Glimpse of metallic hydrogen - Dr. Ross Howie

JANUARY 7, 2016

Driven by the predication of metallic hydrogen, the first and simplest element, there has been 80 years’ worth of combined theoretical and experimental effort to try to reach this predicted state in hydrogen. New breakthrough work from a team including Philip Dalladay-Simpson, and Ross T. Howie, two new scientists of HPSTAR, have carried out static compression on hydrogen and its isotopes above 380 gigapascals; higher pressure than any previous study. The work indicates another new phase, phase V, is detected in both hydrogen and hydrogen deuteride at pressures above 325 gigapascals at room temperature. Phase V could provide a glimpse of the theoretical predicated metallic hydrogen. This breakthrough discovery is published on January 07, 2016, in Nature.

The quest for metallic hydrogen at high pressures represents a longstanding problem in condensed matter physics. By now there has been more than five phases found in hydrogen since 1935, while metallic hydrogen remains illusive. Researchers continuously need to improve techniques and challenge experimental difficulties to gain even higher pressure conditions.

There are three major technical drivers in this pursuit: theoretical calculations and dynamic and static compressions. Static compression of hydrogen to very high pressure is technically very challenging. Dr. Ross Howie of HPSTAR and his colleagues from The Univeristy of Edinburgh have improved resistively heated diamond-anvil cell techniques and methods to solve hydrogen containment, enabling them to reach pressure over 300 GPa in pure hydrogen and its isotopes.

Using in situ high-pressure Raman spectroscopy, Howie et al., found phase IV (Phys. Rev. Lett. 108, 125501, 2012), which was the first solid phase of hydrogen to be discovered over a 25 year hiatus. Very recently, Ross and co-workers also investigated the high pressure melting conditions of hydrogen, indicating a liquid phase at unusually low temperature ( Nature Materials) (>200 GPa and 550–1,300 K). Such measurements have long been an experimentally inaccessible "no man's land" for hydrogen.

The new study published in Nature, using similar techniques, carried out high-pressure compression up to above 380 gigapasals to probe new states of hydrogen. It is so far the highest pressure reported on hydrogen from static techniques.The substantial decrease in intensity of vibrational Raman bands, the change of slope of the vibrational-mode frequency with pressure, and changes in position, width and intensity of the low-frequency modes, indicate a transition to a new structure, named phase V of H2 and HD above 325 GPa.

Caption: Proposed phase diagram of hydrogen up to 400 GPa.

“We speculate that phase V might be the onset of metallic hydrogen,” described in the paper,” “then it will bring out series of new questions about the current phase diagram of hydrogen”, said Ross, staff scientist of HPSTAR.

What is the connection between the proposed metallization and phase V is also waiting to be answered.

“Our data could provide invaluable information about the fundamental physics and chemistry that governs the behaviour of the simplest element at high denstities”, said Philip, a new postdoc will join HPSTAR soon.

“The discovery of phase V is another major breakthrough in hydrogen followed by phase IV,” said Dave Mao, HPSTAR director, “If phase V is indeed a precursor to metallic state, then the metallic hydrogen should be possible and closer to be found.”

Other access to the work:

BBC: Science in Action, http://www.bbc.co.uk/programmes/p03dtmfn