The behavior of 4He films on structured substrates

The results of measurements which use third sound as a probe to study capillary condensation and localization effects in thin films (3-25 atomic layers) of superfluid 4He adsorbed on structured substrates at temperatures below 2 Kelvin are reported. The effects of capillary condensation of the film on third sound velocity are studied in detail for several systems, including alumina particles on glass microscope slides, Nuclepore filter material, and etched silicon wafers. Comparison of third sound velocities on such roughened substrates to velocities on smooth reference substrates of each material (i.e., clean glass, nonporous Nuclepore filter material, and smooth silicon substrates) shows velocities as much as six times slower on the rough surfaces. Substrates are fabricated which contain one-dimensional random, periodic, and quasiperiodic arrays of rough silicon regions on otherwise smooth silicon wafers; the difference in sound speeds between the two regions causes the rough regions to act as third should scatterers. Results of wave propagation on the three arrays are reported; band structure is observed on the periodic array, localization effects are observed on the random array, and a modified band structure is observed on the quasiperiodic array