Vortices and Bubbles in Cryogenic Liquids

*非技术摘要*这个基本凝聚态物理项目的主要目标之一是了解覆盖在纳米尺度多孔材料(如长碳纳米管阵列)表面的氦薄膜的超流相变(无摩擦流动的液体变成正常液体的点)的性质。随着温度的升高，这些薄的氦薄膜(大约只有一个原子厚)在绝对零度以上零点几度的温度下失去了无摩擦的流动特性。理论预测，当量化的漩涡(漩涡)在氦薄膜中被热激发，有效地阻止氦沿着一维纳米管流动时，就会发生这种情况。将要进行的实验将检验这一假设。对这一过程的理解可能对高温超导体具有重要的技术意义，高温超导体的超导相变也被认为涉及涡旋激发。将开展理论工作来探索这一联系。这个项目的第二个主要目的是研究气泡在各种液体中坍塌时发出的光。用聚焦的激光脉冲在液体中产生约1 mm大小的气泡。当它们坍塌时，探测到的光爆发只持续了几纳秒。目前尚不清楚光是如何产生的，尽管人们认为它与气泡中气体的压缩加热有关，气泡坍塌时的温度高达约1万华氏度，比太阳表面更热。为了加深我们对这一现象的基本了解，我们将对低温液体、液氮和液氮中气泡的光发射进行研究。还将在室温下的碱性盐溶液中进行研究，在那里已经检测到来自碱原子中原子跃迁的新的光脉冲。从事这些项目的研究生将接受广泛的凝聚态实验室技能培训，包括在极低温度下进行实验的低温技术。本科生也将在本学年参与这项研究，加州大学洛杉矶分校物理系REU计划的一名REU学生将在暑假接受指导。*技术摘要*这个基本凝聚态物理项目的一个主要内容是研究一维、二维和三维几何中的量子化涡旋在超导和超导相变中的作用。利用扭转振荡器技术，将测量吸附在纳米孔陶瓷MCM-41或碳纳米管上的亚单层氦薄膜的超流密度。结果将与理论模型进行比较，理论模型预测当热涡激发的核心尺寸与极薄薄膜中的圆柱体直径相当时，将发生维度交叉。还将开展厚超流和超导薄膜三维相变的理论工作。这将集中在容器壁上的涡旋边界条件如何影响超流性。第二个推力将是研究低温液体和水中破裂气泡的发光。激光诱导气泡坍塌的发光将在各种不同的液体中进行测量，从85Kelvin的液体Ar到在碱-盐溶液上的室温测量，在那里观察到了碱原子的新的线发射脉冲。这些测量的目标是了解产生纳秒光发射脉冲的机制。从事这些项目的研究生将接受广泛的凝聚态实验室技能培训，包括在极低温度下进行实验的低温技术。本科生也将参与这些项目。

****NON-TECHNICAL ABSTRACT****One of the main goals of this fundamental condensed matter physics project is to understand the nature of the superfluid phase transition (the point where a liquid that flows with no friction changes to a normal liquid) of helium films coating the surface of nano-scale porous materials, such as arrays of long carbon nanotubes. As the temperature is raised these thin helium films (only about one atom thick) lose their frictionless flow properties at temperatures only a few tenths of a degree above absolute zero. Theories predict that this occurs when quantized vortices (whirlpools) are thermally excited in the helium film, effectively blocking the flow of the helium along the one-dimensional nanotubes. The experiments to be undertaken will test this hypothesis. An understanding of this process may have important technological implications for the high-Tc superconductors, where the superconducting phase transition is also thought to involve vortex excitations. Theoretical work will be carried out to explore this connection. A second major thrust of this project is to study the light emitted from bubbles collapsing in various liquids. The bubbles of about 1 mm size are created with a focused laser pulse in the liquid. When they collapse a burst of light lasting only a few nanoseconds is detected. Exactly how the light is generated is unknown, although it is thought to be connected with the compressional heating of the gas in the bubble as it collapses, up to about 10,000 degrees, hotter than the surface of the sun. To increase our fundamental understanding of this phenomenon, studies will be undertaken of the light emission from bubbles in the cryogenic liquids, liquid argon and liquid nitrogen. Studies will also be undertaken in alkali salt solutions at room temperature, where a new light pulse has been detected coming from atomic transitions in the alkali atoms. The graduate students working on these projects will receive training in a wide range of condensed-matter laboratory skills, including the cryogenic techniques for doing experiments at very low temperatures. Undergraduate students will also be involved with this research during the academic year, and an REU student will be mentored in the summers under the UCLA Physics Department's REU program.****TECHNICAL ABSTRACT****One thrust of this fundamental condensed matter physics project is the study of the role of quantized vortices in superfluid and superconducting phase transitions in one-, two-, and three-dimensional geometries. Using a torsion oscillator technique, the superfluid density of sub-monolayer helium films adsorbed on either the nanoporous ceramic MCM-41 or carbon nanotubes will be measured. The results will be compared with theoretical models that predict a dimensionality crossover occurs when the core size of the thermal vortex excitations becomes comparable to the cylinder diameter in the very thin films. Theoretical work will also be carried out on the three-dimensional transition of thick superfluid and superconducting films. This will focus on the how vortex boundary conditions at the container walls affect the superfluidity. A second thrust will be studies of the luminescence from collapsing bubbles in cryogenic liquids and in water. Measurements of the luminescence from collapsing laser-induced bubbles will be undertaken in a variety of different liquids ranging from liquid argon at 85 Kelvin to room-temperature measurements on alkali-salt solutions, where a new line-emission pulse from the alkali atoms has been observed. The goal of these measurements is to understand the mechanisms that give rise to the nanosecond pulses of light emission. The graduate students working on these projects will receive training in a wide range of condensed-matter laboratory skills, including the cryogenic techniques for doing experiments at very low temperatures. Undergraduate students will also be involved in these projects.