Using extremely high pressures, experimentalists have created many superconducting hydride compounds including one—carbon sulfur hydride (CSH)—that appears to work at close to room temperature. Theorists have predicted other hydride compounds which could work at lower pressures. There is race to find versions stable at ambient pressure and room temperature.
In 2004, Ashcroft suggested that adding other elements to hydrogen might add a “chemical precompression,” stabilizing the hydrogen lattice at lower pressures. The race was on to make superconducting hydrides. In 2015, researchers including Mikhail Eremets, a physicist at the Max Planck Institute for Chemistry, reported in Nature that a mix of sulfur and hydrogen superconducted at 203 K when pressurized to 155 GPa. Over the next 3 years, Eremets and others boosted the Tc as high as 250 K in hydrides containing the heavy metal lanthanum. Then came Dias’s CSH compound, reported late last year in Nature, which superconducts at 287 K—or 14°C, the temperature of a wine cellar—under 267 GPa of pressure, followed by an yttrium hydride that superconducts at nearly as warm a temperature, announced by multiple groups this year.
Hirsch and others caution that the high-pressure results don’t actually show a key feature of superconductors: the exclusion of magnetic fields. No one has yet figured out how to measure the effect inside a diamond vise.
Eremets, whose team has discovered multiple hydride superconductors, says Hirsch is wrong, at least about the hydrides Eremets has worked on. His group’s 2019 Nature Communications paper on a hydrogen sulfide superconductor contains a plot showing the Tc drops as an external magnetic field is increased, meaning the resistance flattens as expected for a superconductor. “Simply, he missed it,” Eremets says.
To raise Tc and lower pressure, researchers need chemical recipes that either add a bunch of electrons to the hydrogen lattice or lock hydrogen into a stiffer lattice. Researchers have reported success with both strategies, leading to two classes of hydrides with very different 3D structures. The first class includes repeating cagelike structures made from hydrogen atoms, with each cage enclosing an electron-rich metal atom, such as lanthanum or yttrium. The second class adds light elements designed to bind directly with hydrogen to create a continuous network of interlocking atoms.
Theorists have predicted that hydrides such as calcium hydride or actinium hydride should superconduct at close to room temperature—and at a pressure considerably less than that needed for CSH. Still, Boeri says, “I’m not sure we can get to ambient pressure.”
Dias has preliminary evidence for a room-temperature hydride superconductor that is stable down to 20 GPa, less than one-tenth the pressure CSH required. But because he and his team are patenting that discovery, too, and collecting more data, he’s unwilling to say what the material is.
Written By Brian Wang, NExtbigfuture.com
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