In 1935 a prediction by Eugene Wigner and Hillard Huntington that hydrogen would become a metal at sufficiently high pressures started a race—one that’s turned out to be a grueling marathon—to create the elusive phase. Now Marcus Knudson and Mike Desjarlais of Sandia National Laboratories and their colleagues have used Sandia’s Z machine (see Physics Today, June 2014, page 24), shown below, to get liquid deuterium to shed its molecular bonds and transition into an atomic metallic state. The researchers invented a new experimental configuration in which a precisely shaped electromagnetic pulse first shocks the deuterium to somewhere between 20 GPa and 50 GPa and heats it to 800–1400 K and then more gently ramps the pressure beyond 300 GPa while keeping the temperature less than 2000 K. That gentle ramp was crucial because theorists think that the putative insulator-to-metal transition terminates at a critical point somewhere in the vicinity of 2000 K. Optical measurements taken during the experiments showed that near 300 GPa, the liquid suddenly becomes reflective, a sign that the D2 molecules have dissociated and become an atomic metal. Knudson, Desjarlais, and their colleagues point out that theoretical models disagree widely about where exactly the transition line is in temperature–pressure space. If the new results stand up to independent scrutiny, theorists investigating hydrogen metallization will finally have a benchmark target. (M. D. Knudson et al., Science 348, 1455, 2015.)
