Studies of helium films on disordered and patterned substrates
The results of a series of experiments involving rough surfaces, adsorbed helium films, and helium films adsorbed on rough surfaces are reported. A comprehensive study using atomic force microscopy of the surface roughness of thermally deposited CaF2 of varying film thicknesses indicates that the surface roughness of the CaF2 film increases systematically with CaF2 film thickness while the porosity of the film remains constant. The roughness, growth, and dynamic exponents are found to be α = 0.88±0.03, β = 0.75±0.03, and z = α/β = 1.17±0.06 respectively. In addition, we find evidence indicating that the growth dynamics may be dominated by noise described by a power-law probability distribution.
The nature of the superfluid or Kosterlitz-Thouless transition of helium films adsorbed on rough CaF2 surfaces was studied. For T = 1.672 K the signature of decoupled mass observed on crossing the Kosterlitz-Thouless transition as a function of 4He film thickness decreases and becomes increasingly difficult to identify as the surface roughness is increased. A peak in the dissipation, indicative of the onset of superfluidity, changes little with roughness. In a separate experiment with smaller 4He film coverages and thus lower temperatures, the presence of surface roughness does not have a significant effect on the frequency shift at the Kosterlitz-Thouless transition. These results are interpreted as an indication that a varying film thickness, as would be present in higher temperature experiments due to capillary condensation and absent in the lower temperature experiments, induces disordered into the system which in turn broadens the transition.
Continuous third sound experiments on substrates patterned with a disordered and ordered array of rough CaF2 scatterers are reported. In the disordered case we compare the experimental results to theoretical predictions of classical wave localization in such systems. The application of a continuous superfluid flow across the substrate breaks the time-reversal symmetry of the system and is predicted to effect the transmission properties. The application of a flow is found experimentally to have no effect on the transmission properties of the substrates.
The evolution of third sound pulse shapes in the presence of a superfluid flow was also studied.