First Principles Theory of Complex Compounds

University Of California-Davis, Davis CA

Investigators

Abstract

0114818 Pickett Discovery or fabrication of new materials with unexpected and fascinating properties is a continuing, perhaps accelerating, process. A very good example of such a discovery within the past year is that the ferromagnet UGe2 becomes superconducting while remaining magnetic. This type of coexistence was not supposed to happen according to Ginzburg in 1957, and it had not been questioned seriously until the last couple of years (RuSr2GdCu2O8 has related properties for quite different reasons). Another example is the spin glass compound Y2Mo2O7 which is structurally ordered whereas before its discovery structural disorder was expected to be required for spin glass behavior. Such discoveries attract great interest among materials physicists, and seem invariably to impact strongly subsequent developments in the field, viz. superconductivity, Kondo effect, quantum Hall effect. In each of the materials mentioned above, the unanticipated behavior could not have arisen in simpler (either structurally, or in terms of interactions) systems, because the requisite degrees of freedom are not present. Coexistence of superconductivity and magnetism in RuSr2GdCu2O8 , although not at all understood, is allowed only because of the separation of magnetic and superconducting layers; in Y2Mo2O7 it is thought probable that spin glassiness results from a combination of ferromagnetic exchange coupling, strong single ion anisotropy, and frustrating topology of the pyrochlore sublattice that hosts the magnetic Mo ions. This theoretical project is concerned primarily with behavior that involves the spin degrees of freedom in an essential way, and in progressing toward a first principles theory. Novel magnetic phenomena keep appearing: two areas that concern this grant directly are spin gap behavior in low dimensional insulators and half metallic behavior in conductors. The case of CaV4O9 (CaVO) is impressive in the speed in which its behavior has been solved within a five year time span. CaVO has a 1/5-th depleted 'square' lattice of V ions with S=1/2, and displays spin gap (or quantum spin liquid) behavior, the first two dimensional (2D) material to do so. Through a combination of experimental probes and applications of several theoretical methods from a variety of groups, the behavior of CaVO is understood. The objective of the grant is to obtain a microscopic description, based on first principles (parameter-free) methods, that is detailed enough to provide an understanding of the observed behavior. Once the description becomes good enough, the theory may be used in a predictive fashion to determine computationally, rather than experimentally, what a material's behavior will be. Becasue it is possible to 'look into' the simulation in a much more detailed way than is possible experimentally, this approach leads to insights that are not available otherwise, and will enable materials physicists to design new materials that have desired properties. %%% Theoretical research will be conducted on a variety of complex materials of current interest in order to understand their properties from a fundamental approach and, at the same time, to develop techniques which will provide the tools needed to design materials with specified propoerties. ***