Exact Diagonalization Study of Real Materials
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EXACT DIAGONALIZATILON STUDY OF REAL MATERIJALS CHANGFENG CHEN Department of Physics, University of Nevada, Las Vegas, Nevada 89154, USA ABSTRACT We report some recent developments in theoretical modeling of real 3d magnetic materials, particularly interface and ultrathin-film structures. We focus on many-body correlation effects and describe the method of modeling realistic strongly correlated electron systems using and going beyond the local-density-approximation single-particle electronic structures. Grouptheoretical techniques and high-performance computing facilities are systematically used in this work. Fundamental spectroscopic properties of several prototypical 3d transition-metal systems have been obtained and new features induced by many-body correlation have been discovered. Results will be presented and discussed.
INTRODUCTION Many new materials that have attracted considerable attention in recent years, such as oxide superconductors, alkali intercalated C6 0 materials, magnetic thin films and superlattices, and much more, belong to the category of strongly correlated materials systems. Future applications of the novel properties observed in these new materials rely on the understanding of the fundamental roles played by the correlation effects in these systems. From the theoretical point of view, the main challenge is how to properly treat the strongly interacting electrons (or quasiparticles) in these materials. Traditionally, there are two kinds of approach used in the study the electronic structure and related properties, namely the single-particle view (epitomized by band theory) and the strongly correlated on-site view (epitomized by the Hubbard model and its variants). While they are capable of attacking some aspects of the problem, both approaches have some drawbacks. Conventional band theory employing localspin-density approximations can provide a good description for the ground-state properties of a large variety of materials; 1 but it has some intrinsic problem in dealing with strongly correlated electron systems such as d-electrons in the oxide superconductors and many magnetic materials. The Hubbard model has been extensively used in the study of correlated systems. However, most work does not attempt to make connection to ab initio results and thus, while capable of providing important insight into many-body problems, fails to explain some correlation-induced features uniquely associated with specific materials systems. Recently, extensive efforts have been made from both band and many-body point of view to improve the situation. 2 In this contribution, we present a theoretical approach that combines the band and the many-body aspects. We take the results of band calculations as the starting point and include the many-body effect in a periodic small-cluster approach (PSCA). In the PSCA the single-particle and many-body effects are treated on an equal footing. The problem is made tractable by sampling only a few high symmetry points in the Brillouin zone. This is equivalent to selecting a
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