Vortices and Bubbles in Cryogenic Liquids

该奖项根据2009年《美国复苏和再投资法案》(公法111-5)提供资金。*非技术摘要*超流氦在极低温度下可以无摩擦流动，但众所周知，当液体的温度高于临界值时，这种超流状态可以被破坏。人们认为，这种从超流体到正常液体的转变是由量子漩涡引起的，随着温度的升高，量子漩涡在液氦中被热激发，但这个过程的细节仍然不太清楚。这个项目将以一种新的几何结构来研究超流相变，这种形式的液氦薄膜只有几个原子层厚，吸附在直径只有几个纳米的碳纳米管表面。通过在覆盖纳米管的氦膜中传播温度波，将监测超流体的性质(以及因此被激发的漩涡)。揭开这种相变的本质是很重要的，因为最近在高温超导体(电流在没有摩擦的情况下流动的材料)中的实验表明，涡流也参与了从超导体到正常材料的转变，在正常材料中，电流不再能够在没有摩擦的情况下流动。对漩涡如何参与的理解可能会导致对如何提高转变温度的洞察：超导研究的“圣杯”是将转变提高到室温，这将对能源技术产生巨大影响。这项工作将提供一名博士生的论文项目，在低温技术、纳米材料制造和表征以及光学和激光技术方面提供有价值的培训。本科生也将参加实验，让他们体验如何进行研究。量化的涡旋在超流氦的性质中起着至关重要的作用。随着流体温度的升高，越来越多的涡旋被热激发，直到达到临界温度，在那里超流体密度被涡旋驱动到零，定义了向正常液体的相变。这个项目将以一种新的几何结构来研究超流相变，这种形式的液氦薄膜只有几个原子层厚，吸附在直径只有几个纳米的碳纳米管表面。初步测量表明，可以在纳米管上吸附一层超流膜，并且可以通过在覆盖在纳米管上的液氦中传播温度波来探测超流体的性质。超流相变将作为薄膜厚度的函数进行研究，特别注意在起始厚度附近观察到的不寻常的再入行为。将开展的第二个项目涉及使用聚焦激光脉冲在低温液体中产生气泡，以及观察气泡崩溃阶段结束时出现的发光脉冲，在这个阶段，由于压缩气泡中的气体而产生非常高的温度。初步研究表明，这一过程在液氮中发生，研究也将在低温下的醇中进行。这项工作将构成博士研究生的论文项目，提供低温技术、纳米材料制造和表征以及光学和激光技术方面的培训，本科生也将参加实验。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).****NON-TECHNICAL ABSTRACT****Superfluid helium at very low temperatures is known to flow without friction, but it is known that this superfluid state can be destroyed when the temperature of the liquid is raised above a critical value. It is thought that this transition from a superfluid to a normal liquid is caused by quantum whirlpools that are thermally excited in the liquid helium as the temperature is raised, but the details of this process are still not well understood. This project will study the superfluid phase transition in a novel geometry, in the form of a liquid helium film only a few atomic layers thick that is adsorbed on the surface of a carbon nanotube, which is only a few nanometers in diameter. The superfluid properties (and hence the whirlpools that are excited) will be monitored by propagating a temperature wave in the helium film coating the nanotubes. It is important to unravel the nature of this phase transition because recent experiments in high-temperature superconductors (materials where the electric current flows without friction) show evidence that the whirlpools are also involved in the transition from a superconductor to a normal material, where electrical currents can no longer flow without friction. An understanding of how the whirlpools are involved may lead to insights on how to increase the transition temperature: the "holy grail" of superconductivity research is to raise the transition to room temperature, which would have enormous implications for energy technology. This work will provide the thesis project of a Ph.D. student, giving valuable training in low-temperature techniques, nanomaterial fabrication and characterization, and optical and laser techniques. Undergraduate students will also participate in the experiments, giving them experience in how research is carried out.****TECHNICAL ABSTRACT****Quantized vortices are known to play a crucial role in the properties of superfluid helium. As the temperature of the fluid is raised, more and more vortices are thermally excited, until the critical temperature is reached where the superfluid density is driven to zero by the vortices, defining the phase transition to a normal liquid. This project will study the superfluid phase transition in a novel geometry, in the form of a liquid helium film only a few atomic layers thick that is adsorbed on the surface of a carbon nanotube, which is only a few nanometers in diameter. Initial measurements have shown that it is possible to adsorb a superfluid film on the nanotubes, and that the superfluid properties can be probed by propagating a temperature wave in the liquid helium coating the tubes. The superfluid phase transition will be studied as function of the film thickness, with particular attention to an unusual re-entrant behavior that has been observed close to the onset thickness. A second project that will be undertaken involves the generation of a bubble in cryogenic liquids using a focused laser pulse, and observation of the luminescence pulse that occurs at the end of the bubble collapse phase, where very high temperatures are generated from the compression of the gas in the bubble. Initial studies showed this process occurs in liquid nitrogen, and studies will also be undertaken in alcohols at low temperature. This work will constitute the thesis project of a Ph.D. graduate student, providing training in low-temperature techniques, nanomaterial fabrication and characterization, and optical and laser techniques, and undergraduate students will also participate in the experiments.