Optical properties of the superconducting hydrides and of topological materials

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The discovery of materials that superconduct at room temperature remains a highly desirable goal of condensed matter physics. It would revolutionize the use of MRI imagers in hospitals, the electrical grid for power transmission and magnetic levitation for high speed travel. The recent discovery of superconductivity with a critical temperature Tc = 200 K in H3S under 150 GPa pressure, and subsequent optical evidence that the driving mechanism is the electron-phonon interaction represents a quantum leap forward. The systematic study of other hydrides and related materials, some at ambient pressure is central to this area of research. We proposed to provide theoretical support to guide experimentalists in this search and to interpret their results particularly as it relates to mechanism and how it might be manipulated to achieve even higher values of Tc. Optics has been shown to be a powerful technique in this effort and we are world experts. A second important recent development in condensed matter physics has been the discovery of topological materials which has introduced new concepts and indeed new ways of thinking about semimetals. It has led to the rapid development of our understanding of the importance of topology in providing electronic systems with entirely new functionalities. This holds the potential for transformative impact on opto-electronic devices, on spintronics, magnetic memories, solar cells, water filtration, on quantum computing and more generally on communications. New materials in this area are discovered at a rapid rate partly because of advances in theory. Powerful new numerical techniques allows us to accurately predict electronic structure and implied properties. We propose to calculate select properties of new topological semimetals including Weyl and Dirac materials as they are discovered in the laboratory, with the aim of increasing understanding of what new and unique properties are possible. Already new Weyl fermions have been found in solid state systems that have no counterpart in high energy particle physics. So fundamental new physics is being discovered which could impact future technologies. The proposed research program will also provide solid training in advanced mathematical, numerical and computing techniques to graduate students, highly desirable and transferable skills to many other areas.