First Principles Theory of Complex Compounds

9802076 Pickett这个奖项是为了支持当前人们感兴趣的各种材料的电子结构和性能的理论研究。最近发现的产生巨大活性的材料的例子包括：化合物Sr14- xCaxCu24O41，它有Cu-O链和阶梯，但没有层，但在特定的压力范围内，在狭窄的x范围内具有超导性；一类自旋为1/2的层状钒酸盐CaVnO2n+1，表现为自旋-佩尔斯行为（n=3）或自旋-间隙行为（n=2和n=4）；Fe-Si系统FenSim，其中n=1, m=2和n=2, m=1成员是相当标准的金属，而n=m=1成员是一个非常不寻常的“近藤绝缘体”，能量缺口为60meV；以及在热管理应用中表现出极好的热电性能的填充方钨矿体系，如Ce1-xLaxCo4Sb12。当然，高温超导铜酸盐由于其明显的d波阶参数、部分由于其短相干长度而产生的复杂通量晶格行为，以及在很大程度上由于微观配对机制的持续难以捉摸而继续引起关注。人们对所谓的“巨磁阻”锰矿材料产生了广泛的兴趣，这些材料显示出大量的电荷、自旋和结构秩序，以及不寻常的场驱动重入行为。在一些情况下，新材料的发展源于理论和实验之间的密切相互作用，不仅理论的作用是对性质的解释，而且还直接表明什么可能是有趣的，在哪里寻找它，等等。许多这些新材料的共同特征是：(1)它们是多组分的，其性质强烈依赖于化学计量；（ii）它们的晶体结构不对称，表明它们具有不寻常的化学键模式；（iii）它们的性质是非常规的，通常随着材料参数的微小变化而变化很大（掺杂水平、温度、压力，甚至离子大小有时都是至关重要的）；（iv）它们涉及流动载体和某种意义上本地化的国家之间的相互作用。正是这最后一个特征，连同其他的复杂因素，提供了这种奇特的行为。对于试图描述系统的理论家来说，单胞的复杂性通常不会造成根本的困难。复杂的金属间化合物、第四元半导体和半导体超晶格通常可以通过结合现象学方法和密度泛函理论中的局部密度近似来处理。同样，简单晶格中的局域原子态（即岩盐结构中的NiO）可能会导致非常有趣的相关电子行为，但它们根本不具备更复杂晶格中存在的自由度。在许多这样的系统模型中，将继续需要哈密顿量来处理它们的热力学和光谱性质。然而，从第一性原理出发，迫切需要解决在组合流动/局部载流子系统中出现的伴随相关电子行为的单元胞的复杂性。这里提出的工作将应用方法的组合-参数化紧密结合哈密顿量，精确的局部自旋密度计算，“超越LDA”创新和精确对角化方法-用于这种复杂的化合物，期望同时增加对观察到的行为的理解并扩展第一原理方法的能力。这项理论研究将调查当前感兴趣的复杂材料，这些材料显示出定位的电子（即固定在原子或分子上的电子）与非定位的电子之间的竞争。这种竞争，连同其他材料的特性，如温度、压力和浓度，可以产生非常不寻常的影响，这些影响是至关重要的，并可能导致技术应用。＊＊＊

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. ***