Hydrogen at Ultra-High Pressure

Harvard University, Cambridge MA

Investigators

Abstract

Technical 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.