Superconductors are heating up, and a world record-holder may have just been dethroned.
Two studies report evidence of superconductivity — the transmission of electricity without resistance — at temperatures higher than seen before. The effect appears in compounds of lanthanum and hydrogen squeezed to extremely high pressures.
All known superconductors must be chilled to function, which makes them difficult to use in real-world applications. If scientists found a superconductor that worked at room temperature, the material could be integrated into electronic devices and transmission wires, potentially saving vast amounts of energy currently lost to electrical resistance. So scientists are constantly on the lookout for higher-temperature superconductors. The current record-holder, hydrogen sulfide, which also must be compressed, works below 203 kelvins, or about −70° Celsius (SN: 12/26/15, p. 25).
The new evidence for superconductivity is based on a dramatic drop in the resistance of the lanthanum-hydrogen compounds when cooled below a certain temperature. One team of physicists found that their compound’s resistance plummeted at a temperature of 260 kelvins (−13° C), the temperature of a very cold winter day. The purported superconductivity occurred when the material had been crushed with almost 2 million times the pressure of Earth’s atmosphere by squeezing it between two diamonds. Some samples even showed signs of superconductivity at higher temperatures, up to 280 kelvins (about 7° C), physicist Russell Hemley of George Washington University in Washington, D.C., and colleagues report in a study posted online August 23 at arXiv.org. Hemley first reported signs of the compound’s superconductivity in May in Madrid at a symposium on superconductivity and pressure.
Another group found evidence of superconductivity in a lanthanum-hydrogen compound under chillier, but still record-breaking, conditions. The researchers crushed lanthanum and hydrogen in a diamond press to about 1.5 million times Earth’s atmospheric pressure. When cooled to about 215 kelvins (−58° C), the compound’s resistance falls sharply, physicist Mikhail Eremets of the Max Planck Institute for Chemistry in Mainz and colleagues report in a paper posted online August 21 at arXiv.org.
It’s not clear what the exact structures of the chemical compounds are and whether the two groups are studying identical materials. Differences between the two teams’ samples might explain the temperature discrepancy. By scattering X-rays from the compound, Hemley and colleagues showed that the material’s structure was consistent with LaH10, which contains 10 hydrogen atoms for every lanthanum atom. Hemley’s team had previously predicted that LaH10 would be superconducting at a relatively high temperature.
The results are “very exciting,” says theoretical chemist Eva Zurek of the University at Buffalo in New York. However, the studies are not conclusive: They have not been peer reviewed and do not yet show an essential hallmark of superconductivity called the Meissner effect, in which magnetic fields are expelled from the superconducting material (SN: 8/8/15, p. 12). But the results agree with the previous theoretical predictions made by Hemley and colleagues. So, Zurek says, “I would hope and suspect that this is indeed … correct.”
The researchers are now working on bolstering their evidence for superconductivity. “Both groups should make more efforts to convince people,” Eremets says.
The requirement of ultrahigh pressures makes the materials unlikely to be useful for applications, but better understanding of high-temperature superconductivity could lead scientists to other, more practical superconductors.
The potential new superconductor and the previous record-holder are both chock-full of hydrogen. Scientists are looking for superconductivity in such hydrogen-rich materials based on the prediction that pure hydrogen, when squeezed to immensely high pressures, will become a metal that is superconducting at room temperature (SN: 8/20/16, p. 18). But metallic hydrogen has proven difficult to produce, requiring pressures even higher than those needed for hydrogen-rich compounds. So scientists are looking for superconductivity in hydrogen-mimicking compounds that are easier to create.
“The picture is very bright for looking at more and more of these materials and finding these astonishingly high superconducting transition temperatures,” Hemley says.