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

小行星9501119 本研究计划的目的是测量电荷密度波（CDW）的瞬态结构响应的驱动电场或温度的突然变化。 这些 数据将 使用 到 测试 现代非平衡统计力学理论 描述在存在随机杂质和缺陷的情况下材料的生长。贯穿这些模型的一个共同主题是推导出流体动力学变量（如质量密度、浓度或温度）的运动方程。 Kardar-Parisi-Zhang方程就是一个例子。 一个 运动方程最近 颇受 注意文学。 从形式上讲， 各种感兴趣的相关函数的运动方程直接从流体动力变量的运动方程得出。尽管问题的性质定义明确，但这些模型中的任何一个都很少有直接的实验测试。 然而，X射线散射测量的瞬态结构响应的CDW的是几乎理想的这样的测试。对于第一阶，可以认为CDW是近似正弦准一维密度波，并且X射线被周期性结构衍射。 详细的运动方程描述了这种密度波的局部相位已经提出。 该项目将通过向样品施加随时间变化的驱动电场或温度，然后使用基于同步加速器的时间分辨高分辨率X射线散射测量CDW的瞬态结构响应来测试这些模型。将进行比较与以前的测量瞬态的电子传输进行现场电流-电压测量。 近年来，描述材料生长的数学模型 受到了广泛的关注，因为 对培育新材料的技术兴趣。 从理论的角度来看，这种兴趣在很大程度上是由于预测材料生长应该表现出类似于系统在相变时表现出的现象（例如， 在 的 关键 温度下 一 材料 成为超导体）。因此，20世纪70年代发展起来的研究相变的成功的理论工具正被用来解决这个新问题。尽管大量的理论工作，有很少的直接实验测试的任何这些模型。X射线散射测量的电荷密度波（CDW）系统的演变几乎是理想的实验测试这些增长模型。 某些金属在低温下会发生CDW。 虽然它们的起源是量子力学的，但在最简单的近似中，CDW可以被认为是原子平均密度的微小但重复的变化。在许多情况下，当向样品施加电场时，CDW通过刚性地"滑动"通过晶体来响应。 该项目将通过向样品施加电场（或温度变化）来测试这些模型，然后使用X射线测量CDW的结构演变。 ***

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. ***