2017-01-27 11:59:10
Hydrogen Squeezed Into a Metal, Possibly Solid, Harvard Physicists Say

11:59, January 27 977 0

Squeezed between two pieces of diamond, hydrogen has been transformed into a metallic form believed to exist inside giant planets like Jupiter, scientists reported on Thursday.

“You can see it becomes a lustrous, shiny material, which is what you expect for a metal,” said Isaac F. Silvera, a professor of physics at Harvard.

If some theoretical predictions turn out to be true, the new state of hydrogen could even be a solid metal that is metastable — remaining solid even after the crushing pressure is removed — and a superconductor, able to conduct electricity without resistance, Dr. Silvera said.

Dr. Silvera and Ranga P. Dias, a postdoctoral researcher, published the findings on Thursday in the journal Science.

But in the small but contentious field of high-pressure physics, some scientists who perform similar experiments were harshly skeptical and wondered how the research passed peer review at a top journal like Science.

“It’s — how should I put it? — the product of Ike’s imagination from the title to the end,” said Eugene Gregoryanz, a physicist at the University of Edinburgh in Scotland.

Paul Loubeyre, a physicist at France’s Atomic Energy Commission, wrote in an email, “The fact that the paper went through illustrates the fact that the reviewing process has some flaws.”

Editors at Science declined to discuss the paper, but in a statement, Jeremy Berg, the editor in chief, said that submissions must pass rigorous review by experts and that only about 7 percent are published.

Dr. Silvera defended his work. “If we did it again, we’d get the same result, I’m certain,” he said.

Hydrogen is the lightest of elements; each atom consists only of one proton and one electron. In the ordinary conditions at Earth’s surface, where the weight of air presses at 14.7 pounds per square inch, hydrogen atoms pair up into simple molecules.

At supercold temperatures, hydrogen molecules first condense into a liquid, then a solid with the molecules intact. When squeezed together, the molecules stack up into a solid form. More than 80 years ago, the physicists Eugene Wigner and Hillard Bell Huntington predicted that hydrogen, under high enough pressures, would turn metallic, with the hydrogen molecules broken apart and the electrons squeezed loose.

Scientists have momentarily transformed hydrogen into a liquid metallic state with violent shock wave experiments. Planetary scientists are convinced metallic hydrogen must exist inside Jupiter, to generate that planet’s powerful magnetic fields.

But so far, no one has convincingly demonstrated the solid metallic hydrogen state predicted by Wigner and Huntington.

In the Harvard experiment, a small amount of hydrogen was placed between two diamond tips about one 850th of an inch in diameter and cooled to -433 degrees Fahrenheit. Initially transparent, the hydrogen became dark as it was squeezed. Then, at nearly 72 million pounds per square inch, the hydrogen became shiny, reflecting 90 percent of light shining on it, the scientists reported.

However, they did not make additional measurements to confirm other metallic properties, like the ability to conduct electricity as metals do, and they did not show that it is solid.

Only a handful of research teams around the world have been attempting to squeeze hydrogen into a metal. While the experiments are not large — the Harvard apparatus is about the size of a soda bottle — the techniques require care and precision. Imperfect diamonds often shatter under pressure. Hydrogen can leak out. Precise measurements are tricky.

Dr. Silvera said his group had developed techniques to polish, etch and coat the diamond surfaces with aluminum oxide to enable the diamonds to squeeze together without breaking.

Still, Dr. Gregoryanz was incredulous that the Harvard scientists could have achieved the ultrahigh pressure they report, which is 20 percent higher than their competitors, on the first try, in October, and that they have not yet attempted to duplicate the feat.

“You make such a big and bold claim,” said Dr. Gregoryanz, who said he typically has one success out of 10 to 15 tries. “Why didn’t you repeat the experiment?”

The pressure estimates are also indirect estimates. “That is probably the weakest point in this paper,” said Reinhard Boehler, a high-pressure scientist at the Carnegie Institution for Science in Washington, D.C., and Oak Ridge National Laboratory in Tennessee.

“Literally there was a screw turned, and a pressure was estimated,” he said. He said the optical measurements were also weak. “If true, fantastic, but there’s a lot of ifs and buts in this publication.”

Dr. Silvera said that he will be performing additional measurements called Raman scattering — shining laser light on it and seeing how the atoms are excited—- and then take it to Argonne National Laboratory in Illinois to probe it with a bright beam of X-rays. Those measurements, if successful, could tell more about the state of the hydrogen.

If the sample survives without breaking, Dr. Silvera said he wanted to then open it up, relieving it of pressure, and see if there is a speck of solid metallic hydrogen still inside. Some theorists have predicted that solid metallic hydrogen, if it can be produced, would remain in that state, in much the same way a diamond stays a diamond instead of reshaping itself into graphite, a lower-energy configuration of carbon atoms.