This individual investigator award will support studies of hydrogen at ultra high pressures and temperatures. Earlier attempts to study hydrogen in the high-pressure high-temperature regime were thwarted by diffusion of hydrogen in the pressure cell materials and loss of sample or embrittlement and failure of cell materials. By utilizing pulsed laser heating this problem has been overcome since the time that the sample is hot and can diffuse is limited to the short time of the pulse. The emphasis will be on studies along and above the melting line of hydrogen. Hydrogen was predicted to have a peak in its melting line and this peak was recently experimentally demonstrated to occur below a megabar. The melting line studies will be extended to higher pressures. At lower pressures hydrogen melts from a molecular solid to a molecular liquid. With increasing temperature above the melting line hydrogen will dissociate and become monatomic with metallic conductivity. With increasing pressure beyond the peak the melting temperature may descend to zero Kelvin and one may observe melting directly from the molecular to the atomic phase. The atomic metallic liquid is expected to demonstrate two-component superconductivity (electrons and protons) as well as superfluidity Metallic hydrogen is predicted to be metastable due to a potential barrier. This barrier between the two phases may be responsible for inhibiting the transition from solid molecular to atomic metallic at low temperature. At high temperature thermal energy may allow the molecular phase to overcome the barrier and make the transition to metallic hydrogen. The broader impact of this research is the development of new methods to study materials under extreme conditions enriching the scientific community. This program involves young researchers at all levels-high school, undergraduate, graduate, and postdoctoral, as they develop to become the scientists of the future.
Non-technical Over 70 years ago Wigner and Huntington predicted that at high pressure hydrogen will transform from a molecular solid to an atomic metallic solid, later predicted to be a possible room temperature superconductor (no resistance to the flow of electricity) that is metastable, i.e., will remain in the metallic phase when pressure is released. Because of its extreme quantum nature, theory is challenged to make accurate predictions of hydrogen's properties and needs experimental guidance. Hydrogen has been pressurized to more than 10 times the predicted transition pressure and remains molecular insulating. Recent theory predicted a peak in the melting line and with pressure increasing beyond the peak the melting temperature could descend to zero Kelvin. Hydrogen would be an atomic metallic liquid with superconductivity of both the electrons and protons. Earlier attempts to study hydrogen in the extreme pressure-temperature regime were frustrated due to the proclivity for hydrogen to diffuse out of the high pressure apparatus or into the materials comprising the apparatus at high temperature. Using a newly developed method of pulsed laser heating, hydrogen can now be studied in this regime. The predicted peak in the melting line has been observed and this research program will extend studies to higher pressures in search of the metallic state. On a broader level, new techniques for high pressure and high-temperature/low-temperature are developed for the scientific community; if metallic hydrogen can be produced and is metastable it will be a high energy density material as well as the most powerful rocket propellant available to man. Students and postdoctoral fellows on all levels, the next generation of scientists, are involved in the developments and research.
Project Outcomes: DMR-0804378; August, 2013 PI: Isaac F. Silvera Project Accomplishments: Hydrogen is the simplest element in the periodic table of elements and in the pure form is found here on earth as molecular hydrogen or H2. Due to important quantum mechanical effects, it turns out that this conceptually simple element is extremely complex in the solid state. Over 75 years ago Wigner and Huntington predicted that by compressing solid molecular hydrogen to very high pressures the molecules would dissociate and it would become a metal. Subsequently it was predicted that metallic hydrogen (MH) may be a room temperature superconductor and that it may be metastable, that is after transforming to the metallic phase, if the pressure is released it would remain metallic. In this form if triggered to recombine to the molecular form by heating, it would be the most powerful rocket propellant known to man and revolutionize rocketry. There are two pathways to MH: quasi-isothermal compression of solid molecular hydrogen to multi-megabar (1 megabar is 100 GPa, the SI unit of pressure) pressures until the molecules dissociate to form atomic metallic hydrogen; and at high pressure, heating of the solid above the melting line until liquid molecular hydrogen dissociates to form liquid metallic atomic hydrogen. A theoretical/experimental phase diagram of hydrogen is shown in Fig. 1. The high temperature transition is predicted to be a first-order liquid-liquid transition and is called the plasma phase transition (PPT). Theoretical predictions for this phase line are shown by the blue lines with the negative slope (showing the range of various predictions). An interesting aspect of the phase diagram is that the melting line of hydrogen has maximum with a negative slope at higher pressures (red line in Fig. 1), and has been predicted to go to zero Kelvin so that hydrogen may be an atomic liquid at megabar pressures and T=0 K. In the figure it is seen that the PPT line is also expected to merge with the melting line so that at higher pressures hydrogen is predicted to melt from solid molecular to liquid atomic. In this research project we have succeeded in using pulsed laser heating to explore the high P,T region of the phase diagram. The hydrogen is pressurized by placing a metallic gasket with a hold in it, about 50 microns in diameter (diameter of a human hair) shown in Fig. 2. The sample is in the open round hole in the center to confine it, placed between two diamonds, and squeezed to high pressure. Since hydrogen is transparent to our pulsed laser, an absorber is embedded in the hydrogen (dark square with a central hole); this is heated by the laser and the hot absorber heats the hydrogen. The 7 spheres around the absorber are ruby balls, used to measure the pressure from their fluorescence spectrum. With this technique we can heat hydrogen to thousands of degrees K. We have been able to extend the melting line to the highest pressures yet achieved and have found evidence of a phase transition that overlaps the theoretical predictions for the PPT. Our data are shown in Fig. 1. We have not yet shown that this phase is metallic, but our recent experiments of the transmission/reflection of hydrogen at temperatures at and above the observed phase transition provide evidence of changes in the optical properties, consistent with metallic behavior. With this evidence we will have produced metallic hydrogen in the high pressure-high temperature form. Other General Activities for Hydrogen Research New Pressure Calibrants We recently reported on a possible new calibrant for high pressure, gold coated silica nanoshells. We have acquired such nano-shells and made some preliminary measurements. We experimented on glass slides and could bond the nanoshells by functionalizing the surface. It is more challenging to attach the nanoshells to diamond. Subsequently we came up with an improved idea for a new pressure gauge, using some of the properties of nano-diamond particles which is much easier to place on a diamond culet. Metallization of GeO Germanium oxide is one of a family of tin oxide and silicon oxide, etc., that undergoes an insulator-metal transition at modest pressures and remains amorphous after it metalizes. It is difficult to synthesize so we had a powder sample made for us by a crystal growing company. We studied it down to liquid nitrogen temperatures for several pressures and found it to metalize at a pressure between 10 and 11 GPa. With one of our collaborators we have carried out an x-ray analysis to show that it is amorphous. The results, resistance versus inverse temperature for several pressures, are shown in Fig. 3. With increasing pressure the resistance decreases until it displays a metallic nature shown on the right hand side.
