Noise Studies of Disordered Materials

本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尚未解决的问题。主要焦点将是混合相巨磁阻（CMR）材料和弛豫铁电体。噪声为许多CMR中涉及的混合相状态提供了一个敏感的探针，允许对小区域进行直接热力学测量，并揭示电流不均匀的程度。随机替代无序、净应变和各向异性应变约束在稳定混合相中的作用将在几种材料中进行研究，特别是在锰矿石中。与传统固溶体材料相同的净掺杂多层材料将提供对淬火随机性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，使它们在比传统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，对于各种松弛剂中玻璃状冻结的机制还没有达成共识。噪声和老化使我们能够在比纳米畴更大或更小的尺度上确定非常规的、玻璃状冻结的形式。通过对一些标准弛豫量的初步结果，提出了一个类似于可重入式玻璃冻结的新模型。对几种不同弛豫材料进行比较研究，可以找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型科学技术和复杂的光刻和表征技术。他们通常会继续在计算机行业的材料领域工作，或者从事进一步的基础研究。虽然有许多发展良好的技术来研究有规律有序的晶体材料，这些材料构成了半导体工业的基础，但目前正在开发用于传感器和其他应用的许多材料是无序的。研究无序材料的技术还没有得到很好的发展，因为每一个地点都与其他地点有所不同。在无序材料中，通常存在许多稍微不同的物理状态，其中材料自发波动，产生低频噪声。这项研究的重点是利用这种噪声作为探测器来研究几种材料的基本物理特性。要研究的主要类型之一将是巨磁阻材料，它可以通过施加磁场从导电性差的非磁性状态驱动到导电性金属状态，从而提供潜在有用的传感器设备。最初的研究表明，这种变化通常是通过导电和非导电区域的拼凑而发生的，而不是通过光滑的均匀变化。噪声提供了一种观察单个斑块行为的方法，从而可以详细了解材料变化如何影响这种转变。另一个主要主题将是弛豫铁电体，它冻结成一个拼凑的区域，其中成千上万的晶体细胞的电偶极子排列在一起，但这些单元随后集体冻结成一个随机的模式。这种随机模式被证明具有与常规模式不同的有用属性。噪音研究（以及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备来制作和表征样品。他们通常会继续在计算机行业的材料领域工作，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.