Hydrogen is the simplest and most abundant element in the universe. But that simplicity belies its often unpredictable nature. A case in point: Unlike the alkali metals that sit below it on the periodic table, hydrogen, even in its solid phase, remains a molecular insulator down to the lowest temperatures.

In 1935 Eugene Wigner and Hillard Huntington predicted that squeezing solid hydrogen to a sufficiently high pressure could cause it to shed its molecular bonds and transform into an atomic metal. The race to find the insulator-to-metal transition in hydrogen was on, but it’s turned out to be a marathon rather than a sprint.

High-pressure experiments are notoriously difficult, and ones on hydrogen even more so. Diamond-anvil cells, the go-to equipment for static-compression experiments, are hampered by hydrogen’s tendency to penetrate into the diamond and cause cracks. Dynamic experiments using shock compression reach higher pressures, but they heat the sample to high temperatures and only access specific values of pressure and temperature that depend on the system’s initial state. Still, experimentalists have subjected hydrogen to pressures of 320 GPa using static techniques and 500 GPa using dynamic methods but have not found the metallic phase.

transition. The group used Sandia’s Z machine to dynamically compress liquid deuterium 1 et al. , Science 348, 1455 (2015). 1. M. D. Knudson, Science, 1455 (2015). https://doi.org/10.1126/science.aaa7471 pressures greater than 300 GPa at temperatures between 1000 K and 2000 K. Their measurements indicate that the liquid abruptly goes from being an insulator to being a metal at about 300 GPa. Now Marcus Knudson and Mike Desjarlais of Sandia National Laboratories and their colleagues at Sandia and the University of Rostock in Germany think they have sighted the elusiveThe group used Sandia’s Z machine to dynamically compressdeuteriumtogreater than 300 GPa at temperatures between 1000 K and 2000 K. Their measurements indicate that theabruptly goes from being anto being aat about 300 GPa.

A shocking squeeze Section: Choose Top of page ABSTRACT A shocking squeeze << Shiny happy deuterium REFERENCES CITING ARTICLES 1 pressures. (See Physics Today, liquid deuterium. The world’s most powerful pulsed-power generator, the Z machine, shown in figure, is best known for inertial confinement fusion and weapons-related research. But under Sandia’s Z Fundamental Science Program, it also serves as a powerful tool to study the properties of materials at extreme temperatures and(See June 2014, page 24 , and the article by Paul Drake, June 2010, page 28 .) For their experiment, Knudson, Desjarlais, and their team used the intense magnetic field that accompanies the Z machine’s electromagnetic pulse to squeezedeuterium. Researchers have caught signs of metallized hydrogen before. In 1996 Samuel Weir, Arthur Mitchell, and William Nellis used a gas gun to fire metal disks at liquid hydrogen and deuterium targets 2 76, 1860 (1996). 2. S. T. Weir, A. C. Mitchell, W. J. Nellis, Phys. Rev. Lett., 1860 (1996). https://doi.org/10.1103/PhysRevLett.76.1860 pressures up to 180 GPa (see Physics Today, pressures greater than 140 GPa, they saw resistivity behavior indicative of a liquid metal. have caught signs of metallized hydrogen before. In 1996 Samuel Weir, Arthur Mitchell, and William Nellis used a gas gun to firedisks athydrogen and deuterium targetsto achieve shock-inducedup to 180 GPa (see May 1996, page 17 ). Forgreater than 140 GPa, they saw resistivity behavior indicative of a However, those observations were at high temperatures—around 3000 K. Theorists think that the putative insulator-to-metal transition line terminates at a critical point somewhere in the vicinity of 2000 K. Most of the hydrogen was probably still in a molecular state, and the metallization was more a crossover than a transition. The Sandia–Rostock team wanted to duck under that critical point and cross directly through the transition line. “The biggest experimental challenge we had was how to reach the high pressures necessary while keeping the temperature low,” explains Knudson. The researchers figured that if they precisely shaped the Z machine’s current pulse, they could combine two typical compression methods: shock and ramp. The initial part of the pulse magnetically launched an aluminum plate at the front face of an aluminum cryocell containing a 150-μm-thick layer of liquid deuterium cooled to 22 K. The shock reverberations from the 2- to 3.5-km/s impact heated the deuterium to between 800 K and 1400 K and drove pressure to 20–50 GPa. The remainder of the pulse, with its rising magnetic pressure, further compressed the sample along a gentler ramp so that the temperature remained below 2000 K, even as the pressure shot past 300 GPa. The team chose to use deuterium rather than hydrogen for their first crack at crossing the insulator-to-metal phase boundary because deuterium’s greater density made it easier to get the shock–ramp sequence right. Now that they have experience with their new technique, Knudson is confident that they can pull off a similar experiment with hydrogen.