Theory of Solids

这笔赠款支持对模拟真实三维实体行为的模型的理论研究。介观物理学的最新发展为研究强无序体系中的电或热输运提供了强大的新的计算机算法。我们将对“电阻率饱和”现象进行研究，以验证“混带”效应的假设，并使超额电导超越玻耳兹曼输运理论的天真预测。将对一系列具有日益复杂的能带结构的模型进行数值研究。晶格热输运中的一个类似问题是玻璃的导热系数。非晶硅的真实原子模型将用介观物理的工具来处理。有许多重要的氧化物材料具有钙钛矿(ABO3)或相关的晶体结构，包括铁电材料、高温超导体、离子导体电解液等。虽然大多数钙钛矿是绝缘体，但也有少数是金属，相当多的是金属到绝缘体的转变作为掺杂或温度的函数。Ba1-xKxBiO_3和La_(1-x)CaxMnO_3系统是特别好的例子。纯端元(x=0)化合物是由于基态对称性的破坏，即BaBiO_3中的电荷有序，LaMnO_3中的轨道有序而具有绝缘性的材料的原型。当掺杂水平x增加到0.2-0.4的临界值时，这两个系统都成为有趣的金属。对于参与这种从金属到绝缘体转变的电子来说，存在着很好的模型。建议对这些材料中的自陷态进行研究。特别是相对端元(x=1)CaMnO_3，当轻掺杂电子(x=1-epsilon)时，将被选作研究自旋极化子效应和晶格极化子效应之间竞争的模型体系。在该模型中，BaBiO_3和LaMnO_3的最低电子激发态都是自陷激子。将研究这些化合物及其新的光学性质(吸收光谱、共振拉曼光谱等)。会被预测到。当重掺杂时，空穴应该自组织成平面反相界带电缺陷(三维版本的“条纹”)。我们将对这些进行研究，并预测其性能。金属-绝缘体的转变将作为掺杂(极化子-玻璃坍塌)的函数被模拟和仔细描述。%这项资助支持氧化物材料的电学和热学性质的理论研究，由于随着添加剂的温度和/或浓度的变化，氧化物材料的复杂行为是非常重要的。这些材料表现出丰富的效应，包括铁电性、高温超导和离子导电，可能导致广泛的应用。

0089492AllenThis grant supports theoretical research on models which mimic the behavior of real three-dimensional solids. Recent developments in mesoscopic physics have provided powerful new computer algorithms for studying electrical or heat transport in strongly disordered systems. The phenomenon of "resistivity saturation" will be studied, in order to test the hypothesis that "band-mixing" effects are involved and cause the excess conduction beyond the naive prediction of Boltzmann transport theory. A series of models will be studied numerically with increasingly complex band structures. An analog problem in lattice heat transport is the thermal conductivity of glasses. A realistic atomistic model for amorphous silicon will be treated using the tools of mesoscopic physics.There are many important oxide materials with the perovskite (ABO3) or related crystal structures, including ferroelectrics, high temperature superconductors, ionic conductor electrolytes, etc. Although most perovskites are insulators, a few are metals, and quite a few have metal-to-insulator transitions as a function of doping or of temperature. The systems Ba1-xKxBiO3 and La1-xCaxMnO3 are particularly nice examples. The pure end-member (x=0) compounds are prototype examples of materials which are insulating because of a broken symmetry in the ground state, namely charge ordering in BaBiO3 , orbital ordering in LaMnO3 . Both systems become interesting metals when the doping level x increases to a critical value 0.2-0.4. Good models exist for the electons which participate in this metal-to-insulator transition. It is proposed to study self-trapped states in these materials. In particular, the opposite end-member (x=1) CaMnO3 , when lightly doped with electrons (x=1-epsilon) will be chosen as a model system for study of the competition between spin-polaron effects and lattice-polaron effects. The lowest electronic excitations of both BaBiO3 and LaMnO3 in this model are self-trapped excitons. These will be studied and their novel optical properties (adsorption spectra, resonant Raman spectra, etc.) will be predicted. When more heavily doped, holes should self-organize into planar anti-phase boundary charged defects (the three-dimensional version of "stripes"). These will be studied and the properties predicted. The metal-insulator transition as a function of doping (polaron-glass collapse) will be modeled and carefully characterized.%%% This grant supports theoretical research on the electrical and thermal properties of oxide materials which are of great fundamental interest due to their complex behavior as the temperature and/or concentration of additives is changed. These materials exhibit a wealth of effects including ferroelectricity, high temperature superconductivity and ionic conduction which may lead to widespread applications.***