Superconductors escape Flatland

A flat, two-dimensional flow of electric current has long been thought essential to the secret of how high-temperature superconductors work. But new research shows that an iron-based superconductor allows current to flow in three dimensions.

For at least some high-temperature superconductors, the mechanism that enables electrons to flow with zero resistance doesn’t depend on the electrons moving along the boundary between layers in the material, the new research shows. In previous experiments on copper-based superconductors, the free-flowing current always occurred at these 2-D boundaries, so most theorists have thought this 2-Dness is somehow essential to how this category of superconductors works.

Finding a three-dimensional high-temperature superconductor “was very surprising for us,” says Huiqiu Yuan, a condensed matter physicist at Zhejiang University in Hangzhou, China, who coauthored the study published in the Jan. 29 Nature. “The two-dimensionality is not a necessary condition for getting a high superconducting transition temperature.”

These 3-D high-temperature superconductors could also be more useful than their 2-D cousins. When superconducting electric current is confined to a 2-D plane, strong magnetic fields disrupt the current. But superconductors are often used specifically for the purpose of creating strong magnetic fields. This limitation is a major reason why copper-based superconductors haven’t become widely adopted. High-temperature superconductors capable of 3-D flow wouldn’t have the same problem.

“Three dimensionality would help a great deal,” comments Jan Zaanen, a condensed matter theorist at Leiden University in the Netherlands, who wrote a commentary on the research in the same issue of Nature.

Cool a superconductor below its transition temperature, and suddenly electric current will flow with zero resistance. While this unimpeded flow of electrons makes superconductors useful for loss-free transmission of electric power and for very powerful electromagnets in MRI machines and levitating trains, conventional superconductors have transition temperatures near absolute zero (–273º Celsius) and so require expensive liquid helium to keep them cold.

High-temperature superconductors carry resistance-free current at temperatures high enough to allow cooling with liquid nitrogen instead, which is far cheaper and more widely available than liquid helium. Until recently, all known high-temperature superconductors were copper-based materials, and even after more than 20 years of research, scientists still don’t fully understand how these compounds work.

Yuan and his colleagues studied a relatively new kind of superconductor that is iron based. This class has generated a lot of excitement since its 2006 discovery because iron-based superconductors have a very different chemistry than those that are copper-based. So physicists have hoped that comparing the two families of superconductors would reveal some common, essential mechanism for high-temperature superconductivity.

“There’s a widespread belief that these [iron-based superconductors] share this basic secret” with copper-based conductors, Zaanen says. “It is very important to know that the mystery of [high-temperature] superconductivity is not limited to two dimensions.”

Yuan’s team used a strong magnetic field to force the electric current to flow in various directions through a superconductor made from iron, arsenic, potassium and barium. Regardless of the direction of the magnetic field, the current flowed about as well.

While the discovery does appear to show that iron-based superconductors aren’t limited to 2-D current, the implications for copper-based superconductors are unclear. If the two classes of materials do share the same mechanism for high-temperature superconductivity, then perhaps some copper-based compounds could be capable of 3-D current flow as well.

But the research might also suggest that the two kinds of superconductors actually have different mechanisms.

Both interpretations are possible, Zaanen says, but “I wouldn’t bet any longer on theories that require two-dimensionality, because there are good reasons to think that they [both copper-based and iron-based superconductors] share the secret.”