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.
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 elusive transition. The group used Sandia’s Z machine to dynamically compress liquid deuterium1 to 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.
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