Room-Temperature Superconductivity Study Retracted

An anonymous reader quotes a report from Science Magazine: 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 (-70C). 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 (15C) -- 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,' reports Science. '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.'

Dias and Salamat posted a paper to arXiv in 2021 containing the raw susceptibility data and purported to explain how the background was subtracted, but 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 accused the data of being 'fabricated,' noting suspicious similarities to data in a 2009 paper on superconductivity in europium under high pressures. It too was later retracted.

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