Collective Mode Dynamics in Superfluid 3He

这个实验项目与超流体3He中Cooper对的内部自由度有关，这在很大程度上解释了这种液体的迷人行为，包括集体模式的发生。在超声实验中观察到的集体模式有时被称为库珀对振动。本课题的实验主要处理3He B两种具有较好特征的集体模态的非线性激励，即实压实模态和压实模态。这些研究将利用这一事实，即在准粒子散射率很小的低温下，这些模式的线宽变得非常窄。计划中的调查将强调饱和度研究和声子回波的搜索，类似于脉冲核磁共振效应。将采用大激励脉冲的脉冲飞行时间研究。此外，孤子行为的研究，特别是自诱导的透明度将采用类似的技术进行。由于压缩模式与超声之间存在较大的耦合，因此对压缩集体模式的研究将更加有利。此外，采用实验方法寻找磁场中压缩和实压缩集体模式塞曼亚能级之间跃迁对应的磁共振信号。%%%%这个实验项目致力于在低温下冷却同位素质量为3的氦原子形成的液体中存在的超流体行为。当液氦冷却到4.2开氏度（约-270摄氏度）以下时，就会变成一种超流体，它没有粘度，允许持续的流动模式，比如在容器内旋转。由质量为3的同位素o - f -氦形成的超流体的性质更有趣，这也是本实验项目的主题。在液氦3中，超流体与氦原子的“配对”有关，并且氦原子对具有相互旋转运动的特征，与传统超导体中电子对没有“轨道角动量”形成鲜明对比。氦3超流体中这种非常规配对的结果包括集体模式，称为压扁模式和实压扁模式，这是本研究的主题，部分将使用核磁共振方法进行。液氦3是已知最简单的液体之一，是对凝聚态物质性质进行深入了解的试验场。即使它很简单，它也提供了复杂的行为，如果正确理解，可以洞察非常不同的材料的行为，包括高温超导体，其中非常规配对也可能发生在超流体（超导）阶段。这项实验工作的一部分是由研究生和博士后学生进行的，他们在物理学和其他领域也接受过优秀的培训，追求使用核磁共振和其他复杂的方法进行低温测量。这对从事工业研究、政府实验室或学术界的职业生涯是一个极好的准备。＊＊＊＊

w:\awards\awards96\num.doc 9701710 Lee This experimental project is related to the internal degrees of freedom of Cooper pairs in superfluid 3He, which are largely resonsible for the fascinating behavior of this liquid including the occurrence of collective modes. The collective modes observed in ultrasound experiments are sometimes known as Cooper Pair Vibrations. The experiments in this project mainly deal with the non-linear excitations of two well characterized collective modes of 3He B, namely the real squashing mode and the squashing mode. The studies will take advantage of the fact that the line widths of these modes become very narrow at low temperatures where the quasi-particle scattering rates are small. The planned investigations will emphasize saturation studies and a search for phonon echoes, in analogy to pulsed nuclear magnetic resonance effects. Pulse time of flight studies with large excitation pulses will be employed. In addition, a search for soliton behavior and in particular self induced transparency will be undertaken using similar techniques. Because of the large coupling between the squashing mode and ultrasound, it is expected that it will be more advantageous to study the squashing collective mode. In addition, an experimental approach to find magnetic resonance signals corresponding to transition between Zeeman sublevels of both the squashing and real squashing collective modes in magnetic fields. %%%% This experimental project is devoted to the superfluid behavior present in the liquid formed by cooling helium atoms of isotope mass three, at low temperatures. Liquid helium when cooled below 4.2 Kelvin degrees (about -270 Centigrade) becomes a superfluid, which flows with no viscosity, permitting persistent flow patterns such as rotating inside a container. The superfluid formed by the isotope o f helium of mass 3 is even more interesting in its properties, which are the subject of this experimental project. In liquid helium three the superfluid is associated with "pairing" of the helium atoms, and the pairs of helium atoms have a characteristic rotational motion about each other, in contrast to pairing of electrons in conventional superconductors in which the pairs do not have "orbital angular momentum". The consequences of this unconventional pairing in the helium three superfluid include collective modes, known as squashing and real squashing modes, which are the subject of this research, which will be carried out in part using nuclear magnetic resonance methods. Liquid helium three, one of the simplest liquids known, is a testing ground for developing a thorough understanding of the properties of condensed matter. Even with its simplicity it offers complex behavior which may, if correctly understood, provide insight into the behavior of very different materials, including high temperature superconductors, in which unconventional pairing may also occur in the superfluid (superconducting) phase. This experimental work is carried out in part by graduate and postdoctoral students who receive an excellent training in physics and in other areas also, in the pursuit of the cryogenic measurements using nuclear magnetic resonance and other sophisticated methods. This is an excellent preparation for a career in industrial research, government laboratories or in academia. ****