Vortices and Bubbles in Cryogenic Liquids

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

****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.