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

本研究计划的目的是测量电荷密度波（CDW）对驱动电场或温度突然变化的瞬态结构响应。这些数据将用于检验现代非平衡统计力学理论，这些理论是用来描述存在随机杂质和缺陷的材料生长的。贯穿这些模型的一个共同主题是推导流体动力学变量（如质量密度、浓度或温度）的运动方程。卡尔达-帕里西-张方程是这种运动方程的一个例子，最近在文献中受到了相当大的关注。形式上，我们感兴趣的各种相关函数的运动方程直接从流体动力变量的运动方程推导出来。尽管这个问题的性质很明确，但对这些模型的直接实验测试却很少。然而，CDW的瞬态结构响应的x射线散射测量几乎是这种测试的理想方法。对于一阶，可以认为cdw是近似正弦的准一维密度波，x射线被周期结构衍射。提出了描述密度波局部相位的详细运动方程。该项目将通过对样品施加时间相关的驱动电场或温度来测试这些模型，然后使用基于同步加速器的时间分辨高分辨率x射线散射来测量CDW的瞬态结构响应。通过进行原位电流-电压测量，将与先前的电子输运瞬态测量进行比较。近年来，由于对新材料生长的技术兴趣，描述材料生长的数学模型受到了相当大的关注。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的生长应该表现出类似于相变系统所表现出的现象（例如，在材料变成超导的临界温度下）。因此，20世纪70年代发展起来的研究相变的成功理论工具正被用于研究这个新问题。尽管在理论上做出了巨大的努力，但对这些模型的直接实验测试却很少。电荷密度波（CDW）系统演化的x射线散射测量对于其中一些生长模型的实验测试几乎是理想的。cdw在某些金属的低温下发生。虽然它们的起源是量子力学，但在最简单的近似中，CDW可以被认为是原子平均密度的一个小而重复的变化。在许多情况下，当电场作用于样品时，CDW的反应是刚性地“滑动”穿过晶体。该项目将通过对样品施加电场（或温度变化）来测试这些模型，然后使用x射线来测量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. ***