Localization in two dimensional third sound systems

The results of third sound and quartz crystal microbalance measurements on thin superfluid 4He films (4–16 atomic layers) exploring the effect of surface roughness, disorder and localization are reported. The effect of surface roughness on 4He adsorption and third sound speed is studied in detail for several well parameterized CaF₂ surfaces. The third sound velocity and the 4He adsorption on CaF₂ both show a hysteretic behavior with the adding and removing of 4He. Adsorption to the CaF₂ can be as much as ten times larger than on similar smooth surfaces and the third sound travelling on this thicker 4He film propagates up to three times slower.; Substrates are fabricated that contain two-dimensional random and ordered collections of disk shaped CaF₂ regions on an otherwise smooth background. The difference in the third sound speed and 4He adsorption on the two regions causes the CaF₂ regions to act as third sound scatterers. The frequency dependence of the third sound propagation through the arrays is reported: pass bands and band gaps are observed on the ordered array and classical wave localization effects are observed on the disordered array.