Dec 02, 2019
Chinese scientists in Institute of Solid State Physics (ISSP) under Hefei Institutes of Physical Science announced they have successfully synthesized fluid metallic hydrogen under extreme high-pressure and high-temperature conditions which were created by a research platform developed themselves.
The team applied diamond anvil cell and pulsed laser heating method in their labto create extreme P-T conditions above 150 GPa-3000 K, which was essential to transform insulating hydrogen gas to fluid metallic hydrogen.
By using pulsed ultrafast broadband-laser probes, the researchers detected successive phase transitions from transparent molecular hydrogen to absorptive semiconductor and reflective fluid metallic hydrogen, revealing exact optic and electrical properties during hydrogen metallization which had not been well described.
As is well known to all, hydrogen is the most common element in the universe, it exists as diatomic molecule at normal conditions on our planet. However, under extreme conditions, changes occur in hydrogen molecule to enable transition from transparent molecule to reflective atomic state, then metallic hydrogen got.
Metallic hydrogen has the highest energy density making itself predicted as room-temperature superconductor which has been evidenced by hydrogen-rich compounds with Tc as high as 260 K.
Due to its exactly great physical significance and potential applications, the metallic hydrogen is regarded as the pearl in the crown in high pressure field.
However, when it comes to the obtaining of metallic hydrogen, there are many challenges lying ahead.
How to realize the extreme pressure above 400 GP which is the precondition to obtain conductive hydrogen is one of them. After all pressure in the core of our earth is only 360 GPa. And further, the metallic hydrogen is really sensitive to any even tiny vibrations and energy disturbance which makes the detection much more difficult.
The team addressed the challenges by exploring the electronic states of hydrogen and deuterium in the P-T range where the insulator-metal was previously reported but not sufficiently characterized in experiments.
To overcome the challenges in sustaining hydrogen at these conditions and probing it spectroscopically, they applied microsecond pulse laser heating in combination with pulsed broadband-laser probing.
The study showed clearly the transition in P-T space included several stages where hydrogen transformed from a transparent insulating state, to an optically absorptive narrow-gap semiconducting state, and finally to a metallic state of high reflectance. The reflectance spectra also suggested much longer electronic collision time than previously inferred, implying that metallic hydrogen at the conditions studied was not in the regime of saturated conductivity (Mott-Ioffe-Regel limit).
The results confirmed the existence of a semiconducting intermediate fluid hydrogen state en route to metallization.
The team believed their work would stimulate further thinking on hydrogen metallization process.
This work acknowledged the funding support from the Natural Science Foundation of China.
By applying microsecond single- to several- pulse laser heating in a diamond anvil cell in combination with pulsed broadband-laser probing detected via a streak camera we determined onset of metallic optical reflectance in fluid hydrogen and deuterium above 3500(1000) K at 150-172 GPa. (Image by JIANG Shuqing)
The phase diagram of hydrogen (deuterium) at high pressure-temperature conditions. the fluid metallic hydrogen was observed above 150 GPa-3000 K along a phase boundary with a negative slope. (Image by JIANG Shuqing)
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