Third sound on CaF2 films of varying roughness

The propagation of pulsed third sound on a series of CaF2 substrates with varying roughness has been studied. The third sound speed is reported over a large range of He-4 film thickness including a detailed study near the onset of superfluidity. The shapes of the received third sound pulses are also reported over a large range of He-4 film thickness. Hysteresis is seen in the third sound speed on the roughest surfaces at low He-4 coverage, likely caused by capillary condensation of fluid on the surface structure. The hysteresis loops change with the roughness of the surface.