Hydrogen at Ultra-High Pressure

凝聚态物理的一个突出问题和挑战是在超高压下在金属相中产生氢。金属氢(MH)被认为是一种室温超导体，可能是亚稳态的，也就是说，当压力释放时，它仍然是金属的。如果亚稳的MH将是已知最强大的火箭推进剂。有两条途径可以获得MH：在高压下，氢可以在熔线上方被加热到一个新的相，即等离子体相变，或PPT，在那里它在液体中解离成为原子金属液体。第二种途径是沿着准等温线加压，直到它解离，变成原子金属。超高压是在钻石顶压室中产生的，必须扩展到约5兆巴的压力才能产生预测的相。已经开发了加工钻石的技术，以增加氢样品的压力和温度的有效范围。利用脉冲激光，兆巴压力范围内的氢样品被加热到几千开尔文的温度，并观察到了相变。将进行测量以显示该相是金属相或PPT相。最近的准等温实验揭示了在~2兆巴以上的新相，但不是金属相。在新阶段以外的更高压力下研究氢的实验将由博士后研究员、研究生和本科生在他们的教育发展中进行。非技术摘要宇宙中最普遍的元素是氢；在地球上发现纯氢以双原子分子的形式结合在一起，作为透明的分子绝缘体在非常低的温度下液化和固化。75年来，人们一直推测，当加压时，分子会解离，形成具有壮观性质的金属原子固体或液体。其中包括室温超导电性，绝对零度和数百万大气压下液体的存在，超流性。此外，金属氢被预测为亚稳定的，也就是说，当释放压力时，它将留在金属相中，就像高压下由碳制成的钻石在压力释放时不会恢复到石墨一样。如果是亚稳定的，金属氢将是已知最强大的火箭燃料，并将给火箭技术带来革命性的变化。使用金刚石压腔产生高压和光学研究技术的压力范围将扩大，目标是实现压力高达400万至500万大气压，并用激光将样品加热到数千度。这个项目的成功，由博士后研究员、研究生和本科生以及可能的高中生进行，将扩大我们对元素周期表中这个最简单但最复杂的原子的知识和理解。

****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.