By: Felipe Flores
The generation of metallic hydrogen by Professor Isaac Silvera and postdoctoral fellow Ranga Dias, PhD represents a crucial advance in the field of high-pressure physics. Originally theorised in 1935 by physicists Eugene Wigner and Hillard Bell Huntington, the ongoing project was finally brought to fruition at Harvard University in January 2017. Both Silvera and Dias agreed to be interviewed for this article.
What was the key to the discovery?
Generating a sample of metallic hydrogen has been an ongoing project for many years in several laboratories, and the Silvera lab has finally succeeded. An essential component to Silvera and Dias’ success was achieving the correct pressure to obtain a transition to a metallic state. While the necessary density of the metallic phase was predicted accurately in the 1930s, the predicted pressure to achieve a metallic phase at the time was around 25 GPa, while modern predictions placed the figures at 400-500 GPa (1,2). Indeed this recent success was obtained with a pressure of 495 GPa, a pressure never achieved before in hydrogen experiments (3). Achieving such pressure had its challenges: “The diamonds we use to contain the samples tend to break or allow diffusion,” explained Dias; “the hydrogen sample can diffuse into the diamond and cause defects, which will weaken the diamond and make it break before reaching the ultra-high pressure needed ” (4). The key therefore, lies in modifying the diamond to make it sustain the immense force necessary for this experiment’s success. Dias summarized the special technique as “adding a diffusion barrier to a very polished diamond with as little defects as possible” (4). Once the scientists achieved the ultra-high pressure and observed a phase transition, they measured the reflectivity of the sample to be consistent with that of a metal, as well as obtained a density in agreement with theoretical predictions.
Why is the discovery so exciting?
Theoretical calculations predict metallic hydrogen to be a metastable material as well as a room-temperature superconductor (5,6). Metastability means the material would remain in the same metallic state even after the high pressure is released (just like diamond is a metastable form of carbon). If such is the case, metallic hydrogen could potentially be an extremely efficient way of storing energy, for instance to be used as a rocket propellant. Superconductivity means the material could carry currents without any resistance or energy loss. Such a property has only been achieved at extremely low temperatures in other materials, while metallic hydrogen is theorized to behave this way at 17˚C, far higher than other candidates. If both properties are confirmed to be true, we might see a revolution in electronics and transportation. For instance, magnetic levitation vehicles could become more accessible, electronics could become more efficient, and space travel could become cheaper and to farther distances.
What is the current status of the experiment?
Unfortunately, the sample was accidentally destroyed, a common fate in high-pressure physics, as both Professor Silvera and Dias explained (7). In order to create the metallic hydrogen, Silvera and Días followed procedures necessary ensure not to break the deice’s compressing diamonds. Upon submitting their paper, they kept the hydrogen at a low temperature and at a high pressure until the acceptance of their work, in case the pair was instructed to perform other tests. The crystals were slowly developing defects while held under the stress of a high pressure during the evaluation of the paper, and when Silvera and Dias later shone a low-energy laser upon the sample, the diamonds broke and the metallic hydrogen was lost. “It was a surprising that the diamonds broke with such a low-energy laser,” (7) said Professor Silvera. However, it is also uncommon to retain samples compressed for such a long time. “The accumulation of defects over all that time was probably responsible,” added Silvera (7).
The discovery has not been met absent of criticism and skepticism, especially on the reproducibility of the experiment. The Silvera lab is currently reproducing their experiment, although with a few tweaks to the procedure, such as the use of a different type of diamond. Another central criticism was the pair’s use of alumina to create a diffusion barrier around the hydrogen, as some thought the metallic nature of the sample could stem from the aluminium casing. However, Dias is not concerned about this possibility, explaining that even at pressures such as 400 GPa there has been no observed change in aluminium, and as such it is highly doubtful that it would be the culprit. Additionally, Silvera and Dias utilised a layer of only 48 nanometres of aluminium in their experiment, which, they say, is so thin it could not be the source of metallic properties they measured.
How do you feel about the future of the experiment?
“Hopeful,” said Silvera; “optimistic,” said Dias (4,7). If their success is repeated, the scientists will transport their sample to Argon Labs, near Chicago, in order to x-ray the metallic hydrogen sample, examine its structure, and determine whether it is indeed the desired metallic phase and whether it is metastable, amongst other things. While the future of metallic hydrogen is yet to be determined, the confident attitudes of Silvera and Dias are encouraging for expectant scientists around the world as physics embraces its newest exciting discovery.
Felipe Flores ‘19 is a sophomore in Quincy House studying Human Developmental and Regenerative Biology and Physics.
 Wigner, E.; Huntington, H. B. J. Chem. Phys. 1935, 3, 764-770.
 Silvera, I.; Cole, J. J. Phys. Conf. Ser. 2010, 215, 012194.
 Dias, R.; Silvera, I. Science. 2017, 355, 715-718.
 Flores, F. Interview with Ranga Días, PhD. Apr. 4, 2017.
 Ashcroft, N. W. Phys. Rev. Lett. 1968, 21, 1748-1749.
 Brovman, E. G et al. Sov. Phys. JETP. 1972, 35, 783-787.
 Flores, F. Interview with Prof. Isaac Silvera. Apr. 7, 2017.
Categories: Spring 2017