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 any need for special cooling systems. Although it was just a speck of carbon, sulfur, and hydrogen forged under extreme pressures, the hope was that someday the material would lead to variants that would enable lossless electricity grids and inexpensive magnets for MRI machines, maglev railways, atom smashers, and fusion reactors.
Faith in the result 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 Dias’s team had claimed. “There have been a lot of questions about this result for a while,” 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 longtime critic of the study, says the retraction does not go far enough. He believes it glosses over what he says is evidence of scientific misconduct. “I think this is a real problem,” he says. “You cannot leave it as, ‘Oh, it’s a difference of opinion.’”
The retraction was unusual in that Nature editors took the step over the objection of all nine authors of the paper. “We stand by our work, and it’s been verified experimentally and theoretically,” Dias says. Ashkan Salamat, a physicist at the University of Nevada, Las Vegas, and another senior member of the collaboration, points out the retraction does not question the drop in electric resistance—the most important part of any superconductivity claim. He adds, “We’re confused and disappointed in the decision-making by the Nature editorial board.”
The retraction comes even as excitement builds for the class of superconducting materials called hydrides, which includes the carbonaceous sulfur hydride (CSH) developed by Dias’s team. Under pressures greater than at the center of the Earth, hydrogen is thought to behave like a superconducting metal. Adding other elements to the hydrogen—creating a hydride structure—can increase the “chemical pressure,” reducing the need for external pressure and making superconductivity reachable in small laboratory vises called diamond anvil cells. As Lilia Boeri, a theoretical physicist at the Sapienza University of Rome, puts it, “These hydrides are a sort of realization of metallic hydrogen at slightly lower pressure.”
In 2015, Mikhail Eremets, an experimental physicist at the Max Planck Institute for Chemistry, and colleagues reported the first superconducting hydride: a mix of hydrogen and sulfur that, under enormous pressures, exhibited a sharp drop in electrical resistance at a critical temperature (Tc) of 203 K (–70°C). That was nowhere near room temperature, but warmer than the Tc for most superconducting materials. Some theorists thought adding a third element to the mix would give researchers a new variable to play with, enabling them to get closer to ambient pressures—or room temperatures. For the 2020 Nature paper, Dias and colleagues added carbon, crushed the mix 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 is not considered sufficient. The gold standard is to provide evidence of another key attribute of superconductors: their ability to expel an applied magnetic field when they cross Tc and become superconducting. Measuring that effect in a diamond anvil cell is impractical, so experimentalists working with hydrides often measure a related quantity called “magnetic susceptibility.” Even then they must contend with tiny wires and samples, immense pressures, and a background magnetic signal from metallic gaskets and other experimental components. “It’s like you’re trying to see a star when the Sun is out,” Hamlin says.
The study’s magnetic susceptibility data were what led to the retraction. The team members reported that a susceptibility signal emerged after they had subtracted 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—an assumed background rather than a measured one. But Salamat says relying on a user-defined background is customary in high-pressure physics because the background is so hard to measure experimentally.
In response to some of the criticisms, Dias and Salamat in 2021 posted a paper to the arXiv physics preprint server. It contained raw susceptibility data and purported to explain how the background was subtracted. “It raised more questions than it answered,” says Brad Ramshaw, a quantum materials physicist at Cornell University. “The process of going from the raw data to the published data was incredibly opaque.”
Hirsch, a firebrand who has criticized other hydride superconductivity claims, has 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,” Hirsch says. He also noted suspicious similarities to data in a 2009 Physical Review Letters paper on superconductivity in europium under high pressures. That study, which shared one author with the Nature paper, was retracted last year because of inaccurate magnetic susceptibility data.
In preprints, Hirsch kept hammering on the Dias study—so forcefully that in February, he was temporarily banned from posting to arXiv. He also complained to the University of Rochester, which in two inquiries found no evidence of scientific misconduct. This month, Hirsch and another critic, Dirk van der Marel, a condensed matter physicist at the University of Geneva, published their conclusion that the susceptibility data in the Dias study are “pathological.” Van der Marel is heartened by the Nature retraction. “It is good to know you are not alone in believing something is seriously wrong,” he says.
Dias says the team plans to resubmit the paper to Nature without any background subtraction; he says the raw data alone show the change in magnetic susceptibility. Salamat also notes that Hirsch and Van der Marel are not high-pressure experimentalists. “We believe that some of their actions have veered into personal attacks,” he says. “We’re just not going to have people throw mud at us from a distance.” Dias sent “cease and desist” letters to Van der Marel and to Hirsch’s department chair and dean at UCSD.
Eremets says the Dias study might still be right about CSH. But he has tried at least six times to replicate the results and failed. Although Dias’s team has shared the basics of its experimental protocol, Eremets says they have been less forthcoming in the details, such as what type of carbon they used in their CSH mix. Boeri agrees. “There are a lot of people who are a lot more careful, 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 CSH replication published in July. Critics question its independence, however, because 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 has co-founded a company, Unearthly Materials, to pursue commercial room-temperature superconductors. At conferences this summer, Dias has presented claims of superconductivity in new hydride compounds. Although he declined to comment on those claims until they are published, he says, “We’ve moved on from the 2020 work.” Salamat adds, “We’re on the precipice of a new era of high-temperature superconductivity.”
Eremets is skeptical that Dias’s new superconductors will stand up to scrutiny. “How is this possible? Everything he touches turns to gold.” But he is confident that patient work of science, underpinned by painstaking replication, will sort the real promise of hydrides from the questionable claims. “Science is not afraid of these things,” he says. “The truth, sooner or later, will come.”
Source: Science Mag