Electron and Lattice Dynamics Across Phase Transitions in Triangular Lattices and Atom Chains on Surfaces

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****NON-TECHNICAL ABSTRACT****Low-dimensional electronic materials with layered or chain-like crystal structures are at the core of some of the most exciting discoveries in modern materials research. Nobel prize winning discoveries of novel quantum-mechanical effects in artificially structured semiconducting materials (1985, 1998), giant magneto-resistance (the change in electrical resistance due to the presence of a magnetic field) in stacked metal layers (2007), and the existence of electrical conductivity with no energy loss ("high-temperature superconductivity") in low-dimensional compounds made from copper and oxygen (1987) are just a few examples of transformational discoveries that define the frontiers of condensed matter science. In particular, the fascinating properties of technologically important oxide materials are rooted in the correlated motion of electrons, arising from the delicate interplay between the electron's charge, its magnetic property known as "spin" or "magnetic moment", and atomic vibrations. This project aims to study these delicate interactions in low-dimensional model systems, such as atom chains or single-atom layers on surfaces. These extreme low-dimensional systems also exhibit the rich physics associated with correlated electron motion but they are easier to control and analyze than bulk oxide materials. The key aspect of this project will be the education and training of two PhD students and a postdoctoral research associate in the use of advanced scientific instrumentation and analytical problem solving. These are qualities and experience that provide an excellent preparation for careers in academia and high-tech industry. The project both nurtures and expands the fundamental knowledge base and workforce for materials innovations that may ultimately drive technological and economic development.****TECHNICAL ABSTRACT****Surfaces and interfaces function as ideal platforms for studying low-dimensional electron systems and phase transitions. This project will focus on the triangular surface phases of group IV elements on Si(111) and Ge(111) surfaces with odd electron counts; and on quasi one-dimensional atom chains on silicon surfaces with nested Fermi surfaces and strong spin-orbit coupling. These surface phases exhibit the rich physics arising from competing electron-electron interactions, electron-phonon coupling, magnetic interactions, broken symmetries and geometrical frustration, and are amenable to first principles calculations and theoretical modeling of the many-body interactions. The nature and driving forces of the electronic phase transitions in these systems will be studied using the unique combination of high-resolution angle-resolved photoemission and helium atom scattering. The strong coupling between the electronic and phonon excitations will be disentangled using a novel pump-probe scheme for time-resolved photoemission. Finally, the nature and strength of the many-body interactions and electronic phase diagram will be explored using chemical doping and pressure tuning via epitaxial strain. This program provides an excellent setting for the education and training of two PhD students and a postdoc at the frontiers of condensed matter physics. These learning opportunities will be enhanced by direct access to sophisticated instrumentation and an international network of theory collaborators. Successful implementation not only advances the frontiers of modern condensed matter science but also fosters fundamental knowledge that will eventually facilitate the creation of novel functional materials by design.