High Pressure Physics
Our research is aimed at producing metallic hydrogen in the laboratory. But first a short discussion of high-pressure techniques.
The field of high-pressure physics started to intensify in the first part of the last century. P.W. Bridgman at Harvard developed many of the techniques that led to the rapid development of this field of physics, for which he was awarded the Nobel Prize.
Today, there are two methods of studying materials under pressure: static and dynamic high pressure. In the static technique, the sample is under pressure for extended periods of time, and can be either cooled in a cryostat or heated in an oven or laser heated. Although earlier work was carried out with large presses, some of them requiring enormous loads to produce high pressures on large samples, the modern trend has been to use diamond anvil cells (DACs) which achieve much higher pressures. In this case the samples are quite small, of order a few to a few hundred microns thick and 20 to 500 microns in diameter. Smaller samples go to higher pressures. The DAC is easy to set up and use by a single investigator and has optical windows to the sample—the diamonds. Pressures as high as 5 megabar have been reported. This is the method that we use: static pressure generated in a DAC (see fig. 1).
Fig. 1. DAC used for megabar research. The diamond anvils are in the lower part of the DAC; these are shown on the right (diamonds have dimensions 2-3 mm). The cable bundle is for measuring temperature, load, heating, etc. The DAC is about the size of a coke-bottle; it is long and narrow so that it easily fits into a cryostat with optical windows. A pressure of millions of bars can be applied to a sample by the simple finger-turning of a screw.
Hydrogen Under Pressue
Background: One of the important challenges to condensed matter physics has been to transform hydrogen to the metallic state by compressing it to very high densities. This problem was first set forth by Wigner and Huntington in 1935 who proposed that at a sufficiently high density the molecules at the lattice sites would dissociate to form an atomic lattice that would be a metal. In more than three quarters of a century since, the predicted pressure for metallization has risen from 0.25 megabar to the 4-6 megabar (400-600 GPa) region as experimentalists have probed to ever-higher pressures and theorists have refined their calculational techniques. The challenge to theorists is to incorporate the large zero-point energy into their calculations, whereas the popular density functional theory (DFT), does not incorporate this. The challenges to experimentalists is to achieve higher pressures. Everybody loves the challenge of high temperature superconductivity. The prediction by Ashcroft in 1968 that metallic hydrogen might be a room temperature superconductor spurred on the developments to produce this material.
There are two pathways to metallic hydrogen at static pressures. The first is to compress along a quasi-isotherm, temperatures ~1-400 K, until the insulator-metal transition (IMT) occurs. It was realized by Ramaker, Kumar, and Harris that even in the molecular state, hydrogen might become metallic by a band overlap mechanism. It is also predicted to be a high temperature superconductor. The second pathway is to heat above the melting point at high pressure until the molecules dissociate. This liquid-liquid transition to atomic metallic hydrogen is a first order phase transition with a critical point. Modern calculations predict it to have a negative slope (temperature vs pressure) with a critical point around 2000 K (see fig. 2). It was discussed in general for liquids early on by Landau and Zeldovich and is called the plasma phase transition, or PPT.
Recently the focus has shifted to the PPT and the melting line. The melting line was predicted by Bonev and coworkers to have a peak so that as pressure is increased beyond the peak, the melting temperature may descend to zero Kelvin. In this atomic metallic state, hydrogen would be a two-component superconductor, electrons and protons, possibly at room temperature. Another important predictions is that metallic hydrogen would be metastable so that if the pressure was released it would remain in the atomic metallic phase. This would have important implications for energy with dissipation free power lines and rocketry, as metastable metallic hydrogen would be the most powerful rocket propellant in existence, enabling interplanetary travel.
Quasi-isothermal measurements on hydrogen have been carried out to pressures above 300 GPa and a new non-metallic phase has been observed. The samples have remained transparent and there is no indication of metallization. We have recently observed a transition believed to be the PPT, discussed below.
What are we doing? We are developing techniques to push the pressure of hydrogen and its isotopes (deuterium and HD) to the 4+ megabar range using DACs. We have also developed the technique of pulsed laser heating of samples, using a laser pulse of order 100 ns that can easily melt metals, etc.
Our research has always featured developing an understanding of the physics of hydrogen as the pressure bar has been raised. We have observed a number of phase transitions in high-pressure hydrogen and its isotopes and studied these systems by optical means such as Raman scattering, infrared absorption, and optical absorption. Recently we have laser heated high-pressure hydrogen to a few thousand degrees and observed a new phase (see Fig. 2), believed to be the PPT . The next step is to show that this phase is metallic.
Fig. 2. The experimental/theoretical phase diagram of hydrogen showing recent measurements of the melting line and a phase transition that is in agreement with predicted phase lines for the PPT, from different calculations
Some Past Accomplishments
In hydrogen (and its isotopes) we have observed lines separating two phases, the Broken Symmetry Phase (BSP) and the hydrogen A-phase (sometimes called phases II and III). Neither are metallic. In our research program we have studied a number of materials. We have metallized xenon and hydrogen iodide. We use a variety of techniques, mostly those used on larger macroscopic sized samples, including NMR, but adopted to the samples in DACs. One of our focuses is hydrogen and the possibility of metallization. This is discussed in more detail, below.
 V. Dzyabura, M. Zaghoo, and I. F. Silvera, "Evidence of a liquid–liquid phase transition in hot dense hydrogen," Proc. of the National Academy of Sciences, vol. 110, pp. 8040-8044, 2014.