In 2020, Ranga Dias, a physicist at the University of Rochester, and his colleagues published a sensational result in Nature, featured on its cover. They claimed to have discovered a room-temperature superconductor: a material in which electric current flows frictionlessly without the need for special cooling systems. Although it was just a grain of carbon, sulfur and hydrogen forged under extreme pressures, the hope was that one day the material would lead to variants that would enable lossless power grids and inexpensive magnets for MRI machines, maglev railways, atom breakers and fusion. reactors.
Faith in the outcome is now evaporating. On Monday Nature retracted the study, citing data issues other scientists have raised over the past 2 years that have undermined confidence in one of two key signs of superconductivity that Dias’ team had claimed. “There have been a lot of questions about this result for some time,” says James Hamlin, an experimental condensed matter physicist at the University of Florida. But Jorge Hirsch, a theoretical physicist at the University of California, San Diego (UCSD), and a longtime reviewer of the study, says the retraction doesn’t go far enough. He thinks it glosses over what he says is evidence of scientific misconduct. “I think that’s a real problem,” he says. “You can’t just leave it like, ‘Oh, it’s a difference of opinion.'”
The retraction was unusual in that sense Nature the editors took the plunge over the objection of the nine authors of the article. “We stand by our work, and it has been verified experimentally and theoretically,” says Dias. Ashkan Salamat, a physicist at the University of Nevada, Las Vegas, and another senior member of the collaboration, points out that the retraction does not challenge the drop in electrical resistance, the most important part of any superconductivity claim. He adds: “We are confused and disappointed with the decision-making by the Nature Editorial Committee.”
The retraction comes even as excitement mounts for the class of superconducting materials called hydrides, which includes carbon sulfur hydride (CSH) developed by Dias’ team. Under pressures above those at the center of the Earth, hydrogen behaves like a superconducting metal. Adding other elements to hydrogen, creating a hydride structure, can increase the “chemical pressure”, reducing the need for external pressure and making superconductivity accessible in small laboratory vises called diamond anvil cells . As Lilia Boeri, a theoretical physicist at La Sapienza University in Rome, says, “These hydrides are a kind of realization of metallic hydrogen at a slightly lower pressure.”
In 2015, Mikhail Eremets, an experimental physicist at the Max Planck Institute for Chemistry, and his colleagues reported the first superconducting hydride: a mixture of hydrogen and sulfur that, under enormous pressures, exhibited a sharp drop in resistance electrical at a critical temperature (Tc ) of 203 K (–70 °C). It was far from room temperature, but hotter than Tc for most superconducting materials. Some theorists believed that adding a third element to the mix would give researchers a new variable to play with, allowing them to approximate ambient pressures or ambient temperatures. For 2020 Nature paper, Dias and his colleagues added carbon, ground the mixture in a diamond anvil cell, and heated it with a laser to create a new substance. They reported that tests showed a sharp drop in resistance at a Tc of 288 K (15°C) – roughly room temperature – and a pressure of 267 gigapascals, about 75% of the pressure at the center of the Earth. .
But in a field that has seen many superconducting claims come and go, a drop in resistance alone isn’t considered enough. The gold standard is to provide proof of another key attribute of superconductors: their ability to expel an applied magnetic field as they pass through Tc and become superconductors. Measuring this effect in a diamond anvil cell is impractical, so experimenters working with hydrides often measure a related quantity called “magnetic susceptibility”. Even then, they have to contend with tiny wires and specimens, immense pressures, and background magnetic signal from metal joints and other experimental components. “It’s like trying to see a star when the sun is out,” Hamlin says.
Magnetic susceptibility data from the study led to the retraction. Team members reported that a susceptibility signal appeared after subtracting a background signal, but they did not include raw data. The omission frustrated critics, who also complained that the team relied on a “user-defined” background—one that was assumed rather than measured. But Salamat says relying on a user-defined background is customary in high-pressure physics because the background is so difficult to measure experimentally.
In response to some of the criticism, Dias and Salamat published an article in 2021 on the physical preprint server arXiv. It contained raw susceptibility data and claimed to explain how background noise was subtracted. “It raised more questions than it answered,” says Brad Ramshaw, a quantum materials physicist at Cornell University. “The process of going from raw data to published data was incredibly opaque.”
Hirsch, an arsonist who has criticized other claims of hydride superconductivity, made stronger accusations. He says some of the published data presented by Dias and Salamat could be represented by a smooth polynomial curve, impossible for noisy laboratory measurements. “I think they were fabricated,” says Hirsch. He also noted suspicious similarities to data from a 2009 survey. Physical examination letters article on superconductivity in europium under high pressures. This study, which shared an author with the Nature paper, was withdrawn last year due to inaccurate magnetic susceptibility data.
In preprints, Hirsch kept hammering Dias’ study, so forcefully that in February he was temporarily banned from publishing on arXiv. He also complained to the University of Rochester, which in two investigations found no evidence of scientific misconduct. This month, Hirsch and another reviewer, Dirk van der Marel, a condensed matter physicist at the University of Geneva, published their conclusion that the susceptibility data from the Dias study is “pathological.” Van der Marel is encouraged by the Nature retraction. “It’s good to know you’re not the only one who believes something is seriously wrong,” he says.
Dias says the team plans to resubmit the document to Nature without any background subtraction; he says the raw data alone shows the change in magnetic susceptibility. Salamat also notes that Hirsch and Van der Marel are not high-pressure experimenters. “We believe some of their actions turned into personal attacks,” he said. “We’re just not going to let people throw mud at us from a distance.” Dias sent “cease and desist” letters to Van der Marel and to the department chair and dean of Hirsch at UCSD.
Eremets says the Dias study might still be right about CSH. But he tried at least six times to reproduce the results and failed. Although Dias’ team shared the basics of their experimental protocol, Eremets says they were less forthcoming about details, such as the type of carbon they used in their CSH mixture. Boeri agrees. “There are a lot of people who are much more cautious, and they share the data, and they share the samples,” she says.
Salamat says colleagues are welcome to come to their labs and observe their methods and protocols. “We have an open door policy.” And he points to a replica of the CSH published in July. Critics question its independence, however, as it was led by Salamat’s group and includes many of the same authors as the Nature paper.
Dias and Salamat are not slowing down. The duo co-founded a company, Unearthly Materials, to develop commercial room-temperature superconductors. At conferences this summer, Dias presented claims of superconductivity in new hydride compounds. Although he declined to comment on these claims until they are published, he says, “We’ve moved on from the work of 2020.” Salamat adds: “We are on the brink of a new era of high-temperature superconductivity.
Eremets doubts that Dias’ new superconductors will stand up to scrutiny. “How is that possible? Everything he touches turns to gold. But he is confident that the patient work of science, backed by careful replication, will distinguish the true promise of hydrides from dubious claims.” I’m not afraid of these things,” he said. “The truth, sooner or later, will come.
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