Though a year has passed since the discovery of a new family of high-temperature
superconductors, a viable explanation for the iron-based materials' unusual
properties remains elusive. But a team of scientists working at the National
Institute of Standards and Technology (NIST) may be close to the answer.
NIST research shows that magnetism plays a key role in iron pnictide superconductors’ crystal structure. (Iron is purple; arsenic is yellow; calcium is green.) Only if the iron’s magnetism is taken into account do calculations of the distance between these crystal layers match up with lab measurements. Magnetism’s importance to their physical properties make it a likely factor in the iron pnictides’ ability to superconduct, say team members. Credit: Yildirim, NIST
The team has found strong evidence that magnetism is a pivotal factor governing
the physical properties of iron pnictides, a group of materials that conduct
electricity without resistance at temperatures of up to 56 kelvin (-217 degrees
celsius). Iron pnictides are composed of regularly spaced layers of iron sandwiched
between other substances. And although -217 might sound pretty cold, they are
the first class of materials found to superconduct at that high a temperature
since the discovery of copper-based superconductors more than two decades ago.
The team’s evidence shows that, without taking magnetism into account,
theoretical calculations of iron pnictides’ inner structure do not line
up with actual lab measurements. Factor in magnetism, though, and these discrepancies
vanish—a decisive difference that, according to theorist Taner Yildirim,
could imply that magnetism is also key to iron pnictide superconductivity.
“Without considering magnetism, for example, the calculated distance
between iron layers—a distance that has been thoroughly measured—comes
out to be wrong,” says Yildirim, of NIST’s Center for Neutron Research.
“However, provided that we consider magnetic spin in our calculations,
we can explain almost all of the iron pnictides’ structural and dynamic
Yildirim gave an invited talk* at the March meeting of the American Physical
Society, where he presented theoretical evidence demonstrating how magnetism
controls basic aspects of iron pnictides as the position of the atoms, the materials’
phase transition, i.e. the sudden changes in the structure with temperature,
and—probably, Yildirim says—their superconducting properties.
“Determining the mechanism of superconductivity in iron pnictide systems
is very important in solving the long-standing mystery of the high temperature
superconductor phenomena in general,” Yildirim says. “Once we have
such an understanding of this strange phenomenon, we can then make predictions
and design new materials with even higher superconductivity temperatures.”
For more on Dr. Yildirim’s work, see www.ncnr.nist.gov/staff/taner/highlights.htm.
For more on iron-based superconductors at NIST, see “Iron-based Materials
May Unlock Superconductivity’s Secrets,” NIST Tech Beat, Nov. 12,
* T. Yildrim. Competing magnetic interactions, structural phase transition,
and the unprecedented giant coupling of Fe-spin state and the As-As interactions
in iron-pnictide. Presented at the March Meeting of the American Physical Society,
March 17, 2009. An abstract is available at http://meetings.aps.org/Meeting/MAR09/Event/96315.