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几个尚未解决的无序凝聚态问题。主要焦点将是混合相巨磁电阻(CMR)材料和弛豫铁电体。噪声为许多CMR中涉及的混合相态提供了灵敏的探头，允许对小区域进行直接热力学测量，并揭示了电流可能不均匀的程度。随机置换无序，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，特别是锰氧化物。净掺杂等同于传统固溶体材料的多层材料将特别直接地测试淬火随机性的作用。弛豫铁电体冻结成局部铁电纳米域的整体无序集合，允许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，对于各种松弛药中玻璃状冻结背后的机理或机制，几乎没有达成共识。噪声和老化使得能够确定比纳米区域的尺度更大和更小的非传统的玻璃状冻结的形式。根据一些标准松弛材料的初步结果，提出了一种新的模型，类似于可再入弹珠玻璃冻结。对几种不同的松弛材料进行比较研究，将找出哪些型号适用于哪些类别。从事这些项目的研究生已经能够使用桌面上的小科学技术和复杂的光刻和表征技术。他们通常在计算机工业的材料方面或进一步的基础研究中工作。尽管有许多研究规则有序晶体材料的成熟技术，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。研究无序物质的技术并不那么发达，因为每个地点都与其他地点略有不同。在无序材料中，通常存在许多略微不同的物理状态，在这些状态下，材料会自发波动，从而产生低频噪声。这项研究的重点是使用该噪声作为探针来研究几种材料的基本物理。将要研究的主要类型之一是巨型磁阻材料，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金属状态，从而提供一种潜在的有用的传感器设备。初步研究表明，这种变化通常是由导电区和非导电区拼凑而成的，而不是平稳的均匀变化。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更改如何影响此过渡。另一个主要话题将是弛豫铁电体，它冻结成一个拼凑的区域，在这些区域中，数千个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，形成一种随机的模式。事实证明，这种随机模式具有与规则模式不同的有用属性。噪音研究(以及缓慢老化与时间关系的相关研究)可以确定导致这种特殊冰冻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上的小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方面或进一步的基础研究方面工作。

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.