Probing Hydrogen above 300 GPa

Even though hydrogen is the most abundant element in the universe, an understanding of its properties under extreme pressure and temperature conditions is not yet complete. Ultimately, the fundamental physics behind the behavior of this seemingly simple element can inform and expand our understanding of matter as a whole. New work from Carnegie scientists has enabled an examination of hydrogen under static pressure-temperature conditions not possible previously [C. S. Zha et al., Phys. Rev. Lett. 108, 146402 (2012)].

Theory has predicted a variety of novel states for hydrogen at high pressure, including the existence of room-temperature superconductivity and a fluid, or even superfluid, ground state. The interplay between theory and experiment has provided stringent tests for theoretical models and computational methods and has made the study of this material a problem of the first rank in condensed matter physics for nearly 80 years. Experimentally, work on hydrogen presents numerous challenges associated with confining the diatomic molecular form under pressure in diamond anvil cells (DACs). With continuous advances in DAC techniques, new findings in the multimegabar pressure range continue to push the limits of current models for its behavior.

The Carnegie team of Chang-Sheng Zha, Zhenxian Liu, and Russell J. Hemley developed new techniques for containing hydrogen in DACs, reaching pressures of 360 GPa (3.6 million times atmospheric pressure) from 12 K to close to room temperature. Synchrotron infrared spectroscopy was carried out at beamline U2A at the National Synchrotron Light Source, Brookhaven National Laboratory. The measurements probed the transmission properties of hydrogen in order to look for the break down of the molecules to form the monoatomic and perhaps metallic state. The key signature of molecular hydrogen, the intramolecular vibration or vibron, persisted to the highest pressures achieved in the experiment, while sample remained transparent in the near to mid-IR range (Figures 1 and 2). The results indicate that the material is semiconducting, and possibly semimetallic, at these high P-T conditions.

The strong transmission of hydrogen in the mid-infrared range contradicts a recent claim that the metallic state of hydrogen was reached below 300 GPa in DAC experiments. Meanwhile, in another paper also published in Physical Review Letters, a team from the University of Edinburgh and including Carnegie’s Alexander Goncharov reports evidence for another phase of molecular hydrogen (near 220 GPa at 300 K). Their work focused on a higher temperature range and somewhat lower pressures, and did not measure the metallic properties. They suggests that the structure of hydrogen in this new phase is a honeycomb
made of six-atom rings, similar to the carbon structure called graphene [R. T. Howie et al., Phys. Rev. Lett. 108, 125501 (2012)].