Theoretical investigation of solid hydrogen and deuterium
Solid hydrogen forms at extreme conditions, under high pressures. Although the hydrogen atom is easy to understand theoretically, when interacting in the solid state it becomes complicated. Up to now, five different solid phases have been confirmed experimentally and theory has predicted numerous competing crystal candidates. The goal is to obtain solid metallic hydrogen which has been predicted theoretically eighty years ago and has since been considered the holy grail of high pressure science. In nature, this form of matter is believed to exist at the core of large planets like Jupiter and Saturn, being responsible for the planets' large magnetic fields. Understanding the different phases of hydrogen is a test for our most advanced theories of quantum mechanics in condensed matter and it is fundamentally important for both planetary and material science. Recently discovered solid phase IV is stabilized by entropy and therefore only exists at relatively high temperatures. Using molecular dynamics (MD) I studied the room temperature behavior of phase IV starting with the ground state candidate structures reported in the literature. Additionally, I devised a velocity projection method for extracting Raman spectra from MD in light of direct comparison to experiment. My results helped establish the true nature of phase IV and validated the structure against experimental data. Applying the same method to the previously proposed C2=c crystal structure, I obtained results that confirm this structure is the best candidate for phase III. Within the last year, a new phase V of solid hydrogen was discovered in Raman experiments. While attempting to identify the crystal structure associated with this new phase, I discovered a manifestation of solid hydrogen in the form of long polymeric chains that could be stabilized by a charge density wave. Here I discuss the possibility of such a state of matter as an intermediate on the path to molecular dissociation of hydrogen. Chains could, however, be a spurious structure - the effect of a subtle non-convergence problem in the MD, which could indicate serious issues with many previous studies reported in the literature. A far more likely candidate for phase V is a structure similar to that of phase IV with a subtle dynamical modification. I will present Raman and phonon results from both static and dynamic calculations to support this claim. I conclude my work on pure solid hydrogen with an instructive model that could explain the entire phase diagram based on simple thermodynamic considerations. All of the assumptions were extracted from our previous ab initio studies through analysis and observations. This model encodes a comprehensive summary of the current understanding of solid hydrogen at high pressures. Raman and infrared spectroscopy have been the methods of choice in most hydrogen studies. Another way to look at the problem is to analyze the behavior of isotopic mixtures: hydrogen-deuterium binary alloys. Using isotopic substitutions, I revealed a textbook effect in hydrogen: phonon localization by mass disorder. The effect might be unique to this element, owing to the large mass ratio between hydrogen and deuterium. Phonon localization explains the complicated Raman spectra obtained experimentally in hydrogen-deuterium mixtures at various concentrations. More recent experimental results claim an unexpected phase transition in mixtures at low temperatures based on splittings in the infrared spectra. Here I will show that the infrared splitting seen experimentally could be induced by mass disorder in phase III and does not necessarily indicate a structural transformation.