Electromechanical and Thermodynamic Properties of Quasi-One Dimensional Metals

由于密度波的非线性行为，准一维金属中密度波的去钉扎和伴随的弹性软化是当前研究的热点。特别是，目前还不清楚软化是密度波固有的，还是由于密度波中的缺陷或磁畴的松弛所致。这项提案涉及专门为解决这一现象而设计的实验。中心实验是测量当三硫化钽的电荷密度波消失时，其静态剪切模数的变化。测量将利用最近开发的灵敏位置探测器。用调频螺旋谐振腔(400 MHz)测量三硫化钽光纤的静态扭转角的变化。扭矩将与磁线“旗帜”连接在样品上。测量准一维金属在不同相变时比热随温度变化的实验；目的是为更好地理解涨落在这些相变中的作用提供基础。要研究的相变包括电荷密度波、自旋密度波、阴离子有序和自旋-Peierls相变。小晶体(通常小于1毫克)的比热将用交流量热法测量，并可应用于小至0.01毫克的晶体。免费测量将包括杨氏模数和磁化率。结果将符合不同的波动模型。%准一维金属只沿一个晶体方向传导，因此表现出一些不寻常和无法解释的现象。许多材料本身是非线性的和/或涉及强烈的涨落效应，本提案描述了一些使用新的、敏感的探测器的实验，这些实验解决了我们在理解这些材料方面长期存在的问题。许多准一维金属在低温下表现出电子密度波失真，其中电荷密度或自旋密度变得周期性地调制。在其中一些材料中，密度波可以在极小的电场下被去除(被迫移动)；这种效应已经成为固体中非线性动力学的范例。当密度波被脱钉时，材料变得弹性软化；目前尚不清楚这种软化只是一种动态效应，是由密度波缺陷的运动引起的，还是由滑动密度波引起的内在效应。这项建议研究了三硫化钽的静态剪切模数的变化，这是一种典型的电荷密度波材料，直接解决了这个问题。研究人员最近开发了一种工作在400 MHz的射频螺旋谐振器腔，作为一种极其灵敏的探头，可以检测到小于1埃的样品运动；该设备将被改装成测量这些新材料的静态特性。如果成功，该技术可能适用于相关研究，如扭矩磁测量。此外，准一维金属还表现出广泛的低温相变。由于涨落效应因其一维特性而增强，所以在理论上对这些相变有相当大的兴趣。然而，一直缺乏高质量的热力学测量来探索这些涨落效应。研究人员将继续使用交流量热法来研究这些材料的比热。特别是电荷密度波、自旋密度波、有序无序和自旋Peierls相变。补充热力学测量将包括杨氏模数和磁化率。这些结果将在当前理论研究的背景下进行分析，以了解波动在这些转变中的作用。

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.