First Principles Theory of Complex Compounds

[01:14818 .皮克特发现或制造具有意想不到的迷人性能的新材料是一个持续的，也许是加速的过程。在过去的一年中，这种发现的一个很好的例子是，铁磁体UGe2在保持磁性的同时变成了超导。1957年，Ginzburg认为这种共存是不可能发生的，直到最近几年才受到严肃的质疑（RuSr2GdCu2O8由于完全不同的原因而具有相关的性质）。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，人们认为自旋玻璃的行为需要结构无序。这些发现引起了材料物理学家的极大兴趣，并且似乎总是对超导、近藤效应、量子霍尔效应等领域的后续发展产生强烈影响。在上面提到的每一种材料中，意外的行为不可能在更简单的系统中出现（无论是结构上的，还是相互作用方面的），因为不存在必要的自由度。在RuSr2GdCu2O8中，超导性和磁性共存，虽然完全不被理解，但这仅仅是因为磁性层和超导层的分离；在Y2Mo2O7中，自旋玻璃性被认为可能是铁磁交换耦合、强单离子各向异性和含有磁性Mo离子的焦绿盐亚晶格的令人沮丧的拓扑结构的综合结果。这个理论项目主要关注的是在本质上涉及自旋自由度的行为，并朝着第一原理理论发展。新的磁现象不断出现：直接关系到这一拨款的两个领域是低维绝缘体中的自旋间隙行为和导体中的半金属行为。CaV4O9 （CaVO）的案例令人印象深刻的是，它的行为在五年的时间跨度内就被解决了。CaVO具有1/5耗尽的V离子“正方形”晶格，S=1/2，并显示自旋间隙（或量子自旋液体）行为，这是第一个二维（2D）材料。通过实验探针和不同群体的几种理论方法的应用相结合，CaVO的行为被理解。拨款的目的是根据第一性原理（无参数）方法获得微观描述，该描述足够详细，可以提供对所观察行为的理解。一旦描述变得足够好，该理论就可以以预测的方式用于计算而不是实验来确定材料的行为将是什么。因为有可能以比实验更详细的方式“研究”模拟，这种方法可以获得其他方法无法获得的见解，并使材料物理学家能够设计出具有理想性能的新材料。%%%将对当前感兴趣的各种复杂材料进行理论研究，以便从基本方法了解其特性，同时开发技术，为设计具有特定特性的材料提供所需的工具

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.***