If hydrogen molecules could be pressured into transforming themselves into a metallic phase, would that material be a useful high-temperature superconductor? A Cornell University theoretical physicist posed just such a scenario in 1968. Now, almost 30 years after surmising it, he may be proved right.
"Suppose hydrogen could be a metal," said Neil W. Ashcroft, the Cornell physicist who speculated on this case, "would it have properties that would distinguish it? I proposed that, yes, lone among the elements, it could be a high-temperature superconductor."
But it took the explosive power of a two-stage gas gun, originally developed for weapons research at the Lawrence Livermore National Laboratory in California, to come up with what may be proof of Ashcroft's model. Scientists at Livermore and Ashcroft reported March 21 at the meeting of the American Physical Society (APS) in St. Louis that they have achieved metallic hydrogen in a form proposed by Ashcroft.
"This has been the holy grail of high-pressure physics," said Ashcroft, a scientist in Cornell's Laboratory of Atomic and Solid State Physics and former chair of the APS Division of Condensed Matter Physics. "We have been after this since the first predictions of a metallic form of hydrogen were made by Eugene Wigner and Hillard Huntington in 1935." The study of hydrogen as a metal is crucial for condensed matter physics. As a low-temperature liquid, it is an insulator. But as a high-density metal, it could be a viable superconductor, which could carry electricity with no or little resistance. At high temperatures, it could be used in applications where heat is generated, such as in controlled fusion for an abundant energy supply. The understanding of dense hydrogen would help in understanding the dynamics of Jupiter, for example, which is thought to have an abundant supply of hydrogen in metallic form.
"Hydrogen is a major player in the universe," Ashcroft noted. "It's very basic to physics and astrophysics. Understanding it in its metallic phase is critical to understanding planetary interiors." If this notion of its properties announced by the Livermore team is correct, then astrophysicists may have to change their models of the internal constitution of Jupiter, and thus, the basic ideas about how the gaseous giant planets formed.
The Livermore experimental physicists -- William J. Nellis and Samuel T. Weir -- a Cornell graduate -- used extremely high pressures to squeeze hydrogen to about 1.4 million atmospheres at very high temperatures -- about 4,000 degrees Kelvin. Their methods have proved accurate in the past for probing properties of physical matter. (One atmosphere is the equivalent pressure at the surface of the Earth.) They found that in the metallic phase, even if just for an instant before the sample was destroyed, the hydrogen molecules appeared to ionize, but remain paired. That means they formed what is called a paired metal, where the ions pair up and cannot be broken apart. In other metals, the crystal structures are usually based on single ions. But that appears not to be the case with hydrogen. "It's very tightly bound. It's a very strange effect. I thought it might be possible 28 years ago and the experimentalists say that they are seeing it now," Ashcroft said.
Another method of achieving high pressure is by squeezing a sample between a very hard surface but not destroying it -- known as a diamond-anvil technique. Arthur Ruoff, Cornell professor of materials science and engineering, routinely has squeezed hydrogen to 2.5 million atmospheres -- (the pressure at the center at the Earth is just over 4 million atmospheres) -- but without the high temperature needed to get the sample to convert into a metal.
"The whole theoretical effort at Cornell has been to look at low temperature properties of dense hydrogen," said Ashcroft, who wrote a recent review paper in Physics World (July 1995, p. 43) describing in part the difficulties in achieving metallic hydrogen.
Byard Edwards, Cornell doctoral student in Ashcroft's group, has been looking at paired state for several years. "We can now say that hydrogen is likely to remain paired in its low- temperature, high-density state," Ashcroft said. "We're on the right track; that's nice to know." Edwards presented a paper at the APS meeting on these studies.
These ionic pairings exist at pressures equal to more than 2 million atmospheres. "This is a fundamentally new way of looking at metal," Ashcroft said. "It seems to remain a paired metal, even in the high temperature liquid state. The robustness of this bond in quite extreme conditions is really quite remarkable."