Hydrogen sulfide—the stuff that makes rotten eggs stink—becomes a superconductor at a record high temperature, physicists in Germany have shown. When solidified, the compound conducts electricity without resistance at 203.5 K. That's still cold—about 70°C below the freezing point of water. But it’s far higher than anything ever achieved before and a big step closer to the lofty goal of achieving superconductivity at room temperature. The team's preliminary claim was circulating for more than a year, but new data clinch the case, says Michael Norman, a theorist at Argonne National Laboratory in Lemont, Illinois. “It's the real deal.”
The result may revive visions of superconductors that work at room temperature and magnetically levitated trains. But there's a catch: Hydrogen sulfide works its magic only when squeezed to more than 100 million times atmospheric pressure, roughly one-third as high as the pressure in Earth’s core. This condition makes it impractical for most applications. “Where does it go from here?” asks Igor Mazin, a theorist at the U.S. Naval Research Laboratory in Washington, D.C. “Probably nowhere.” Even so, the discovery is already altering the course of research in superconductivity.
Scientists know of a few kinds of superconductivity. In so-called conventional superconductivity, a metal such as niobium carries electricity without resistance when cooled to nearly absolute zero, or 0 K. The metal consists of a cagelike array of positively charged ions through which the negatively charged electrons flow. The electrons ordinarily lose energy as they deflect off the rattling ions. But at very low temperatures, the electrons pair. Deflecting an electron then requires breaking a pair. As there isn't enough energy around to do that, the pairs flow freely.
But something must hold the paired electrons together. In a conventional superconductor, that glue is provided by vibrations of the ion lattice called phonons. Phonons hold only so strongly, so the record temperature for an ordinary superconductor was 39 K (or –234.5°C) using the compound magnesium diboride.
However, in the 1980s physicists discovered a family of “high-temperature superconductors,” complex compounds containing copper and oxygen that become superconductors at far higher temperatures, and a decade ago, they found a similar family of iron and arsenic compounds. In those materials, the interactions of the electrons alone appear to provide the glue—although physicists aren’t sure how.
But even with these discoveries, some physicists still hoped to achieve higher transition temperatures with conventional superconductivity. As far back as the 1960s, Neil Ashcroft, a theorist at Cornell University, argued that at high pressures, solid hydrogen should become a superconducting metal. According to Ashcroft, the light hydrogen ions would shake with very high frequency phonons, the key to boosting the transition temperature. For decades, experimenters have searched for such superconductivity by squeezing bits of solid hydrogen between the tips of diamonds.
Alexander Drozdov and Mikhail Eremets, physicists at the Max Planck Institute for Chemistry in Mainz, Germany, and colleagues tried something slightly different last year: They squeezed a tiny sample of hydrogen sulfide and saw its electrical resistance vanish at 190 K, as they reported in December on the preprint server arXiv.org. That bested the record of 164 K for a copper-and-oxygen superconductor squeezed to 350,000 times atmospheric pressure. Some physicists were skeptical.
Now, Drozdov and Eremets have put the doubts to rest by demonstrating a second sign of superconductivity. When exposed to a magnetic field, a superconductor should expel it, as free-flowing currents generate an internal field that cancels the applied one. Drozdov and Eremets see just that effect, as they report online today in Nature. That measurement was a significant feat, as the experimenters’ disk-shaped sample had a diameter smaller than the width of a human hair. The researchers now report that they've reached a transition temperature as high as 203.5 K.
The high transition temperature doesn't present any major mysteries, however. Last November, theorists in China calculated that pressurized hydrogen sulfide should become a superconductor with a transition temperature between 191 K and 204 K—specifically as H2S breaks down to produce H3S, which does the superconducting. “We were lucky because this model immediately began to explain our results,” Eremets says. There is little doubt that, as predicted, the material is a conventional superconductor. When the researchers replaced the lighter hydrogen atoms with heavier atoms of deuterium (hydrogen with a neutron in its nucleus), the transition temperature fell by about 20%—just as expected if phonons provide the glue.
Is the incredible pressure really necessary for this kind of superconductivity? Maybe not, Eremets says. The pressure serves only to turn hydrogen sulfide into a metal, he says. So it may be possible to start with a compound that scientists can turn into a metal by tweaking its composition instead. Mazin is less optimistic. “It's hard to conceive how these conditions could be achieved at ambient pressure,” he says.
But rather than getting rid of the pressure, Norman predicts that researchers will do the opposite and look for new superconductors by squeezing other insulators. “In the last year, this is the big result,” he says. “It's already having an effect on the community.”
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