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Pressurized superconductors approach room-temperature realm

23 August 2018

When subjected to extreme pressure, a compound of lanthanum and hydrogen behaves as a conventional superconductor at temperatures up to 280 K, according to new research.

Though it takes about 2 million times the atmospheric pressure at Earth’s surface to achieve it, researchers may be closing in on room-temperature superconductivity.

A superconducting transition temperature (Tc) of between 260 K and 280 K was realized in a pressurized compound of lanthanum and hydrogen, scientists led by Russell Hemley of the George Washington University report 23 August at the national meeting of the American Chemical Society in Boston and in an arXiv paper. In a separate arXiv paper submitted on 21 August, a group led by Mikhail Eremets of the Max Planck Institute for Chemistry in Mainz, Germany, reports a Tc of 215 K in a lower-pressure compound of the same elements. If confirmed, both measurements would break the previous record Tc of 203 K, which was demonstrated by Eremets’s team in another hydride.

“This is a very exciting development,” says Elisabeth Nicol, a condensed matter theorist at the University of Guelph in Canada. “For those of us who have been working in the field for a very long time, it is like a dream come true to see evidence for superconductivity quite near to room temperature.”

Optical micrograph of the sample
An optical micrograph of the LaH10 sample at 178 GPa. Credit: Russell Hemley

The teams made their claims based on observed plunges in electrical resistance. To secure a place in the record books, Hemley and his colleagues will need to demonstrate that the high-pressure compound expels magnetic fields—the Meissner effect—which is considered the gold-standard signature of superconductivity. That’s especially important in this kind of experiment, as scientists have found it notoriously difficult to definitively characterize tiny samples squeezed between the tips of diamond anvils.

Although a room-temperature superconductor that requires 200 gigapascals of pressure wouldn’t be at all useful outside the lab, it could provide a road map for formulating another material that behaves similarly at ambient pressure.

The superconducting material, LaH10, is the latest pressurized hydride to exhibit high Tc through conventional superconductivity, the kind described by the six-decade-old Bardeen-Cooper-Schrieffer (BCS) theory. The revival of interest in BCS superconductivity stems from work by Neil Ashcroft, who predicted a half-century ago that hydrogen would not only become metallic under pressure but would also be a high-temperature superconductor. In 2004 the Cornell University theorist proposed a more practical means of realizing those properties: Rather than crush pure hydrogen to unleash its metallic abilities, subjecting a hydrogen-rich compound to lesser compression might yield similar results. Chemical bonding would do some of the work in forcing hydrogen atoms together and inducing the coupling of electrons and lattice vibrations, or phonons, that hastens BCS superconductivity.

Following Ashcroft’s blueprint, in 2014 Eremets and his colleagues reported the record Tc of 203 K in samples of hydrogen sulfide subjected to 150 GPa between diamond anvils. Despite initial skepticism, the finding was eventually confirmed via observation of the Meissner effect (see Physics Today, July 2016, page 21).

Meanwhile, other groups have been conducting theoretical and experimental searches for additional hydrides that would mimic metallic hydrogen at high yet attainable pressures. Last year in Proceedings of the National Academy of Sciences, Hemley and his colleagues used density functional theory and a model for predicting crystal structures to assess the properties of hydrogen bonded with the elements lanthanum and yttrium. With tight hydrogen bonding and a structure that’s associated with high Tc, LaH10 stood out as an intriguing candidate. Further calculations suggested that LaH10 would have a Tc of 274–286 K at 210 GPa.

Resistance measurements
The resistance of LaH10 was found to drop to as low as 0.5 μΩ from a value of 30 mΩ at 300 K. Credit: Russell Hemley

Experiment rapidly followed theory. The researchers crushed La and H2 between diamond anvils at room temperature and then laser heated the concoction. X-ray diffraction measurements revealed a distinct shift in structure at about 1000 K and 170 GPa. As predicted, LaH10 emerged with a caged structure of 32 H atoms surrounding each La atom, Hemley’s team reported in Angewandte Chemie last November. The resulting configuration pushed adjacent H atoms to within 1.1 Å of each other, a distance that’s consistent with predictions for metallic hydrogen and thus seemingly ideal for realizing high-temperature superconductivity.

The final step was to measure the electrical properties. Over the past several months, Hemley and colleagues performed several trials in labs at George Washington, the Carnegie Institution for Science, and Argonne National Laboratory. Using a four-point probe on a 5-μm-thick sample pressurized to 190 GPa, they measured a sudden drop in resistance, to about 0.5 μΩ, at 260 K. Analysis of three other samples with a less precise technique yielded evidence of sudden drops in resistance at temperatures as high as 280 K at 200 GPa. A pressurized sample of pure lanthanum showed no such resistance change.

The day after receiving a summary and figures of Hemley’s forthcoming paper, Eremets, who once worked for Hemley as a research scientist, submitted a paper of his own to arXiv. He and his team report a Tc of 215 K in a sample of lanthanum and hydrogen pressurized to 150 GPa.

Neither Hemley’s nor Eremets’s teams tested for the Meissner effect, though both plan to. The need to laser heat the compound in addition to crushing it complicates the process, as does the smaller size of the samples than those used in previous high-pressure superconductivity experiments. Recent controversial claims regarding the production of metallic hydrogen demonstrate the obstacles in yielding consistent results when dealing with material under extreme pressure.

If the record Tc is confirmed, scientists will work to further exploit chemical compression to coax hydride compounds into metallic hydrogen–like states. Researchers can also study LaH10 and related compounds to learn about metallic hydrogen.

Editor’s note, 24 August: The article was updated with a link to the Hemley team’s paper and a quote from Elisabeth Nicol.

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