Arsenic heats up iron superconductors

High-temperature materials set scientific community a-buzzing.

Could economically viable magnetic levitation be back on track? Credit: E. Hoshiko/AP

A new class of high-temperature superconductors has been discovered in a breakthrough that once again has the scientific community whispering that economically feasible magnetic levitation and lossless energy transmission may be possible.

First discovered in 1911, superconductors can conduct electricity with virtually no resistance and can maintain a current without a voltage being applied. But they initially offered little practical application — most of the metals used in the early work, such as mercury, lead and niobium, had to be cooled to a few degrees above absolute zero before they reached their ‘transition temperature’, the point at which electrical resistance disappears.

In 1986, scientists identified the first in a family of copper-based, high-temperature superconductors. Within a few years, they had pushed the transition temperature above 100 K — past the boiling point of liquid nitrogen at 79 K, an economically important benchmark. A record of 138 K was set in 1995. But those temperatures eventually stopped rising, dashing hopes that a room-temperature superconductor was possible.

Interest was rekindled two years ago when researchers at the Tokyo Institute of Technology synthesized a new superconductor based on iron rather than copper. The material also featured oxygen, lanthanum and phosphorus, but its transition temperature was just 4 degrees above absolute zero, no better than the very first superconductor discovered a century before.

Then this February, the same group announced an exciting development. The researchers had replaced phosphorus with another pnicogen, arsenic, in the layered material and — boom — the transition temperature shot up to 26 K (Y. Kamihara et al. J. Am. Chem. Soc. 130, 3296–3297; 2008). Subsequent tweaking has already boosted that temperature above 50 K. “We all were surprised,” says materials scientist Hideo Hosono, who led the study.

The superconductor community is now buzzing. A number of groups around the world are working with the iron-based material, ramping up efforts and trying other pnicogens such as bismuth. Every week, scientists are posting papers on the preprint server arXiv with new proclamations of the material’s properties. “It’s phenomenal, because we’ve broken the tyranny of copper,” says Paul Canfield, a physicist at the Ames Laboratory in Iowa.

The iron-based family might provide a fresh opportunity to engineer superconductors that operate at practical temperatures. It also offers chemists a chance to finally figure out how high-temperature superconductors work.

Conventional low-temperature superconductors were explained in a Nobel prizewinning theory developed by John Bardeen, Leon Cooper and Robert Schreiffer. Vibrations in the metallic crystal lattice squeeze electrons into pairs, overcoming their mutual repulsion. The paired electrons, operating in a single quantum state, move freely without resistance. A secondary property of superconductors is the blocking out of magnetic fields, which leads to their ability to levitate magnets.

A hot sandwich: the superconducting iron and arsenic filling in this crystal is fed electrons from the layers above and below. Credit: Michael McGuire, Oak Ridge National Laboratory

The discovery of the copper-based family of ceramic superconductors raised interesting questions that are still unanswered. “We cannot make sense of it with the theoretical physics we know,” says Jan Zaanen, a condensed-matter theorist at the University of Leiden in the Netherlands. “We’re dealing with a big mystery.” Theorists strongly suspect that magnetism is involved in high-temperature superconductivity. In certain types of metal, the movement of electrons can become ‘jammed’, and the metal becomes an insulator. As the electrons sit in place, they spin, forming tiny magnets — but with opposite polarities that cancel out the net field. Such materials are said to be antiferromagnetic.

Like the copper-based superconductors, the new iron-based ones seem to be antiferromagnetic. They also share a planar structure, where the superconducting ‘action’ occurs in the middle of the chemical sandwich (see diagram). As magnetism can exist at very high temperatures, the hope, barely expressed because it’s so audacious, is that an understanding of the mechanism could lead to a way of designing custom superconductors, ones that work at room temperature. “Maybe, maybe, maybe,” says Hosono.

Much work remains. The researchers have so far worked only with superconducting powders and need to grow crystals, which are more useful for experiments. Copper-based superconductors are easy to work with, but the new family involves a trickier chemistry, and there is the toxicity and volatility of arsenic to consider. Last month, a sealed tube of arsenic exploded at the Argonne National Laboratory in Illinois, when a research group started to synthesize the new material for the first time. No one was hurt, but some academics won’t work with arsenic because they don’t want their graduate students to take risks.

However, any worries or doubts are, right now, outweighed by excitement and renewed enthusiasm for high-temperature superconductivity. “It’s early days,” says Canfield, “but we’ve already doubled the transition temperature.”

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Hand, E. Arsenic heats up iron superconductors. Nature 452, 922 (2008). https://doi.org/10.1038/452922a

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