9731523 Williams This low temperature physics project is composed of three components. (1) The superfluid phase transition of very thin He-4 films adsorbed in porous materials will be studied, using a torsion oscillator to measure the superfluid density. This allows a measurement of the vortex core diameter in the superfluid films. The increase of in the vortex core size with added He-3 will be studied, and also the effect on the vortex structure of pre-plating the substrate with 2-3 atomic layers of hydrogen. Measurements will also be undertaken in a new porous ceramic material, known as MCM-41, where the pore size of the material can be varied between 16 and 100 by changing the fabrication process. Theoretical studies of the vortices in helium films will be carried out, investigating the effect of a finite supercurrent on the vortex pairs at the Kosterlitz-Thouless superfluid transition. (2) A search for sonoluminescence from bubbles acoustically trapped in cryogenic liquids such as nitrogen and oxygen will be undertaken, and the oscillation dynamics of the bubbles will be studied using laser scattering. The cryogenic liquids have the advantages of being pure and inert. They are also more transparent than water in the ultraviolet region, where most of the sonoluminescent energy is emitted. (3) Bubbles will also be created in liquid helium by injecting electrons into the liquid, forming multi-electron bubbles large enough to become trapped in the acoustic fields. The bubble radius as function of the electron number will be measured, and also the oscillation dynamics of the bubble. %%% This low temperature physics project is composed of three components. (1) The superfluid properties of liquid helium films will be studied, with the emphasis on understanding how quantized vortex flow line in the liquid can bring the frictionless superfluid flow to a halt. Measurem ents of the fraction of the liquid helium that is superfluid will allow the size of the vortex structures to be determined. A new type of ceramic materials will be employed as a substrate for the helium films, one whose pore size can be varied between about 16 and 100 . Theoretical studies will be carried out to provide a framework for interpreting the experimental results. This work has applications to the High-Tc superconductors, where it is just now being understood that quantized vortex flux lines are excited near the superconducting transition. These lines have the effect of bringing to a halt the frictionless flow of the electrons. (2) A search for sonoluminescence from bubbles in cryogenic liquids such as nitrogen and oxygen will be undertaken, and the oscillatory motion of the surface of the bubbles will be studied using laser light scattering. The mechanism that results in the 100 picosecond bursts of light from oscillating bubbles in water is still not understood, and it is important to find other liquids where the phenomenon occurs. The noble-gas cryogenic liquid have the important advantages that they are very pure and inert, and are more transparent than water in the ultraviolet region of the spectrum where most of the light is emitted from the bubbles. (3) Bubbles will also be created in liquid helium by injecting electrons into the liquid, forming small bubbles with up to several million electrons inside. The radius of the bubble as a function of the number of electron will be measured, and also the oscillatory motion of the bubble surface. ***
