| This dissertation reports on two different projects---the development of an experiment designed to study weak links in 4He with a sine-like current-phase relationship and a set of experiments exploring dissipation of third sound in thick films of superfluid 4He.; Since Brian Josephson's proposals of phenomena in weakly-linked macroscopic quantum systems over 40 years ago, researchers have been searching for such effects in superfluid 4He. Until recently, it has been believed that technological barriers would prevent the observation of a sine-like current-phase relationship between two weakly linked volumes of superfluid 4He. In this dissertation we report on the fabrication of sub-15nm aperture arrays and preliminary measurements helium flow through the arrays below Tlambda . These measurements show the linear temperature dependence of critical velocity expected in the phase slip regime. Efforts to study the cross-over to the sin &phis; regime are ongoing.; We measured the frequency and line shapes of third sound resonances at temperatures between 0.3 and 2.1K in saturated films approximately 30nm thick, and, from these measurements, calculated the attenuation of the third sound waves. In the past, such measurements have been inconsistent, within as well as among reported work. More recent theory, however, suggests that these variations may point toward vortex-driven dissipation mechanisms. Metastable vortex populations are known to permeate superfluid 4He. Our measurements indicate that attenuation is a strong function of the history of the film, affected by temperature excursions and physical perturbations. We also observe frequency shifting of resonances attributable to trapped circulation and a large increase in dissipation with the introduction of 3He impurities to the film. Taken together with our observation of a dissipation mechanism linear in amplitude, our results provide support for recent proposals of dissipation due to vortex-excitation interactions. |
