By Andrew Grant
SAN ANTONIO — Superconductors are finally heating up. A recent experiment is raising hopes that for the first time in two decades, physicists have set a record high temperature at which a material can transport electrical current with no resistance.
The material, a compound of hydrogen and sulfur, must be compressed at extreme pressures to become superconducting. And the temperature at which it works is still very low: 190 kelvins (–83° Celsius). But confirmation of the finding, which was first reported in a paper posted online in December at arXiv.org, would bring physicists another step toward finding or fabricating a material that exhibits superconductivity at room temperature (roughly 300 kelvins). It would also redirect the efforts of many physicists who have been focusing on copper-based superconducting compounds called cuprates.
For over a century, physicists have dreamed of room-temperature superconductors that could, among other things, help propel high-speed trains that levitate over the tracks. But until now, the highest-temperature superconducting material was a cuprate that had to be cooled to 164 kelvins.
Mikhail Eremets, a high-pressure physicist at the Max Planck Institute for Chemistry in Mainz, Germany, approached the problem from a different angle. He works with hydrogen, and some theorists had suggested that compressed hydrogen-rich materials could become superconducting at relatively high temperatures. So Eremets and colleagues took the noxious, flammable gas hydrogen sulfide, cooled it until it liquefied, and squeezed it between two diamonds.
Readings from electrodes indicated that at pressures exceeding 150 billion pascals, or 1.5 million times standard atmospheric pressure, electrical resistance plummeted even when the sample was well above 100 kelvins. Other measurements corroborated that the compound exhibited superconductivity at up to about 190 kelvins.
At a presentation March 2 at an American Physical Society meeting, Eremets said that about 15 theoretical papers have already come out supporting his team’s results. But Ivan Božović, a condensed matter physicist at Brookhaven National Laboratory in Upton, N.Y., is skeptical, noting that many extraordinary claims of superconductivity have failed to hold up. Many physicists say they’d like to see Eremets’ group or another team determine whether the compound expels magnetic fields, a distinguishing trait of superconductors. Eremets said he just built an instrument to perform that measurement under high pressure. In addition, at least one other lab is trying to reproduce the experiment.
During the wait for confirmation of the discovery, some scientists are already thinking about the implications. Crushing a superconductor to get it working isn’t very practical, but physicists may be able to boost the temperature and reduce the pressure requirements of similar hydrogen compounds by doping them with other elements. “It is the most exciting development in superconductivity since the discovery of the cuprate superconductors,” says James Schilling, a high-pressure experimental physicist at Washington University in St. Louis. That discovery, he notes, won the Nobel Prize for Georg Bednorz and Alex Müller in 1987, the year after they reported their finding.
