Hydrogen at Ultra-High Pressure

****技术摘要****凝聚态物理的突出问题和挑战之一是在超高压下产生金属相的氢。金属氢（MH）被预测为室温超导体，并且可能是亚稳态的，也就是说，当压力释放时，它仍保持金属态。如果亚稳态MH将是已知最强大的火箭推进剂。氢有两种途径：在高压下，氢可以被加热到熔化线以上的一个新阶段，等离子体相变，或PPT，在液相中它解离成为原子金属液体。第二种途径是沿着准等温线加压，直到它解离并成为原子金属。在金刚石砧胞中产生超高压，并且必须扩展到~5兆巴的压力才能产生预测的相。已经开发出处理钻石的技术，以增加氢样品的有效压力和温度范围。使用脉冲激光，在兆巴压力范围内的氢样本被加热到几千开尔文的温度，并观察到相变。将进行测量以显示该相是金属或PPT。最近的准等温实验发现了~2兆巴以上的新相，但不是金属相。研究氢气在更高压力下的实验，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，纯氢以双原子分子的形式结合在一起，在极低的温度下液化和凝固，形成透明的分子绝缘体。75年来，人们一直推测，当加压时，分子会解离，形成具有惊人性质的金属固体或液体原子。这些包括室温超导性，存在于绝对零度温度和数百万大气压下的液体，超流动性。此外，据预测，金属氢是亚稳态的，也就是说，当压力释放时，它将保持在金属相，就像由碳在高压下制成的金刚石在压力释放时不会还原为石墨一样。如果金属氢是亚稳态的，它将是已知最强大的火箭燃料，并将彻底改变火箭技术。利用金刚石砧细胞产生高压和光学技术的研究，压力范围将扩大，目标是实现压力高达400万至500万大气压，并用激光加热样品到数千度。这个项目的成功，将由博士后、研究生和本科生，以及可能的高中生进行，将扩展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.