D. R. Luhman

4He third sound and AFM characterization of rough calcium fluoride substrates

We present resugts from experiments utilizing third sound and atomic force microscopy (AFM) that are designed to test the reproducibility of the surface roughness obtained by CaF2 thermal deposition onto borosilicate glass substrates. Third sound speed vs. 4He film thickness data show reasonable agreement for all samples. Statistical analysis of the AFM measurements confirm that surfaces with similar roughness characteristics (∼10 nm) can be fabricated using a common deposition protocol.

Kosterlitz‐Thouless Transition in 4He Films Adsorbed to Rough Calcium Fluoride

Previous measurements in our lab have shown that the onset of superfluidity at the KT transition, typically seen as a sharp change in the frequency of a smooth‐surface quartz crystal microbalance, becomes less identifiable in the presence of increasing surface roughness or disorder, while the peak in the dissipation is unchanged. Using a series of microbalances coated with increasingly rough CaF2, we have extended our measurements to lower 4He film coverages and thus lower temperatures. We find at lower 4He coverages that the presence of disorder on the substrate has a diminished effect on the frequency shift.

Two-Dimensional Classical Wave Localization in a Third Sound System

Patterned calcium fluoride deposited on glass creates an effective two‐dimensional scatterer of third sound propagating on a thin 4He film. We have utilized a substrate with a periodic arrangement of scatterers and a substrate with a random arrangement of scatterers to investigate classical wave propagation and localization using third sound. We observe pass bands on the periodic substrate while only low frequency modes are observed in the disordered case. The presence of high frequency modes on the ordered substrate and the absence of high frequency modes on the disordered substrate indicates we are observing localization on the disordered substrate. We compare the disordered data to the two‐dimensional localization theory of Cohen and Machta and find reasonable agreement.