Noise Investigations of Condensed Matter Systems

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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?