ARPES Studies of CMR Oxides and Related Materials

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****NON-TECHNICAL ABSTRACT****The colossal magnetoresistive (CMR) oxides are an important set of electronic materials, which present an extreme or colossal change in their electrical resistance upon the application of a magnetic field (hence the term colossal magnetoresistance). These materials have potential for applications - for example as sensors in next-generation magnetic storage devices. However, much of the physics behind CMR and other related effects in these materials is not yet understood. The goal of this project is to advance the understanding of these and related materials using the technique of photoemission spectroscopy - the principles of which were explained by Einstein in his Nobel-winning 1905 paper. The photoemission experiments will be performed with ultraviolet lasers and with synchrotron radiation sources such as the Advanced Light Source, Berkeley Labs, California. Experiments will be performed over a wider parameter range than has been done previously, which is expected to provide much deeper insights into the causes of the behavior of these materials. The inclusion of students (including undergraduates) in this research program is important, as it will provide them with useful skills for future careers in academia, national laboratories, or industry. Providing undergraduates with meaningful research experiences is considered one of the most effective ways to attract talented students and to retain them in careers in science and engineering. ****TECHNICAL ABSTRACT****High resolution angle resolved photoemission (ARPES) will be used to study the electronic structure of colossal magnetoresistive (CMR) oxides and other related materials. An emphasis will be placed on cleaved single crystals of the bilayer manganites, which give the best window into the intrinsic low energy electronic excitations of this general class of materials. Experiments will be performed over a wide parameter range of doping and temperature so as to access and eventually understand the complex and rich evolution of the phase diagram. Effects that are important in this phase diagram are the evolution of ferromagnetism, antiferromagnetism, canted magnetism, and paramagnetism, metals, insulators, charge and orbital order and disorder. Electronic gaps and pseudogaps from this order as well as from electron-phonon coupling and localization effects also are relevant. The cooperation and competition between these many effects is ultimately responsible for the "colossal" responses of the systems to minor changes in their external parameters. The goal therefore is to understand the detailed electronic structure, magnetic structure, and interactions in these various phases, as well as to understand how these phases cooperate or compete with each other. The inclusion of students (including undergraduates) in this research program is important, as it will provide them with useful skills for future careers in academia, national laboratories, or industry. Providing undergraduates with meaningful research experiences is considered one of the most effective ways to attract talented students and to retain them in careers in science and engineering.