The depinning of density waves and the concomitant elastic softening in quasi-one dimensional metals is of current interest due to their non-linear behavior. In particular, it is not understood if the softening is intrinsic to the density wave or is due to relaxation of defects or domains in the density wave. This proposal deals with experiments that are specifically designed to address this phenomena. The central experiment is to measure changes in the static shear modulus of tantalum trisulfide when its charge-density-wave becomes depinned. The measurements will utilize a recently developed sensitive position detector. Changes in the static twist angle of a fiber of tantalum trisulfide will be measured with a frequency modulated helical resonator (400 MHz) cavity. Torque will be applied with a magnetic wire "flag" attached to the sample. Experiments to measure the temperature dependence of the specific heat at the different phase transitions exhibited by quasi-one dimensional metals; the goal is to provide a basis for a better understanding of the role of fluctuations at these transitions. Phase transformations to be investigated include charge-density-wave, spin-density-wave, anion-ordering, and spin-Peierls transitions. Specific heat on small crystals (typically less than 1 mg) will be measured using ac calorimetry and can be applied to crystals as small as 0.01mg. Complimentary measurements will include Young's modulus and magnetic susceptibility. Results will be fit to various fluctuation models. %%% Quasi-one-dimensional metals conduct along only one crystallographic direction and, as a consequence, exhibit some unusual and unexplained phenomena. Many are inherently non-linear and/or involve strong fluctuation effects and this proposal describes some experiments using new, sensitive probes, which address long-standing problems in our understanding of these materials. Many quasi-one dimensional metals exhibit electron density wave distortions, in which the charge or spin density becomes periodically modulated, at low temperature. In some of these materials, the density wave can be depinned (forced to move) with extremely small electric fields; this effect has become a paradigm for non-linear dynamics in solids. When the density wave is depinned, the materials become elastically softer; it is not understood if this softening is only a dynamic effect, caused by the motion of density wave defects, or intrinsic due to the sliding density wave. This proposal studies changes in the static shear modulus of tantalum trisulfide, a prototypical charge-density-wave material, that directly addresses this question. The investigators have recently developed a radio-frequency helical resonator cavity operating at 400 MHz as an extremely sensitive probe that can detect sample motion of less than 1 Angstrom; this apparatus will be adapted to measure the static properties of these novel materials. If successful, the technique may be applicable for related studies, such as torque magnetometry. In addition quasi-one dimensional metals exhibit an wide array of low temperature phase transitions. There has been considerable theoretical interest in these transitions because fluctuation effects are enhanced by their one-dimensionality. However, high quality thermodynamic measurements, which probe these fluctuation effects have been lacking. The investigators will continue using ac calorimetry to study the specific heat of these materials. In particular charge-density wave, spin-density wave, order-disorder, and spin-Peierls transitions will be studied. Complementary thermodynamic measurements will include Young's modulus and magnetic susceptibility. The results will be analyzed in the context of current theoretical investigations to understand the role of fluctuations at these transitions.
