Thanks to a bit of diamond smashing, practical room-temperature superconductivity could be close to reality
Published: Friday, 27 August 2021 02:24
ROCHESTER, NEW YORK—Every year, Ranga Dias pulverizes about $100,000 in diamonds, crushing them to a gray powder he tosses in the trash. It’s worth it. The superhard gems were sacrificed to achieve a goal researchers have chased for generations.
Before their demise, pairs of the 2-millimeter-diameter gems serve as jaws in a miniature vise. In fall 2020, Dias, a physicist at the University of Rochester (U of R), and colleagues used the setup to squash a few specks of carbon and sulfur along with a whiff of hydrogen gas to a pressure near that found at Earth’s center. The force rearranged the elements into carbon sulfur hydride (CSH), reported to be the first substance that can conduct electricity with no electrical losses at room temperature—a chilly room, to be sure. That experiment ended the same way nearly all such efforts do: The diamond jaws exploded, and the world’s only room-temperature superconductor vanished in a puff.
To date, no one else has matched the feat. But other high-pressure physics groups are crushing their own diamonds in the attempt. And those researchers have found several other hydrogen-rich materials that superconduct at relatively warm temperatures. Collectively known as hydrides, the cousins contain hydrogen along with other elements, such as lanthanum or yttrium, and still need to be chilled a little more (or sometimes a lot more) than Dias’s CSH.
Together, the hydrides have revolutionized superconductivity, showing it can happen at temperatures only dreamed of in the past, when superconductors revealed their powers only when cooled below –100°C. But so far, the materials can only be made in fractions of grams and only superconduct when squeezed to outlandish pressures. That limitation makes them wholly impractical for real-world applications. So a new goal has emerged: room-temperature superconductors that retain their magic when the pressure is off. They could enable advances as varied as a class of supercomputers that don’t waste vast amounts of energy as heat or transmission lines that carry massive electrical loads without the usual losses, saving billions of dollars and megatons of carbon emissions.
“This will be a challenge,” says Steven Louie, a superconductivity theorist at the University of California (UC), Berkeley, who doesn’t work on hydrides. “But these are very exciting developments. For a long time, people thought high-temperature superconductors couldn’t reach room temperature.” The hydrides, he says, “show this is not true.”
But not everyone buys the results. “It is easy to be fooled. These are difficult experiments done on extremely small samples under intense pressure,” says Jorge Hirsch, a theoretical physicist at UC San Diego who is a leading critic of claims for hydride superconductivity. “I agree something is going on. But I think it’s not going to turn out to be interesting new physics, but an experimental artifact.”
People thought high-temperature superconductors couldn’t reach room temperature. … [The hydrides] show this is not true.Steven Louie, University of California, Berkeley
“Any time this kind of paradigm shift happens, people will push back,” Dias responds. He agrees with Hirsch that high-pressure experiments are difficult to pull off and can produce results that are hard to interpret. But Dias and fellow hydride researchers are confident in their observations. And with theoretical predictions and preliminary experimental results both suggesting that dramatically lowering the need for pressure might be possible, the scientists press forward.
Despite the diamonds, Dias’s lab is no jewel box. The facility sits on the first floor of a blocky four-story, 1960s-era building. The lab’s heart is a fist-size, copper-colored cylinder called a diamond anvil cell. Once loaded with starting materials and screwed shut, the cell slots into a small blue metal box bolted to a table. The box houses a cryogenic cooler that uses liquid helium to precisely control the temperature. Mirrors on the table steer laser light through windows into the diamond anvil cell.
To get a superconductor, the researchers use a pressurized stream of inert gas to drive the diamonds together, generating a force amplified manyfold at their tips. A green laser aimed through the diamonds triggers chemical reactions that combine carbon, sulfur, and hydrogen atoms into a crystalline solid. The same laser then probes the material with Raman spectroscopy, which reveals which chemical elements are bonded to one another. And tiny wires positioned between the diamonds track the material’s electrical resistance, to detect superconductivity’s hallmark sharp plunge to zero. The experiments run nonstop—sometimes for weeks—until the inevitable cracking sound, when another pair of diamonds turns to dust.
Dias, a native of Sri Lanka, moved to the United States to pursue his Ph.D. at Washington State University, studying how explosions can shock materials into new forms. From there, he moved to Harvard University to do a postdoc with Isaac Silvera, a physicist working to use intense pressures to turn hydrogen gas into a metal, which they suspected might be superconducting. Other researchers had previously claimed to have seen signs of metallic hydrogen, but those results remained controversial. In 2017 in Science, Dias and Silvera said they had succeeded by trapping hydrogen in a diamond anvil cell and dialing the pressure up to 495 gigapascals (GPa), more than 4.8 million times atmospheric pressure, transforming the gas into a silvery solid.
That result generated plenty of praise—and pushback. “The word garbage cannot really describe it,” another expert said at the time. Critics cautioned that, among other things, the Harvard pair only published a single result, without replication. But Dias and Silvera have stuck by their results. And Silvera says he hopes to publish additional positive results soon.
Even if metallic hydrogen is real and confirmed to be superconducting, a substance that only survives at millions of atmospheres of pressure won’t make much of a real-world impact. That’s why proving superconductivity in hydrides—which show promise for superconductivity at lower pressures—is a much more important challenge. “It would have a huge impact,” Dias says.
Discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes, the original superconductors were chunks of elemental metals, such as mercury and niobium, cooled to a few degrees above absolute zero. At such frigid temperatures, electrical resistance vanishes because electrons no longer bump into atoms as they travel and give up a bit of their energy to heat.
New materials, long-sought goal
Using 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 (⬤). Now, the race is on to find versions stable at ambient pressure and room temperature.0 100 200 300 CSH LaH 10 YH 9 H 3 S Pressure (gigapascals) 100 200 300 Desirableregion Superconducting temperature 0 Ambient pressure and room temperature LaBH 8 H S Hydride structuresThe atomic structure hasn’t been determined for all hydride superconductors. But so far they fall into two general classes, with examples shown below. Sulfur hydride (H 3 S) In this group of hydrides, hydrogen atoms (white) bind directly to other light atoms, such as sulfur (yellow). Lanthanum hydride (LaH 10 ) In this class, hydrogen atoms (white) form a cage surrounding electron-rich atoms, such as lanthanum (blue). H La
In 1933, German physicist Walther Meissner noted an added feature of superconductors: They expel magnetic fields. The phenomenon remained a mystery until 1957, when physicists John Bardeen, Leon Cooper, and Robert Schrieffer explained those first superconductors. Their “BCS theory” suggested an electron whizzing through a superconducting metal deforms the material’s atomic lattice, drawing positive atomic nuclei toward it ever so slightly. That redistribution of charges around an electron pulls another electron behind it, akin to how the weight of one person on a bed draws a second person toward them. That effect explains the resistance-free flow. The exclusion of magnetic fields, known as the Meissner effect, is a byproduct of the free-flowing electrons: When an external magnetic field meets a superconductor, the field induces an electric current to pirouette within the superconductor, which generates its own magnetic field that cancels out the external field.
The next major advance came in 1986, when J. Georg Bednorz and K. Alexander Müller, physicists in IBM’s Zurich Research Laboratory, discovered that copper oxide ceramics, known as cuprates, could superconduct at a relatively balmy 30 K. That “critical temperature,” or Tc, was soon pushed up in other cuprates to above 77 K, a temperature achievable with liquid nitrogen. That finding opened the door to broader applications because liquid nitrogen is far cheaper than the liquid hydrogen needed to chill the first superconductors. By 2005, researchers had pushed the Tc as high as 166 K (or –107°C), under high pressure, with a mercury-based cuprate. But then the march up the thermometer stopped.
Another potential route to high-temperature superconductors beckoned: hydrogen. In 1968, Neil Ashcroft, a theoretical physicist at Cornell University, suggested putting hydrogen under intense pressure would turn the gas into a solid lattice able to superconduct. His ideas languished because vises weren’t yet producing the necessary pressures; not until design refinements came along could groups including Silvera’s try to make solid metallic hydrogen. And then a new line of research opened up.
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. “It’s all moving very fast,” says Lilia Boeri, a theoretical physicist at the Sapienza University of Rome.
Too fast, some researchers argue. 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. “They have experiments that involve clever techniques to show the magnetic field is reduced inside,” says Marvin Cohen, a theoretical physicist at UC Berkeley. But that’s not good enough for many scientists in the field. “I’m old fashioned,” says Paul Chu, a physicist at the University of Houston and a veteran of battles over superconductivity in the cuprates. “I always like to see the Meissner effect. I haven’t seen it.”
Another problem is the resistance data, Hirsch says. When an external magnetic field is applied to a superconductor, the Tc typically drops, with a greater drop for stronger magnetic fields. In a graph of temperature versus electrical resistance, the curve normally showing the drop in electrical resistance at Tc flattens out as the external field increases. And higher temperature superconductors typically show a flatter slope. But in the hydrides, the highest temperature superconductors of all, “they don’t see this,” Hirsch says. “Either this is a nonstandard superconductor, or it is not a superconductor.”
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.
Dias adds that in some hydrides, the resistance may not behave as expected because the tiny samples in the diamond cell may be highly purified. He says the resistance curve flattening in other superconductors as an external magnetic field increases is actually an artifact of impurities, which can affect a material’s properties. Highly pure superconductors, such as the classic superconductor magnesium diboride, show a sharper drop. “Our results are in line with MgB2,” Dias says.
Either this is a nonstandard superconductor, or it is not a superconductor.Jorge Hirsch, University of California, San Diego
But Hirsch has another concern about Dias’s CSH material: Its magnetic susceptibility, a measure of how much a material becomes magnetized in an applied magnetic field, does not behave like other superconductors’. To test magnetic susceptibility, researchers apply a magnetic field to a potential superconductor as it is cooled. In standard superconductors, magnetic susceptibility drops as the material is cooled below Tc and stays low as the temperature continues to drop. Dias, however, reported that CSH’s magnetic susceptibility drops as the material cools below Tc but then rises again as cooling continues. “A superconductor doesn’t do that,” says Hirsch, who concludes that either the data are wrong or the material isn’t superconducting. He has asked the authors, the editors at Nature, and the National Science Foundation, which funded the work, to supply raw data, but so far has not been given access. “This has me very frustrated,” Hirsch says.
Dias says his team is filing patent applications, so his lawyers have asked him to withhold the data for now. He adds that the apparent rise in magnetic susceptibility below Tc is an artifact. That increase isn’t unusual in high-pressure experiments, Dias says, because they produce a background signal that can overwhelm the experimental signal.
Dias and others concede one of their critics’ points: So far, the Meissner effect is impossible to show inside a vise. However, the researchers say a technique that measures a property known as AC susceptibility accomplishes much the same thing. The technique involves using tiny magnetic coils next to the sample to create an oscillating magnetic field and then watching for any induced voltage change in the material. As superconductors are cooled below Tc, the voltage typically shows a drop. And the hydrides do exactly that. “All the evidence we are seeing is in line with the BCS model,” Dias says.
Eva Zurek, a theoretical chemist at the University at Buffalo, and Louie are confident hydrides will pass muster as superconductors, noting that multiple groups have reported evidence of superconductivity in sulfur hydride (H3S), lanthanum hydride, and yttrium hydride. “To me, that’s convincing,” Zurek says.
However, even other hydride experimentalists are hesitant to sign off on Dias’s room-temperature superconductor, which remains a one-off result. “We’ve been trying to replicate this for the past 6 months, but so far we haven’t seen it,” Eremets says. Katsuya Shimizu, a physicist at Osaka University who has made H3S, is also cautious. “I don’t believe it before I repeat it,” Shimizu says. Dias urges patience. “It will take time to resolve all this,” he says. “It took years for us to get there. You can’t expect others to get there in a few weeks.”
If the CSH results do hold up, that raises an important question: “Is it possible to get to ambient [pressure]?” Dias asks. “That’s what we’re all pushing for now.”
The first order of business will be to find other hydrides that superconduct at room temperature. In normal superconductors, Tc depends on two main factors. First is the abundance of electrons in the material that aren’t stuck to individual atoms but are free to conduct. Typically, the more conducting electrons, the better. Second is how fast atoms in the crystalline lattice vibrate—the lattice vibrations essentially bind pairs of electrons, and faster is better. Hydrogen, the lightest atom, vibrates fastest, but a lattice of hydrogen atoms isn’t very rigid and easily falls to bits. So scientists have to apply external pressure to keep it from flying apart.
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 (see graphic, above).
Dias and colleagues, including Suxing Hu, a theoretical physicist at U of R, believe CSH forms such an interlocking grid. In a 13 May preprint, researchers including Russell Hemley, a chemist at the University of Illinois, Chicago, shone x-rays at a CSH sample made by Dias’s group in a diamond cell. Hemley’s group saw evidence of a regular crystallization pattern of sulfur atoms—at least up to 178 GPa, when the diamonds broke. Hu and colleagues then used Hemley’s data to model a structure for CSH. The team found that CSH most likely adopts the same cubelike structure as H3S, which superconducts at a temperature 80 K lower than CSH does. Because carbon tends to form stronger bonds to its neighbors than sulfur or hydrogen do, carbon may be what’s holding the lattice together at room temperature.
To reduce pressure further, Hu, Dias, and others suggest adding more carbon, or perhaps boron, another strong bonder. In fact, Boeri and colleagues published a paper in Physical Review B on 15 July predicting that lanthanum borohydride (LaBH8) could superconduct at 126 K under just 50 GPa of pressure. Other 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 is more upbeat. At the March meeting of the American Physical Society, he said he had 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. If it holds up, he predicts, “it will be a significant step forward. I think we’ll get there soon.”
He’s banking on it. He recently formed a company called Unearthly Materials to manufacture and commercialize hydride superconductors, if any can be found to work near ambient pressures. If researchers can ease the pressure a bit more, to below 10 GPa, they can ditch the diamond anvil cells. The door would then be open to making larger samples of hydrides, allowing research groups to enter the fray even without access to cells, and enabling straightforward probes for the Meissner effect, the superconductivity signal that skeptics are still waiting to see.
What began with crushed diamonds could turn into a gold rush.
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