Polaron-Induced Local Distortions in Correlated Electron Systems

这是一个新的项目，提出了x射线吸收精细结构（XAFS）测量具有金属绝缘体跃迁和巨磁电阻（CMR）效应的系统，如La1-xCaxMnO3。在这些氧化物中，铁磁转变温度Tc附近的大磁电阻现在归因于磁性、电子系统和局部晶格畸变之间的一种新的耦合，在最近的一篇论文中，我们已经表明，从金属到绝缘行为的转变与局部畸变幅度的快速增加密切相关，这些畸变被认为是极化的起源。然而，原子位移的性质、对称性和范围尚不清楚，需要对这些不寻常的输运效应进行建模。XAFS测量作为温度和磁场的函数将使我们能够探测金属原子局部结构扭曲的细节，并提供这一重要信息。这是一个新项目，提出了对La1-xCaxMnO3等系统的x射线吸收精细结构（XAFS）测量，这些系统在磁性和电学行为方面表现出新的特征，似乎与材料的原子结构有关。最近的理论模型表明，在磁性消失的同一温度下，原子的排列变得扭曲，并且认为这些扭曲有助于观察到对电流流动的电阻的大幅增加。我们最初的研究支持这样一个模型，但是关于原子排列的细节还不清楚。我们将使用x射线吸收技术来研究给定原子在不同温度和磁场下的畸变。我们需要确定哪些原子被扭曲了，以及扭曲的范围和程度，以测试和改进这种不寻常行为的模型。微观原子排列和电性能之间的这种新联系涉及到新的物理学，并可能导致新的应用，因为这种材料可能有助于制造计算机磁盘上读写信息的设备。＊＊＊

9705117 Bridges This is a new project which proposes X-Ray absorption fine structure (XAFS) measurements on systems exhibiting metal- insulator transitions and Colossal Magnetoresistance (CMR) effects, such as La1-xCaxMnO3. In these oxides, the large magnetoresistance near the ferromagnetic transition temperature, Tc, is now attributed to a novel coupling between magnetism, the electron system and local lattice distortions and, in a recent paper, we have shown that the transition from metallic to insulating behavior correlates well with a rapid increase in the amplitude of the local distortions which are believed to be polaronic in origin. However, the nature, symmetry and extent of the atomic displacements are not yet known and are needed for modeling these unusual transport effects. XAFS measurements as a function of temperature and magnetic field will enable us to probe the details of the local structural distortions about the metal atoms and provide this crucial information. %%% This is a new project which proposes X-Ray absorption fine structure (XAFS) measurements on systems such as La1-xCaxMnO3 which exhibit novel features in their magnetic and electrical behavior that appear to be related to the atomic structure of the material. Recent theoretical models suggest that the arrangement of the atoms become distorted at the same temperature at which the magnetic properties disappear and it is suggested that these distortions contribute to the large observed increase in the resistance to the flow of electrical current. Our initial study supports such a model, but details about the arrangements of the atoms are not yet known. We will use x-ray absorption techniques to investigate the distortions about a given atom at various temperatures and magnetic fields. We need to determine which atoms are distorted and the range and magnitude of the distortions to test and improve the models of this unusual behavior. This novel connection between the microscopic atomic arrangement and the electrical properties involves new physics and may lead to new applications since such materials are potentially useful to make devices for reading and writing information on computer disk. ***