Noise Studies of Disordered Materials

这项凝聚态物理研究将低频噪声和老化技术应用于无序凝聚态的几个未解决的问题。 主要焦点将是混合相巨磁阻（CMR）材料和弛豫铁电体。 噪声提供了一个敏感的探针的混合相状态参与了许多CMR，允许直接热力学测量的小区域，并揭示了在何种程度上的电流可以是不均匀的。 随机置换无序，净应变，和各向异性应变约束稳定混合相的作用将在几种材料，特别是锰氧化物。 具有与常规固溶体材料相同的净掺杂的多层材料将提供对猝灭随机性的作用的特别直接的测试。 弛豫铁电体冻结成局部铁电纳米畴的整体无序集合，使它们能够在比常规铁电体更宽的温度范围内保持有用的高介电和压电系数。 然而，在各种弛豫剂中，这种玻璃态冻结背后的机制或机制几乎没有共识。 噪音和老化允许确定的形式的非常规的，玻璃冻结的尺度上更大和更小的比那些的nanodomains。 一个新的模型，类似于一个可重入的spingglass冻结，建议一些标准松弛的初步结果。 几种不同的弛豫材料的比较研究将整理出哪些模型适用于哪些类别。 从事这些项目的研究生已经能够使用桌面小科学技术和复杂的光刻和表征技术。 他们通常从事计算机工业的材料研究或进一步的基础研究。尽管有许多成熟的技术可以研究规则有序的晶体材料，这些材料构成了半导体工业的基础，但许多正在开发的用于传感器和其他应用的材料是无序的。 研究无序材料的技术还没有得到很好的发展，其中每个站点都与其他站点略有不同。 在无序材料中，通常存在许多略微不同的物理状态，材料在这些状态之间自发波动，从而产生低频噪声。 这项研究的重点是使用这种噪音作为探针来研究几种材料的基本物理。 要研究的主要类型之一将是巨大的磁阻材料，其可以通过施加磁场从不良传导的非磁性状态驱动到传导的金属状态，提供潜在有用的传感器设备。 最初的研究表明，这种变化通常是由导电和非导电区域拼凑而成的，而不是由平滑的均匀变化发生的。噪波提供了一种查看单个面片行为的方法，因此可以详细查看材质的变化如何影响此过渡。 另一个主要的主题是弛豫铁电体，它冻结成一个拼凑的区域，其中成千上万个水晶细胞的电偶极子排列在一起，但这些单元随后以随机的模式集体冻结。 这种随机模式具有不同于常规模式的有用特性。 噪声研究（以及缓慢老化与时间的相关研究）可以确定驱动这种特殊冻结的相互作用的规模和类型。 从事这些项目的学生已经能够使用桌面小型科学技术和复杂的大型设施来制作和表征样品。 他们通常会在计算机行业的材料方面工作，或者从事进一步的基础研究。

This condensed matter physics research applies low frequency noise and aging techniques to several unsolved problems of disordered condensed matter. The primary foci will be mixed-phase colossal magnetoresistive (CMR) materials and relaxor ferroelectrics. Noise provides a sensitive probe of the mixed-phase states involved in much CMR, allowing direct thermodynamic measurements of small regions and revealing the extent to which the current flow can be inhomogeneous. The roles of random substitutional disorder, of net strain, and of anisotropic strain constraints in stabilizing mixed phases will be investigated in several materials, particularly manganites. Multilayer materials with net doping equal to that of conventional solid-solution materials will provide a particularly direct test of the role of quenched randomness. Relaxor ferroelectrics freeze into an overall disordered collection of locally ferroelectric nanodomains, allowing them to retain useful high dielectric and piezoelectric coefficients over a broader temperature range than do conventional ferroelectrics. However, there is little consensus on the mechanism or mechanisms behind this glassy freezing in various relaxors. Noise and aging allow determination of the forms of unconventional, glassy freezing on scales both larger and smaller than those of the nanodomains. A new model, analogous to a reentrant spinglass freezing, is suggested by initial results on some standard relaxors. Comparative studies of several dissimilar relaxor materials will sort out which models are applicable to which categories. The graduate students working on these projects have been able to use both table-top small-science techniques and sophisticated lithography and characterization techniques. They have typically gone on to work in the materials side of the computer industry or in further basic research.Although there are many well-developed techniques for studying regularly ordered crystalline materials, which have formed the basis of the semiconductor industry, many of the materials now under development for use in sensors and other applications are disordered. The techniques for studying disordered materials, in which every site is a bit different from every other site, are not so well developed. In disordered materials, there are typically many slightly different physical states among which the material spontaneously fluctuates, giving rise to low-frequency noise. This research focuses on using that noise as a probe to study the basic physics of several materials. One of the main types to be studied will be colossal magnetoresistive materials, which can be driven from a poorly conducting non-magnetic state to a conducting metallic state by application of a magnetic field, providing a potentially useful sensor device. Initial studies show that this change typically happens by a patchwork of conducting and non-conducting regions, rather than by a smooth homogeneous change. Noise provides a way of seeing the behavior of the individual patches, and thus allows a detailed look at how changes in materials affect this transition. The other main topic will be relaxor ferroelectrics, which freeze into a patchwork of regions in which the electrical dipoles of many thousands of crystalline cells line up together, but these units then collectively freeze in a random-looking pattern. That random pattern turns out to have useful properties unlike those of regular patterns. Noise studies (and related studies of slow aging vs. time) allow determination of the scale and type of the interactions driving this peculiar freezing. The students working on these projects have been able to use both table-top small-science techniques and sophisticated large-scale facilities for making and characterizing samples. They have typically gone on to work in the materials side of the computer industry or in further basic research.