Noise Investigations of Condensed Matter Systems

在本提案中，涨落技术将用于探索无序凝聚态中的几个主题。巨磁阻（CMR）跃迁表现出突出的非高斯噪声，这将用于确定介观域的热力学性质，介观域通常负责磁阻效应。各种CMR材料将被研究。这些畴性质对介观CMR器件的设计有重要影响。弛豫铁电体，具有巨大介电系数和非线性压电的无序材料，将被研究，以确定是什么影响负责阻止长程有序的形成，从而保持有用的电学性质。老化实验和介观噪声实验应该能够确定弛豫顺序形成的长度尺度，以及确定随机各向异性、随机矢量场和随机转变温度在弛豫状态中的作用。畴壁响应外场的不规则运动（巴克豪森噪声）的一般问题已为人所熟知多年，但直到最近才有很好的理论方法来研究无序和系统畴壁力之间的相互作用。将进行实验，以区分空间相关无序的影响和域壁上的集体效应，以及测试有关噪声的尺度特性与无序强度的想法。因为这样的测量是“小科学”，每个参与的学生（包括研究生和本科生）都有机会从事材料制备，微型电子设备的制造（使用材料研究实验室的先进设备），复杂的材料表征设备（在MRL），计算机编程，模拟电子学和低温技术。随着凝聚态物理测量技术的改进，不仅可以从材料的平均特性（如电阻）中提取有用的信息，还可以从平均值周围的小波动（如电噪声）中提取有用的信息。这种噪声技术对于大量无规则有序的普通材料（如玻璃）特别有价值，因为在这些材料中，平均特性往往不能传达有关无序的重要信息。例如，在最近开发的一类材料中，其电阻随磁场变化而变化很大，其平均响应掩盖了噪声所揭示的一个事实——即材料的电阻率并不是以连续的方式均匀地变化，而是由一组微小的域组成，这些域在不同的磁场下突然切换。这些发现对于规划这些材料在小型计算机部件中的应用至关重要。因为在这个项目中进行的测量是“小科学”，每个参与的学生（包括研究生和本科生）都有机会从事材料制备，微型电子设备的制造（使用材料研究实验室的先进设备），复杂的材料表征设备（在MRL），计算机编程，模拟电子学和低温技术。具有这种背景和技能水平的学生在工业中需求量很大，特别是在磁存储设备的开发方面

In this proposal fluctuation techniques will be used to explore several topics in disordered condensed matter. Colossal magnetoresistive (CMR) transitions exhibit prominent non-Gaussian noise, which will be used to determine thermodynamic properties of the mesoscopic domains which are typically responsible for the magnetoresistive effect. A variety of CMR materials will be investigated. The design of mesoscopic CMR devices is crucially affected by these domain properties. Relaxor ferroelectrics, disordered materials with huge dielectric coefficients and non-linear piezo-electricity, will be investigated to determine what effects are responsible for preventing the formation of long-range order and hence for preserving the useful electrical properties. Aging experiments and mesoscopic noise experiments should be able to determine the length scale on which relaxor order forms, as well as determining the roles of random anisotropy, random vector fields, and random transition temperatures in the relaxor state. The general problem of irregular motion of domain walls (Barkhausen noise) in response to external fields has been familiar for many years, but only recently have there been good theoretical approaches to the interplay between disorder and systematic domain-wall forces. Experiments will be conducted to distinguish between effects of spatially correlated disorder and of collective effects in the domain walls, as well as to test ideas relating scaling properties of the noise to the strength of the disorder. Because such measurements are "small science", each student involved (both graduate and undergraduate) gets a chance to work on material preparation, fabrication of tiny electrical devices (using advanced facilities at the Materials Research Lab), sophisticated materials characterization facilities (at the MRL), computer programming, analog electronics, and cryogenic techniques.%%%As measurement techniques in condensed matter physics have improved, it has become possible to extract useful information about materials not only from their average properties (e.g. electrical resistance) but also from the small fluctuations around the average values (e.g. electrical noise). Such noise techniques are particularly valuable for the large number of common materials (e.g. glasses) which are not regularly ordered, because in such materials the average properties often fail to convey important information about the disorder. For example, in a recently developed category of materials whose resistance changes by a large factor in response to magnetic fields the average response obscures a fact revealed by the noise--that the material does not uniformly change its resistivity in a continuous fashion but rather consists of a collection of tiny domains which abruptly switch at different fields. Such findings are crucial in planning applications of these materials in small-scale computer components. Because the measurements undertaken in this project are "small science", each student involved (both graduate and undergraduate) gets a chance to work on material preparation, fabrication of tiny electrical devices (using advanced facilities at the Materials Research Lab), sophisticated materials characterization facilities (at the MRL), computer programming, analog electronics, and cryogenic techniques. Students from such backgrounds and skill levels are in high demand in industry, particularly in the development of magnetic storage devices.***