Noise Investigations of Condensed Matter Systems

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Most materials exhibit slow fluctuations, with a broad distribution of characteristic rates, in a variety of electrical, mechanical, magnetic, and optical parameters. Despite the ubiquity of a simple form (1/f) for the frequency spectrum of such fluctuations, a wide variety of mechanisms are involved, as has been demonstrated in past experiments in this project. These fluctuations are of interest for two general reasons: First, they limit the performance of a variety of sensors and devices. Second, they provide a useful tool to study fundamental properties of materials, especially glasses, which exhibit partially frozen random structures. This proposal applied fluctuation techniques to investigate several materials with some of the most common forms of frozen disorder. These include amorphous silicon, charge-density wave systems, several types of spin-glasses, spin-density wave magnetic domains, domain fluctuations in ferromagnets, and the new giant-magneto-resistive materials. The work will emphasize particularly the interplay between the chaotic behavior of some driven systems and the frozen randomness present in equilibrium. %%% New technologies often require new materials and increasingly involve the use of very small structures. In small structures such properties as the electrical resistivity of materials are generally not constant, but fluctuate in time. Such fluctuations are often the most important factor in limiting the miniaturization of devices. Past experiments in this project have shown that a wide variety of mechanisms lie behind these fluctuations in different materials. This proposal explores the origins of the fluctuations in some technically important materials, including amorphous silicon and the new giant-magneto-resistive materials. Particular attention is paid to developing a general understanding of the very common but poorly understood types of materials known as glasses, which exhibit partially frozen random structures.