Anomalous attenuation of piezoacoustic surface waves by liquid helium thin films*

We report on the experimental observation of an anomalously high attenuation of high frequency surface acoustic waves (SAW) by thin films of liquid ⁴He [1]. The piezoelectric SAW propagate along the surface of a lithium niobate substrate, which is covered by liquid helium films of varying thicknesses. When the thickness of the helium layer is much larger than the wavelength of the SAW on the substrate the SAW attenuation is dominated by emitting compressional waves into the bulk liquid, in good agreement with theory and previous measurements. However, for sufficiently thin helium films, we find that the SAW attenuation significantly exceeds that measured with the substrate submerged in bulk liquid. Possible mechanisms for this enhanced attenuation are discussed.

1. H. Byeon et al., arXiv:1807.10808 (2018)