Noise Investigations of Condensed Matter Systems

将使用9623478魏斯曼非高斯噪声统计来探索无序凝聚态中一些重要问题的静力学和动力学。确定了六个中心主题。第一个项目将解决一些新的超导材料中的无序磁通钉扎问题。这包括研究磁通在大电流下松散，破坏超导电性的过程。第二个项目将研究小磁区之间的不均匀电导在确定巨磁电阻机制中的作用。第三个主题将研究一些隐藏的长程有序对玻璃相“固体度”的影响，例如远低于总退火温度的大规模集体重排。第四个主题是研究局域各向异性对不同金属自旋玻璃介观行为的影响。第五个项目将研究两种机制中的哪一种在决定大多数磁体的磁化过程的不均一性方面是最重要的，这两种机制是简单的磁畴壁钉扎还是集体磁畴壁重排。弛豫铁电行为的性质以及与类自旋玻璃现象的比较将是最后要研究的问题。几乎所有的真实材料都不是完全结晶的，而是有一些无序的。在许多情况下(例如玻璃)，这种无序是材料结构性质的基础，在其他情况下(例如磁场中的超导体)，它对于理解动力学行为是至关重要的。一般来说，无序会造成这样的情况，即系统可以通过热能或在外部驱动的影响下，在多种状态之间进行切换。这样的开关会在可测量的特性中产生噪声。这个项目的目标是阐明某些无序结构是如何形成并对外部驱动做出反应的，主要是通过使用噪声作为探针。要解决的中心问题是：(1)在玻璃中，是否有某种隐藏的长程有序对玻璃相的“坚固性”负责，尽管该结构具有明显的随机性？在什么温度下，玻璃中会发生大规模的集体重排？(2)在玻璃、自旋玻璃的磁性相对中，哪些参数决定了集体行为的范围和形式？在技术上重要的弛豫铁电体中，是什么决定了相同的性质？(3)大多数磁体中的潜在无序是如何造成磁化过程的不均匀的？是简单地钉扎磁畴壁，还是集体的磁畴壁重排很重要？(4)在作为磁记录和其他应用前景的场传感器研究的巨型磁阻材料中，磁阻效应的主要机制是什么？小磁区之间的不均匀电导是关键过程吗？

w:\awards\awards96\*.doc 9623478 Weissman Non-Gaussian noise statistics will be used to probe statics and dynamics of some important problems in disordered condensed matter. Six central topics are identified. The first project will address the pinning of magnetic flux by disorder in some of the new superconducting materials. This includes a study of the process by which the flux breaks loose, destroying superconductivity, at high currents.The second project will investigate the role of inhomogeneous conductance among small magnetic regions in determining the mechanism for colossal magnetoresistance. The third topic will study the effect on the "solidity" of the glass phase of some hidden long-range order, such as large-scale collective rearrangements well below the temperature at which gross annealing sets in. The forth topic consists in the study of the effect of local anisotropy in determining the qualitatively different mesoscopic behavior of different metallic spin-glasses. The fifth project will study which of the two mechanisms, simple pinning of magnetic domain walls or collective domain-wall rearrangements, is the most important in determining the unevenness of the magnetization process in most magnets. The nature of relaxor ferroelectric behavior and comparison to spin-glass-like phenomenon will be the last issue to be studied. %%% Almost all real materials are not perfectly crystalline, but rather have some disorder. In many cases (e.g. glasses), this disorder is fundamental to the structural properties of the material, in other cases (e.g. superconductors in a magnetic field) it is crucial to understanding the dynamical behavior. Generally, disorder creates situations in which there are multiple states among which the system can switch, either by thermal energy or under the influ ence of external driving. Such switches create noise in measurable properties. The goal of this project is to clarify how certain disordered structures form and respond to external driving, mostly by using noise as a probe. The central issues to be addressed are: (1) In glasses, is some hidden long-range order responsible for the "solidity" of the glass phase, despite the apparent randomness of the structure? At what temperature do large-scale collective rearrangements occur in glasses? (2) In the magnetic relative of glasses, spin- glasses, what parameters determine the range and form of the collective behavior? In technological important relaxor ferroelectrics, what determines the same properties? (3) How does the underlying disorder in most magnets create the unevenness of the magnetization process? Is a simple pinning of magnetic domain walls involved, or are collective domain-wall rearrangements important? (4) In the colossal magnetoresistive materials, investigated as promising field sensors for magnetic recording and other applications, what is the main mechanism of the magnetoresistive effect? Is inhomogeneous conductance among small magnetic regions the key process?