Anomalous attenuation of piezoacoustic surface waves by liquid helium thin films

TL;DR

This study investigates the unexpectedly high attenuation of high-frequency surface acoustic waves caused by thin liquid helium films on a lithium niobate substrate, revealing new insights into wave-liquid interactions at nanoscale thicknesses.

Contribution

It demonstrates anomalous attenuation effects of surface acoustic waves by thin helium films, extending understanding beyond bulk liquid interactions and suggesting new mechanisms at play.

Findings

Attenuation increases significantly with thin helium films beyond bulk liquid effects.

Attenuation aligns with theory for thick helium layers, diverges for thin layers.

Discussion of potential mechanisms for enhanced attenuation in thin films.

Abstract

We report on the observation of an anomalously high attenuation of high frequency surface acoustic waves by thin films of liquid $^{4}$He. The piezoelectric acoustic waves propagate along the surface of a lithium niobate substrate, which is coated with varying amounts of liquid helium. When the thickness of the helium layer is much larger than the wavelength of the surface acoustic wave on the substrate its attenuation is dominated by the excitation of compressional waves into the liquid, in good agreement with theory and previous measurements. However, for sufficiently thin helium coverage, we find that the acoustic wave attenuation is significantly increased beyond that measured with the substrate submerged in bulk liquid. Possible mechanisms for this enhanced attenuation are discussed.