First Principles Theory of Complex Compounds

发现或制造具有意想不到和令人着迷的特性的新材料是一个持续的、也许是加速的过程。在过去的一年里，这种发现的一个很好的例子是铁磁UGe2在保持磁性的同时变成超导。另一个例子是自旋玻璃化合物Y2Mo2O7，它的结构是有序的，而在它被发现之前，结构无序是自旋玻璃行为所必需的。这样的发现引起了材料物理学家的极大兴趣，似乎总是强烈地影响着该领域的后续发展，即。超导，近藤效应，量子霍尔效应。在RuSr2GdCu2O8中，超导电性和磁性共存，虽然根本不被理解，但仅仅是因为磁性和超导层的分离才被允许；在Y2Mo2O7中，人们认为自旋玻璃化可能是铁磁交换耦合、强烈的单离子各向异性和承载磁性Mo离子的焦绿石亚晶格令人沮丧的拓扑结构的综合结果。这个理论项目主要涉及本质上涉及自旋自由度的行为，并正在向第一原理理论发展。新的磁现象不断出现：两个与这一赠款直接相关的领域是低维绝缘体中的自旋能隙行为和导体中的半金属行为。CaV4O9(Cavo)的案例令人印象深刻，其行为在五年内得到解决的速度令人印象深刻。通过结合实验探索和来自不同群体的几种理论方法的应用，我们了解了CAVO的行为。赠款的目标是基于第一原理(无参数)方法获得足够详细的微观描述，以提供对观察到的行为的理解。一旦描述变得足够好，该理论就可以用预测性的方式来通过计算而不是实验来确定材料的行为。由于有可能以比实验更详细的方式研究模拟，这种方法带来了其他方法无法获得的见解，并将使材料物理学家能够设计具有所需性能的新材料。将对当前感兴趣的各种复杂材料进行理论研究，以便从根本上了解它们的性能，同时开发技术，提供设计具有特定性能的材料所需的工具。*

0114818PickettDiscovery 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.***