Livermore Scientists Achieve Metallic Hydrogen

The first confirmed formation of a metallic state of hydrogen was announced at the March Meeting by scientists at Lawrence Livermore National Laboratory. Metalic hydrogen was achieved in a sample of fluid hydrogen, using a two-stage gas gun to create enormous shock pressure on a target containing liquid hydrogen cooled to 20K. Future experiments will be aimed at learning more about the dependence of metallization pressure on temperatures achieved in liquid hydrogen, which is vital for laboratory applications.

"Metallization of hydrogen has been the elusive Holy Grail in high-pressure physics for many years," said William Nellis, one of three Livermore researchers involved in the project, of the achievement. "This is a significant contribution to condensed matter physics, because a pressure and temperature that actually produce metallization have finally been discovered."

Hydrogen atoms constitute the bulk of the universe's ordinary matter, so scientists have long sought to understand the properties and phases of this simplest of elements. Squeezing hydrogen atoms until they surrender their electrons has been tried ever since Eugene Wigner and Hillard Huntington predicted in 1935 that hydrogen would metallize at sufficiently high pressure. Virtually all predictions have been made for solid hydrogen at low temperatures near absolute zero. The Livermore results were surprising because they looked at liquid hydrogen at relatively high temperatures, for which no predictions had been made.

It was long thought that the road to metallic hydrogen lay with crystalline hydrogen rather than with the disordered fluid phase. According to Neil Ashcroft of Cornell University, dynamic shock techniques to achieve high pressures were first introduced in 1942. Optical evidence of a new phase of hydrogen has been previously reported by scientists at the Carnegie Institute of Washington's Geophysics Laboratory, using an experimental approach that involves crushing microscopic-sized samples of crystalline hydrogen between diamond anvils, achieving pressures up to 2.5 Mbar, but without establishing metallic character. Metallic character is most directly established by electrical conductivity measurements, which are not yet possible in diamond anvil cells at such high pressures.

Thus, Nellis was somewhat surprised when he succeeded at lesser pressures with fluid hydrogen. His team used a two-stage gas gun to compress samples of liquid H2 and D2. In the first stage, gunpowder was used to drive a piston down the pump tube, compressing hydrogen gas ahead of it. At sufficient pressure, the hydrogen breaks through a rupture valve and accelerates a projectile down the second-stage barrel, generating a strong shock wave on impact with an aluminum sample container. Upon impact with the cooled liquid hydrogen, the shock pressure first drops, then reverberates many times between parallel sapphire anvils until the final pressure, density and temperature are reached. The temperatures achieved keep hydrogen in the form of molecules, rather than allowing them to break into atoms.

The Livermore team was able to make direct electrical measurements on a 1-inch-wide sample. A trigger pin in the target produces an electrical signal when struck by the initial shock wave. This is used to turn on the shock electronic conductivity data recording system, to determine if metallization has occurred. They observed that the sample's resistivity fell with increasing pressure, leveling off at a low value at pressures above 1.4 Mbar, about a million times Earth's atmospheric pressure.

In studying the span from insulator to conductor, physicists look at the energy gap, the difference between the highest filled electron energy level and the next available energy level, a level at which the electron is free to flow as part of an electrical current. In hydrogen at ambient pressures, the gap is 15 eV, big enough to qualify hydrogen as an insulator. In his shock-compression experiment, Nellis lowers the gap to only 0.3 eV, which is comparable to the thermal energy of the fluid.

Some of the theorists who predicted metallic hydrogen also believed the substance would remain metallic after the enormous pressures required to produce it were removed, and Ashcroft theorized as early as 1968 that it might be a superconductor. But the metallization events at Livermore occurred too brief a period of time to detect these effects if they occurred.

Properties of metallic hydrogen is of interest to astronomers who model the interiors of gas giants like Jupiter and Saturn, which are expected to harbor vast reservoirs of metallic fluid hydrogen. For example, the results suggest that the boundary between the core and mantle of Jupiter is continuous, rather than the discrete barrier predicted in the past. In addition, the Earth-based measurements indicate that the conductivity of Jovian hydrogen is 100 times smaller than that predicted by other models, but that the extent of the magnetic core is much larger. The results might account for the relatively large magnetic field of Jupiter.

Metallic hydrogen's light weight might also have interesting implications for materials science. "The potential uses of metallic hydrogen are fascinating to contemplate, but they are far down the road, and we've only reached the first mile post on that road," said Nellis.