Bilayer bolometers to Kosterlitz -Thouless: A study of helium films
The work that is presented here is divided into two main sets of experiments. The first set of experiments deals with attempts to fabricate tunable third sound detectors. With different fabrications, the production of such thermometers results in transition temperatures quite different from those that can be obtained by the use of and manipulation of a single pure metal. We were able to reduce the transition temperature in a reasonably controlled manner by changing the Al/Au thickness ratio. The sensitivity of the bilayer detectors appear to be more uniform for various bias currents than the pure metal film detectors. This is advantageous when using the bias current to fine tune the transition temperature of a given detector.
The purpose of the second set of experiments was to study helium films on substrates that contain a form of "ordered disorder", in this case a well ordered array of cylindrical pores. The main goals were to see what effects the pores had on the Kosterlitz-Thouless transition and to attempt to characterize the distribution in the size of the pores. This was done using quartz crystal microbalance and third sound techniques.
As a major result, we found that the Kosterlitz-Thouless transition appears to be very robust with samples of various quality. The transition was observable in all of the samples studied and always appeared to scale with temperature as expected. The results that provided the least answers were the mass adsorption studies where helium was added to or removed from a quartz crystal microbalance. The samples demonstrated frequency shifts that were a function of helium film thickness. The effects occurred while adding and removing helium and they were observable at different temperatures. Even plain quartz crystals, which were found to be rougher than expected, showed large scale frequency shifts that suggest there is more happening on these substrates than expected. In some cases clear evidence of hysteresis was present and these samples are worth further investigation.