Newly published results offer compelling evidence that hydrogen is a metal at extremely high pressures. But is the research enough to convince the field at large that metallic hydrogen exists?
Theorists have long predicted that, at very high pressures (over 4 million times the pressure of the atmosphere at Earth’s surface), hydrogen should exist as a metal, a material that conducts electricity. Researchers have hunted for this material, at times ending up with dubious results. But in the past few years, teams in the U.S., France, and Germany have made strides at confining and compressing hydrogen and probing its properties at high pressure. A new paper finding the strongest evidence yet of metallic hydrogen has been peer reviewed and published—but search isn’t over yet.
We covered this research when it first appeared on the arXiv physics preprint server last June. To recap, physicist Eugene Paul Wigner predicted 80 years ago that, as you crank up the pressure, hydrogen should turn into a solid and eventually into a metal. Creating these incredible pressures to confirm the theory is extremely difficult. The team behind the new research, led by Paul Loubeyre at France’s Atomic Energy Commission, compressed hydrogen between the tips of diamonds using a device called a diamond anvil cell, then observed the sample using infrared radiation at the SOLEIL synchrotron in France.
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At pressures above 300 GPa (aka 300 billion pascals; the Earth’s atmosphere at sea level has a pressure of only 101,325 pascals), the hydrogen became solid and opaque to visible light. At 425 GPa and 80 degrees above absolute zero (0 Kelvin, the temperature where all matter has the minimum amount of heat), the hydrogen became opaque to infrared light—it reflected infrared wavelengths. This is evidence that the hydrogen had transitioned from a solid state into a metallic state.
Loubeyre told Gizmodo this week that they reversed the process, performing more measurements after loosening the pressure between the diamonds to confirm that the hydrogen hadn’t seeped out from between the tips. The key to their results, Loubeyre said, was twofold: First, they used a relatively new kind of diamond anvil cell, called a toroidal diamond anvil cell, with specially formulated diamond tips that can withstand higher pressures than traditional diamond anvil cells. Second, their experiment at SOLEIL combined a microscope with a wavelength-measuring spectrometer, allowing them to measure multiple properties of the hydrogen in the diamond simultaneously.
This week, the team published the peer-reviewed results of that research in the journal Nature, a sign that they’ve held up to scrutiny. The results are a close-to-definitive proof of the creation of metallic hydrogen, according to a Nature commentary by Serge Desgreniers, a University of Ottawa physicist who was not involved in the work.
But announcing a discovery isn’t like flipping a light switch; in many cases, it’s more like raising a dimmer. Even now that the paper has gone through peer review, the scientists don’t claim that they’ve observed metallic hydrogen, as demonstrated by the published paper’s title, which says only that they’ve seen evidence of the “probable transition to metal hydrogen.” Limits to the sensitivity of their equipment meant they couldn’t rule out the existence of a small bandgap—a small amount of input energy required to turn the material into a conductor—and if such a bandgap were truly there, then they haven’t demonstrated the creation of a metal yet. They write that it’s unlikely for the bandgap to be there, but announcing a discovery requires covering all of your bases.
One researcher not involved in the study, Zack Geballe from the Carnegie Institution for Science’s Geophysical Laboratory, told Gizmodo that the work is a milestone that will lead to “further exploration of the metallic properties of this hydrogen and whatever else exists at higher pressure.”
Geballe also said it was time for electrical measurements, literally putting electrodes on either side of the material and measuring current through it. These are difficult measurements, since they rely on putting tiny electrodes on the tip of a diamond and in contact with teensy amount of high-pressure solid hydrogen. Another paper, from a team led by Mikhail Eremets, performed conductivity measurements on their own hydrogen sample and found that from around 350 GPa up to 440 GPa, hydrogen stayed a molecular solid, meaning its atoms were still bonded together, rather than sitting as nuclei in a network of mobile electrons, though a further transition into a metal may have occurred at pressures of 480 GPa.
Loubeyre told Gizmodo that these papers measure pressure slightly differently and are likely compatible—that, based on their own calculations, what Eremets’ team measured at 440 GPa might have actually occurred at 390 GPa, and the groups may be observing the same effects. But the search continues.
Loubeyre’s work is exciting and provides robust evidence that hydrogen begins to take on metal-like properties at high pressures, as was predicted so many decades ago. It also demonstrates that making a conclusive claim, such as “we’ve created metallic hydrogen,” is incredibly difficult in science and requires lots of evidence and checking between multiple teams.
“I think the community recognises that the measurements are exceptional quality for this pressure range, and we’re proud of that,” Loubeyre told Gizmodo. “But now that we can go into this pressure range, we’ll be able to do many more measurements.” The research could keep scientists busy for another decade as they continue to elucidate high-pressure hydrogen’s weird properties.