Electron Spectroscopy of Emergent Quantum Structures

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Strongly correlated electron materials exhibit a remarkable range of quantum ground states, e.g., insulating, metallic, magnetic, superconducting, depending on modest changes in chemical composition, temperature or pressure. Of great recent interest are the behaviors of a system poised between two stable zero temperature ground states, i.e. at a quantum critical point. This individual investigator project uses the techniques of photoemission and inverse photoemission spectroscopies to measure the single-particle electronic structures of selected strongly correlated electron materials displaying novel quantum behaviors, and seeks to construct the paths whereby these behaviors emerge from the underlying electronic structure or transform from one order to another. During the course of the next project period, the program will focus on two main themes, (A) quantum criticality in relation to (1) Non-Fermi Liquid (NFL) behavior generally and (2) electron fractionalization in low dimensional materials, and (B) certain novel or paradigmatic phase transitions. If successful the work will demonstrate the need for an overarching theoretical picture linking quantum criticality, fractionalization and NFL behavior that transcends dimensionality and material type. The experiments are performed both in a home laboratory, and at various national and international synchrotron facilities. The experimental data are analyzed by comparison to theories which treat the Coulomb interactions in different ways, and which provide a link between the spectra and the electrical or magnetic properties. The program relies on strong collaborations with other groups for well-characterized samples to measure, for determining electrical, thermal and magnetic properties of new materials, and for expert theoretical advice and advanced calculations. Thus, it builds human bridges across geographic, institutional and disciplinary boundaries. It also brings Ph.D. students into close contact with scientists in a variety of professional roles at a variety of institutions around the world, and trains them in the techniques of collaborative work.The electrical, magnetic and mechanical properties of materials are "emergent collective behaviors" of the underlying quantum mechanics of their electrons and constituent atoms. A principal goal of solid state physics and materials science is to elucidate this emergence. Achieving this goal would imply the ability to engineer a material that is optimum for any particular application. The mainstay of the current understanding of electrons in solids is known as the "Fermi liquid theory." This theory explains why electrons in solids can often be described in a simplified picture that appears to ignore the large repulsive forces electrons are known to exert on one another. There is a growing appreciation that this theory probably fails for entire classes of materials and there is the suspicion that the failure has to do with unresolved competition between different possible emergent behaviors. This individual investigator grant supports an experimental program aimed at using a technique called electron spectroscopy to measure and quantify the underlying quantum mechanical behaviors of electrons in materials that manifest this unresolved competition in various ways. If successful the work will pinpoint essential ideas that must be combined to take the goal of engineering materials to the next level of sophistication. The experiments are performed both in a home laboratory, and at various national and international synchrotron facilities. The program relies on strong collaborations with other groups for well-characterized samples to measure, for determining electrical, thermal and magnetic properties of new materials, and for expert theoretical advice and advanced calculations. Thus, it builds human bridges across geographic, institutional and disciplinary boundaries. It also brings Ph.D. students into close contact with scientists in a variety of professional roles at a variety of institutions around the world, and trains them in the techniques of collaborative work.