Measurements of Vortex Diffusivity in 2d Superfluid Helium-4

128 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1987.We measure for the first time the characteristics of vortex induced superfluid flow dissipation at temperatures well below the Kosterlitz-Thouless superfluid transition temperature. Our studies are with films of coverage .25 < (sigma)(,s) < 1.9 superfluid atomic layers and with .55 < T(,KT) < 1.9(DEGREES)K. We use a third sound resonator combined with a new detection technique('17) which allows us to measure the vortex dynamics in the superfluid. Using a model developed by Gillis, et al('16,32), we fit the vortex induced dissipation to the data using as our adjustable parameters the vortex diffusivity D and the free vortex creation time (tau)(,free). We find D to be dependent on superfluid thickness and temperature (for 90 mK < T < 600 mK), but to be independent of superfluid flow velocity, frequency of the oscillating velocity field, and, with the exception of the thinnest film on Neon, of the ('4)He-substrate van der Waals strength. However we find the free vortex creation time (tau)(,free) to be a strong function of the van der Waals strength. Measurements on Ar yield (tau)(,free)('(TURN))T('-1.3) while on Ne (tau)(,free)('(TURN))T('-5.2). For the thinnest film on Ne ((sigma)(,s) = .60 layers) D is anomalously large yet temperature independent, and (tau)(,free) is immeasurably large. We postulate that surface roughness may affect the diffusivity of this thin film.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD