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.
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.
Our objectives were to perform high resolution ARPES to study the electronic structure of manganite colossal magnetoresistive oxides, topological insulators, and a variety of strongly spin-orbit coupled iridate materials. We accomplished all of our specific objectives. We made major progress in understanding the electronic structure of the colossal magnetoresistive oxides – combined with our previous ARPES results on these materials there is now a very significant body of electronic structure data across a wide range of materials, and a relatively complete picture now exists. We then started work on other, related materials, especially the strongly spin-orbit coupled iridate materials. We made very good progress on this new class of materials as well, with our work including the first ARPES study on a pyrochlore iridate, the first study on a hole-doped iridate, and other studies on the effect of the dimensionality on some of these materials. Our results have been disseminated to communities of interest through published papers and via talks at conferences and workshops. We’ve published 8 papers, have posted 2 others on the arXiv, and have a few more under review or development. For studies of the manganites we showed the strong dynamical effects of the bistripe localization, including a re-entrant phase transition. Another key manganite paper showed for the first time that there is a minority-spin component to the Fermi surface, i.e. these are not true half-metals. Recent work on topological insulators studies the in-plane orbital states and their effects on the coupling to the spin degrees of freedom. The P.I. was lead scientific organizer of a major international workshop (CORPES11, held in Berkeley) and was co-organizer of the follow-up (CORPES13, held in Hamburg). Other activities included multiple talks to student groups about getting into graduate school and a talk to the general community about "Einstein’s photon". The participants have obtained training in correlated electron materials, advanced synchrotron and laser-based instrumentation, and advanced analysis methods. 3 students obtained PhD’s from work that was directly supported by this program, two undergraduate students did REU work associated with this project, and two undergraduate students completed honors theses based upon this grant.
