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.
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.”