Kinetics of Charge-Density Waves: Time Resolved X-ray Scattering Studies

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9501119 Brock The objective of this research program is to measure the transient structural response of a charge-density-wave (CDW) to sudden changes in either the driving electric field or the temperature. These data will be used to test modern nonequilibrium statistical mechanical theories developed to describe the growth of materials in the presence of random impurities and defects. A common theme running through many of these models is the derivation of an equation of motion for a hydrodynamic variable such as mass density, concentration, or temperature. The Kardar-Parisi-Zhang equation is an example of such an equation of motion which has recently received considerable attention in the literature. Formally, the equations of motion for the various correlation functions of interest follow directly from the equation of motion for the hydrodynamic variable. In spite of the well-defined nature of the question, there are very few direct experimental tests of any of these models. However, x-ray scattering measurements of the transient structural response of the CDW's are nearly ideal for such tests. To first order, one can consider CDWs to be nearly sinusoidal quasi-one-dimensional density waves and x-rays are diffracted by the periodic structure. Detailed equations of motion describing the local phase of this density wave have been proposed. This project will test these models by applying a time- dependent driving electric field, or temperature to the sample and then measuring the transient structural response of the CDW using synchrotron based, time-resolved, high-resolution x-ray scattering. Comparison will be made with previous measurements of transients in the electronic transport by performing in-situ current-voltage measurements. %%% In recent years, mathematical models describing the growth of materials have received cons iderable attention because of technological interests in growing novel materials. From the theoretical, point of view, this interest is due, in large part, to predictions that materials growth should exhibit phenomena similar to those exhibited by systems at phase transitions (e.g., at the critical temperature where a material becomes superconducting). Consequently, the successful theoretical tools developed in the 1970's to study phase transitions are being brought to bear on this new problem. In spite of the large theoretical effort, there are very few direct experimental tests of any of these models. X-ray scattering measurements of the evolution of a charge-density-wave (CDW) system are nearly ideal for experimental tests for some of these growth models. CDWs occur at low temperatures in certain metals. Although their origin is quantum mechanical, in the simplest approximation, a CDW can be thought of as a small but repetitive variation in the average density of atoms. In many cases, when an electric field is applied to the sample, the CDW responds by "sliding" rigidly through the crystal. This project will test these models by applying an electric field (or change in temperature) to the sample and then use x-rays to measure the structural evolution of the CDW. ***