Has the quest for room temperature superconductivity finally succeeded? Researchers at the University of Rochester (U of R), who previously were forced to retract a controversial claim of room temperature superconductivity at high pressures, are back with an even more spectacular claim. This week in Nature they report a new material that superconducts at room temperature—and not much more than ambient pressures.

“If this is correct, it’s completely revolutionary,” says James Hamlin, a physicist at the University of Florida who was not involved with the work. A room temperature superconductor would usher in a century-long dream. Existing superconductors require expensive and bulky chilling systems to conduct electricity frictionlessly, but room temperature materials could lead to hyperefficient electricity grids and computer chips, as well as the ultrapowerful magnets needed for levitating trains and fusion power.

But given the U of R group’s recent retraction, many physicists won’t be easily convinced. “I think they will have to do some real work and be really open for people to believe it,” Hamlin says. Jorge Hirsch, a physicist at the University of California, San Diego, and a vociferous critic of the earlier work, is even more blunt. “I doubt [the new result], because I don’t trust these authors.”

The U of R group, led by physicist Ranga Dias, caused a sensation in 2020 when it reported superconductivity in a tiny speck of carbon, sulfur, and hydrogen (CSH), created by squeezing materials between the tips of two diamonds to millions of times atmospheric pressure. Scientists had made other hydrogen-rich superconductors, known as hydrides, but they had to be chilled to 250 K (–23°C) or lower. CSH superconducted at 287 K, the temperature of a wine fridge.

But other researchers could not replicate the CSH results and complained that the study’s recipe was vague and incomplete. Others found fault with the way the U of R group measured the material’s magnetic behavior, a key signature of superconductivity. Ultimately, Nature retracted the paper in September 2022 over the objections of all its authors.

On 22 February, Dias and his colleagues doubled down on their original claim. In a preprint posted on arXiv they reported synthesizing a new version of CSH that superconducts at a slightly lower 260 K, but at only about half the previous pressure. “This should clear up any questions regarding CSH,” says co-author Russell Hemley, an x-ray crystallographer at the University of Illinois, Chicago, who helped determine the material’s structure.

Now comes the even more promising substance: nitrogen-doped lutetium-hydride (LNH). To make it, Dias’s team loaded a thin lutetium foil in a diamond vise and injected a mix of hydrogen and nitrogen gas. By ramping the pressure up to 2 gigapascals (nearly 20,000 times atmospheric pressure) and baking the mix at 200°C for up to 3 days, they forged a bright blue crystalline fleck, one that survived even after the pressure was eased.

When they dialed the pressure back up to as little as 0.3 gigapascals, the blue fleck turned pink as the electrical resistance plunged to zero. The substance reached a peak superconducting temperature of 294 K—7° warmer than the original CSH and truly room temperature—at pressures of 1 gigapascal. Magnetic measurements also showed the sample repelled an externally applied magnetic field, a hallmark of superconductors. The paper, the authors say, went through five rounds of review.

“This is the most detailed study of a hydride ever,” says Ashkan Salamat, a physicist at the University of Nevada, Las Vegas, and one of the study’s senior authors. Others agree the results look impressive. “It looks believable,” says Alexander Goncharov, a physicist at the Carnegie Institution for Science. “If it is correct, the paper is a tour de force using all the different techniques,” Hamlin says.

But LNH raises as many questions as it answers. “It sort of contradicts everything I would expect of hydrides,” says Lilia Boeri, a theoretical physicist at Sapienza University of Rome. In the conventional theory of superconductivity, vibrations in a material’s crystalline lattice act as glue between pairs of electrons, enabling them to conduct without resistance. Boeri’s calculations and others’ suggest ambient pressure hydride superconductors can exist, but only at colder temperatures, about 125 K. Above that, she says, the vibrational glue loses its grip, and only intense pressure can “stiffen” the lattice and cause electrons to pair up.

Dias and his colleagues argue this is where the nitrogen in their new material comes in. Nitrogen atoms are tiny compared with lutetium. They believe nitrogen atoms might be wriggling in between lutetium atoms, forming a cagelike structure that stiffens the rest of the lattice. He and his colleagues have yet to confirm that structure. But Dias speculates it “provides the stability for superconductivity to occur at lower pressure.”

To solve the riddle, the U of R team “should do everything they can to help other groups reproduce it,” says Mikhail Eremets, a physicist at the Max Planck Institute for Chemistry, whose team discovered the first hydride superconductor in 2015 but failed to replicate the CSH results. “If they will not it will be a disaster.” But this level of cooperation doesn’t appear to be in the cards. Dias says Unearthly Materials, a company he and Salamat founded, is trying to commercialize the new hydride. “We are not going to distribute this material considering the proprietary nature of our process and the intellectual property rights that exist,” Dias said via email.

Other physicists aren’t pleased. “It’s a completely unscientific behavior,” Boeri says. Hamlin says he won’t commit a student to replicating the work unless the U of R group shares samples and raw data.
Salamat says the raw data are available online. As for sharing samples, the paper provides a detailed recipe, he says. “People can go ahead and make it for themselves.”

Eremets plans to try. Because LNH can be made at lower pressures that don’t require diamond vises, “this will be much easier to check by many groups,” he says. Hemley, who is helping the U of R group determine LNH’s structure, agrees. “It’s a whole different ballgame now,” he says.

Source: https://www.science.org/content/article/revolutionary-blue-crystal-resurrects-hope-room-temperature-superconductivity