Ultrahigh Pressure Studies of Hydrogen and its Isotopes

凝聚态物理中一个突出的问题和挑战是确定和理解高密度固体氢的相和性质。许多相已经被预测，特别是维格纳-亨廷顿跃迁到原子金属态，这可能是一个高tc超导体。此外，详细的电子能带结构计算预测高密度分子相应该是金属的，并表现出高温超导性。目前实验已知存在三种相：低压相、110 GPa下的破对称相和150 GPa下的a相。该研究项目将继续研究固体氢及其同位素氘和HD，直至兆巴压力。为了寻找氢的金属相，将致力于将压力扩展到400gpa范围。将测量a相的直流电导率作为温度和压力的函数，以确定氢在这些压力下是否为金属，或者如果不是，将确定带隙作为压力的函数。新开发的高质量、无缺陷和极低杂质的人造金刚石砧细胞将用于这些研究。这些改进的dac有望克服传统设计固有的局限性。光谱学技术将用于寻找和探索新的相线。导电性能将通过直流电导率测量或以德鲁德自由电子谱为特征的光谱学来研究。高压相及其性质的确定将是理论和实验的一项重大成就。研究生和博士后研究助理将参与这项研究，从而获得知识和技能，为他们在工业、学术界或政府部门的就业做好准备。凝聚态物理中最突出的问题和挑战之一是确定和理解高密度固体氢的原子结构和性质。氢在零下260摄氏度结冰。该固体密度低，肉眼可见。在高密度下，分子之间的距离更近，预计固体会变成金属，并可能成为高温超导体。据预测，在更高的密度下，分子会解离，因此在晶格位置上，固体是由原子而不是分子组成的。这种向原子金属氢的转变及其伴随的超导性早在65年前就被预测到，但迄今为止还没有被探测到。现在人们认为，要达到产生原子金属氢所需的密度，需要300万到400万大气压的压力。这项研究的目标是用所谓的金刚石砧细胞（dac）产生这种压力，这种细胞是专门设计用来克服传统dac的局限性的。电学和光谱学方法将用于寻找氢原子金属态的证据。原子金属氢的产生将是一项重大成就，不仅在科学上，而且很可能在技术上也是一项重大成就，因为预测其存在的同一理论也预测，即使在释放压力之后，它在室温下也会是超导体。这些预测是当前关于极端条件下氢的性质的争论的核心。该项目的成功成果不仅将解决当代凝聚态物理学的一个突出问题，而且将为科学发现和应用开辟新的途径。本研究是在研究生和博士后研究助理的协助下进行的。他们将因此获得凝聚态物理当代领域的知识和技能，这将为他们在工业、学术界或政府部门的生产性就业做好准备

One of the great outstanding problems and challenges to condensed matter physics is a determination and understanding of the phases and properties of solid hydrogen at high density. A number of phases have been predicted, in particular the Wigner-Huntington transition to the atomic metallic state, which may be a High-Tc superconductor. In addition, detailed electronic band structure calculations predict that the high-density molecular phase should be metallic and exhibit high temperature superconductivity. Currently there are three phases known to exist experimentally, the low-pressure phase, the broken symmetry phase at a pressure of 110 GPa, and the A-phase at 150 GPa. This research program will pursue the study of solid hydrogen and its isotopes, deuterium and HD, to multi-megabar pressures. An effort will be dedicated to extend the pressure to the 400 GPa range in the search for the metallic phases of hydrogen. The DC electrical conductivity of the A-phase will be measured as function of temperature and pressure in order to determine whether hydrogen is metallic at these pressures or, if not, will determine the band gap as a function of pressure. Newly developed diamond anvil cells (DACs) with high quality, flaw-free and very low impurity synthetic diamonds will be use in these studies. These improved DACs are expected to overcome the limitations inherent in conventional designs. Spectroscopic techniques will be used to search for and explore new phase lines. The conductive properties will be investigated either by DC conductivity measurements or optical spectroscopy, characterized by a Drude free-electron spectrum. A determination of the high-pressure phases and their properties will be a major accomplishment for theory and experiment. Graduate students and post doctoral research associates will participate in this research and will thereby acquire knowledge and skills that prepare them for employment in industry, academia, or government.%%%One of the great outstanding problems and challenges to condensed matter physics is the determination and understanding of the atom structure and properties of solid hydrogen at high density. Hydrogen freezes at -260 degrees centigrade. The solid has low density and is transparent to the eye. At high density the molecules are closer and the solid is predicted to become a metal and possibly become a high-temperature superconductor. At still higher densities the molecules are predicted to dissociate so that the solid is made up of atoms, rather than molecules, at the lattice sites. This transition to atomic-metallic hydrogen with its accompanying superconductivity was predicted over 65 years ago but has so far eluded detection. It is now believed that pressures in the range of 3 to 4 million atmospheres are required to achieve the densities needed to produce atomic-metallic hydrogen. It is the goal of this research to generate such pressures with so-called Diamond Anvil Cells (DACs) that are specially designed to overcome the limitations of conventional DACs. Electrical and optical spectroscopic methods will be used to search for evidence of the atomic-metallic state of hydrogen. The production of atomic-metallic hydrogen would be a major accomplishment, not only scientifically, but very likely also for technological reasons, for the same theory that predicts its existence also predicts that it will be a superconductor at room temperature, even after the pressure is released. These predictions are at the center of current debate about the properties of hydrogen under extreme conditions. A successful outcome of this project will not only settle one of the outstanding problems of contemporary condensed matter physics but will also open up new avenues for scientific discoveries and applications. This research is conducted with the assistance of graduate students and postdoctoral research associates. They will thereby acquire knowledge and skills in a contemporary area of condensed matter physics that will prepare them for productive employment in industry, academia, or government.***