Friday, June 06, 2008

A New Superconductor

Researchers investigate why iron arsenide materials become superconducting at relatively high temperatures.

By Prachi Patel-Predd

A new class of high-temperature superconductors, discovered earlier this year, behaves very differently than previously known copper-oxygen superconductors do. Instead, the new materials seem to follow a superconductivity mechanism found previously only in materials that are superconducting at very low temperatures, Chia-Ling Chien and his colleagues at Johns Hopkins University report in an online Nature paper.

The insight is an important step toward understanding how superconductors work, and it could help researchers design even better materials. High-temperature superconductors could lead to cheaper MRI machines; smaller, lighter power cables; and far more energy-efficient and secure power grids. Utilities, for example, could use superconducting magnets to store energy at night, and then use it at peak demand hours in the mornings and evenings.

Superconducting materials conduct electric current without any losses when they are chilled below a certain temperature, called the critical temperature. Niobium alloys, used to make superconducting magnets for MRI machines, are superconducting only below 10 K. Copper-oxygen compounds, or cuprates, which were discovered in the late 1980s, are superconducting at much higher temperatures of 90 to 138 K. At these temperatures, cheap, easy-to-use liquid nitrogen can be employed as a refrigerant. (Cuprates are not used for MRI magnets because it is difficult and expensive to make wires from them.) And some manufacturers are making nitrogen-cooled superconducting cables for transmission lines.

No resistance: New superconductors contain alternating layers of iron arsenide (orange and red) and rare earth metal oxides (blue and gray) doped with fluorine (green). Iron arsenide compounds become superconducting at relatively high temperatures of 55 K, and researchers are now beginning to decipher their superconducting mechanism.
Credit: Hideo Hosono, Tokyo Institute of Technology

But researchers have long tried to find materials with even higher critical temperatures. "The holy grail is operating [superconductors] at room temperature," says physicist Jeffrey Lynn, who studies superconductors at the National Institute of Standards and Technology. Superconducting power cables, MRI machines, and energy storage devices would be cheaper and smaller if they did not need cooling.

The new iron arsenide superconductors have shown potential for achieving high critical temperatures. Scientists at the Tokyo Institute of Technology first reported in a February paper in Journal of the American Chemical Society that a lanthanum iron arsenide material becomes superconducting at 26 K. Since then, Chinese researchers have pushed the critical temperature up to 55 K. That is not nearly as high as the superconducting temperatures for cuprates, but Johns Hopkins's Chien says that "this is a new material to explore, and one hopes we will get even higher temperatures."

The new material's chemical structure makes it particularly exciting. It contains oxides of rare earth metals sandwiched between layers of iron arsenide. The structure allows for a lot of tinkering that tweaks the material's properties, Lynn says. Researchers can, for instance, replace the iron, arsenic, or rare earth metals with other elements. In fact, Chinese researchers replaced the lanthanum in the original Japanese material with other rare earth metals, such as samarium, to raise the critical temperature above 50 K. "There are a lot of different types of chemical substitutions that you can try," Lynn says. "They're actually more flexible than cuprates."

The new superconductors could also have another crucial advantage, says David Christen, who leads superconductor research at Oak Ridge National Laboratory. While cuprate power cables have to be fabricated as specially designed flat tapes, it might be easier to make wires from iron arsenide semiconductors. "These materials could be more practical than cuprates if it turns out that they're easier and less expensive to make," Christen says.

Researchers are also hoping that iron arsenides will help unlock the mystery of how high-temperature superconductors work. That will be key for designing materials with even higher critical temperatures. In superconductors that work at very low temperatures, such as niobium and lead, electrons form pairs below the critical temperature. Atoms or defects in the crystal do not have the energy needed to break the pair and deflect the electrons. So the electron pair zips around the material unimpeded, giving rise to superconductivity. But this pairing theory does not hold for high-temperature copper-oxygen materials.

In their Nature paper, Chien and his colleagues show evidence suggesting that the pairing theory might hold for the iron arsenide superconductors. "The pairing of electrons is the soul of the superconductor," Chien says. "If the new materials follow the [pairing] theory, then . . . we will be able to understand the materials a little bit easier."

More evidence from experiments done with many different iron arsenide compounds will be needed to confirm how the superconductors work, says Pengcheng Dai, a physics professor at the University of Tennessee, in Knoxville. The Johns Hopkins work is "just one piece of the puzzle," he says. Indeed, while the pairing mechanism of iron arsenides might be different than that of copper-oxygen compounds, the two materials also have similarities. In a recent online paper, also published in Nature, Dai and Lynn showed that the two materials share key magnetic properties. And both materials also have a similar layered structure.

It might be too early to say just how useful the iron arsenide superconductors will be. For now, Dai says that researchers are excited about having broken the 22-year monopoly of cuprates and about having a new high-temperature superconductor to play with.