Hydrogen at Ultra-High Pressure

Harvard University, Cambridge MA

Investigators

Abstract

****Technical Abstract**** One of the outstanding problems and challenges to condensed matter physics is to produce hydrogen in the metallic phase at ultra high pressures. Metallic hydrogen (MH) has been predicted to be a room temperature superconductor, and may be metastable, that is, it remains metallic when the pressure is released. If metastable MH would be the most powerful rocket propellant known. There are two pathways to MH: at high pressure hydrogen can be heated above the melting line to a new phase, the plasma phase transition, or PPT, where in the liquid phase it dissociates to become an atomic metallic liquid. The second pathway is to pressurize along a quasi-isotherm until it dissociates and becomes atomic metallic. Ultrahigh pressures are produced in diamond anvil cells and must be extended to pressures ~5 megabar to produce the predicted phases. Techniques have been developed for processing diamonds to increase their useful range of pressure and temperature with hydrogen samples. Using pulsed lasers, samples of hydrogen in the megabar pressure range have been heated to temperatures of a few thousand Kelvin and a phase transition has been observed. Measurements will be made to show that this phase is metallic or the PPT. Recent quasi-isothermal experiments have revealed a new phase above ~2 megabar, but not metallic. Experiments to study hydrogen to much higher pressures, beyond those of the new phase will be carried out by postdoctoral fellows, graduate, and undergraduate students in their educational development. ****Non-Technical Abstract**** The most prevalent element in the universe is hydrogen; pure hydrogen is found on earth bound together as diatomic molecules, that liquefy and solidify at very low temperatures as a transparent molecular insulator. For over 75 years it has been speculated that when pressurized, the molecules would dissociate to form an atomic solid or liquid that is metallic, with spectacular properties. These include room temperature superconductivity, existence of a liquid at absolute zero of temperature and millions of atmospheres pressure, superfluidity. Moreover metallic hydrogen is predicted to be metastable, that is, it would remain in the metallic phase when pressure is released, just as diamond made from carbon at high pressure does not revert to graphite when pressure is released. If metastable, metallic hydrogen would be the most powerful rocket fuel known and would revolutionize rocketry. Using diamond anvil cells to generate high pressures and optical techniques of study the pressure range will be extended, aiming to achieve pressures up to 4 to 5 million atmospheres and heating samples with lasers to thousands of degrees. Success in this program, carried out by postdoctoral fellows, graduate, and undergraduate students, and where possible high school students, will extend our knowledge and understanding of this simplest, but most complex atom in the periodic table of the elements.