First Principles Theory of Complex Compounds

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9802076 Pickett This award is in support of theoretical research on the electronic structure and properties of various materials of current interest. Examples of recently discovered materials that have generated great activity include: the compund Sr14- xCaxCu24O41, which has Cu-O chains and ladders but no layers, yet superconducts for a narrow range of x in a specific range of pressure; the class of spin-1/2 layered vanadates CaVnO2n+1, which show spin-Peierls behavior (n=3) or spin-gap behavior with a wide range of gaps (n=2 and n=4); the Fe-Si system FenSim, for which the n=1, m=2 and n=2, m=1 members are rather standard metals whereas the n=m=1 member is a very unusual `Kondo insulator' with an energy gap of 60meV; and the filled skutterudite systems such as Ce1-xLaxCo4Sb12 which show superb thermoelectric behavior for heat management applications. The high temperature superconducting cuprates, of course, continue to cause fascination due to apparent d-wave order parameter, the complex flux lattice behavior arising in part from their short coherence length, and in no small part due to the continuing elusiveness of the microscopic pairing mechanism. A widespread fascination has arisen from the so-called `colossal magnetoresistance' manganite materials which show numerous charge, spin, and structural orderings, and unusual field-driven reentrant behavior. In several cases the development of novel materials has resulted from close interplay between theory and experiment, with not only interpretation of properties being the role of theory but also the direct indication of what promises to be interesting, where to look for it, and so on. Features that are common to many of these novel materials are: (1) they are multicomponent, with properties being strongly dependent on stoichiometry; (ii) their crystal structures have low symmetry, indicative of unusual chemical bonding patterns; (iii) their properties are unconventional, and often vary considerably wit h small changes in material parameters (doping level, temperature, pressure, even ionic size is sometimes crucial); and (iv) they involve the interplay of itinerant carriers and states which are in some sense localized. It is this last feature, in concert with the other complications, that provides the peculiar behavior. Complexity of the unit cell alone usually does not cause fundamental difficulty for theorists trying to descibe the systems. Complex intermetallics, quaternary semiconductors, and semiconductor superlattices can usually be handled well by a combination of phenomenological methods and the local density approximation within density functional theory. Likewise, localized atomic states in simple cell lattices (viz. NiO in the rocksalt structure) may lead to very interesting correlated electron behavior, but they simply do not possess the degrees of freedom that are present in more complex lattices. In many of these systems model Hamiltonians will continue to be required to address their thermodynamic and spectral properties. There is however serious need to address the complexities of the unit cell concomitantly with the correlated electron behavior that arises in combined itinerant/loccalized carrier systems from first principles. The work proposed here will apply a combination of methods - parametrized tight-binding Hamiltonians, precise local spin density calculations, `beyond LDA' innovations, and exact diagonalization methods - to such complex compounds with the expectation of simultaneously increasing understanding of the observed behavior and extending the capabilities of first principles methods. %%% This theoretical research will investigate complex materials of current interest which display a competition between electrons which are localized, i.e., fixed to atoms or molecules, and ones which are not. This competition, along with other materials properties such as temperature, pressure and concentration, can produce highly unusua l effects which are of fundamental interest and which may lead to technological applications. ***