Study of Excitations in Cuprate Superconductors and Related Systems by Inelastic Scattering of Visible, Ultraviolet, and X-Ray Photons

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w:\awards\awards96\num.doc 9705131 Klein This experimental research project focuses on the unusual electronic properties of those cuprate compounds which are "parent compounds" of the cuprate superconductors. These compounds exhibit correlated electron behavior, and in particular antiferromagnetism. In the superconducting phases, the residual antiferromagnetism is poorly understood. This experimental project will clarify the residual antiferromagnetism by scattering experiments using visible, ultraviolet, or x-ray photons, to study excitations wherein an electron from an occupied initial state makes a transition to an unoccupied final state, accompanied by a redistribution of charge. The objectives are (1) to understand how electronic charge excitations couple to electronic spin excitations (magnons) and to lower energy electronic charge excitations, which may include superconducting gap excitations; (2) to validate inelastic x-ray scattering techniques for studying electronic charge excitations at finite momentum transfer; (3) to better understand electronic excitations in conventional metals and semiconductors through use of inelastic x-ray scattering techniques. The experiments will employ a combination of low energy and high energy photon probes. These include low energy optical absorption; resonance Raman scattering, especially resonant two-magnon scattering; UV excited Raman scattering, and two new techniques, inelastic s-ray scattering spectroscopy (IXSS) and resonant inelastic x-ray scattering spectroscopy (RIXSS), which provide data at finite momentum transfer. The work is expected to advance knowledge of both strongly-correlated electronic systems and of inelastic x- ray scattering. %%% This research project experimentally studies the unusual behavior of the cuprate compounds which when "dop ed" become high temperature superconductors. These "parent compounds" exhibit unusual behavior, including antiferromagnetism, which is to some extent retained in the superconducting phases. It is believed that a better understanding of the normal and parent phases of these materials is necessary for understanding of the unusually strong superconductivity when suitably doped. The present experimental project will address the important questions concerning the antiferromagnetic behavior, by a combination of measurements of the scattering of photons (visible, ultraviolet, and x-ray) by these compounds. A portion of the work will utilize the Raman effect, and another portion will utilize a new advanced experimental technique, inelastic x-ray scattering, which is now available at the Advanced Photon Source, a world-class synchrotron light source which is located near the Argonne National Laboratory. The work will advance knowledge of both strongly-correlated electronic systems and also of the inelastic x-ray scattering process. The study of these materials and their unusual magnetism is valuable as a testing ground for theories of magnetic, optical, and electrical properties of the cuprates, which can then be more confidently applied to other systems; and in the search for novel properties and new compounds which may find new application in technology. This research project is interdisciplinary in nature and involves both graduate and postdoctoral students who will be excellently trained to enter positions in industry, government or education. ***