Metallic hydrogen makes its debut, maybe

Hydrogen gas squeezed at tremendous pressures has transformed into a metal in the laboratory. So say a pair of scientists in Germany, whose bold claim is being met with skepticism.

Many scientists have tried to make metallic hydrogen since its existence was first predicted in 1935. The exotic substance is thought to form at high pressures, such as those in Jupiter’s core. It may be a superconductor at room temperature, useful for making wires that carry electricity with little loss of current. And NASA hopes to one day put it to work as a rocket fuel that would be more powerful than anything around today. 

“Making metallic hydrogen is often considered the Holy Grail for high-pressure physics,” says Mikhail Eremets, a physicist at the Max Planck Institute for Chemistry in Mainz, Germany, who with Ivan Troyan reported the results online November 13 in Nature Materials.

To see if hydrogen could be made to conduct electricity, Eremets and Troyan squeezed a room temperature sample of the gas between two diamonds. At record-breaking pressures more than 2.3 million times that of Earth’s atmosphere, the hydrogen became opaque and reflective. Its resistance to the flow of current dropped to one ten-thousandth that of hydrogen at lower pressures.     

That’s evidence that the gas changed into something else, say the researchers. To show that this new substance was a metal, they cooled it from room temperature to 30 kelvins. The resistance rose slightly, but the material remained conductive.

“[The finding] has stimulated a lot of activity and is going to have a big impact on attempts to produce metallic hydrogen,” says Isaac Silvera, a physicist at Harvard University who has been trying to make the stuff for more than 30 years.

But while Silvera agrees that the gas changed under pressure, he and other physicists aren’t convinced that the hydrogen changed into the long-sought metal.

“People have thought they created metallic hydrogen before, and they turned out to be wrong,” says William Nellis, a physicist now at Harvard. In 1996 he and colleagues at Lawrence Livermore National Laboratory in California used shock waves to make hydrogen that conducted electricity (SN: 4/20/96, p. 16) but survived only for a fraction of a second, not long enough to definitively prove that it was a metal.

Nellis worries that interactions between hydrogen and the equipment used in the new experiment — the metal electrodes inserted into the hydrogen or the epoxy in the gasket holding it in place — may have muddied the German group’s measurements. Hydrogen is extremely reactive at high pressures.

Arthur Ruoff, a physicist at Cornell University, says the increase in resistance when the hydrogen was cooled doesn’t make sense. In a typical metal, resistance would decrease. For exotic types of conductors, it can increase — but that increase should have been more dramatic than the 20 percent rise measured by Eremets and Troyan, says Ruoff.  

To satisfy his critics, Eremets plans to refine this tricky experiment. But he’s not surprised that the result has proven controversial.

“Hydrogen attracts so much attention in our field,” says Eremets. “Of course there will be a lot of emotion, of course there will be a lot of demands.”