's unprecedented thermal expansion property - that may prove extremely useful for technologies utilising both superconductivity and thermal expansion applications.
