Pressure dependent attenuation peaks for quartz tuning forks in superfluid 4He at mK temperatures

Pure superfluid He-4 at very low temperatures is thought to be free of mechanisms producing dissipation for slowly moving objects, and the superfluid merely contributes an effective mass to the motion without any anticipated dependence on temperature below 0.1 K. To our great surprise, we found that the smallest commercially available quartz tuning fork oscillators with tines shorter than 2 mm and resonance frequency 32 kHz experienced attenuation by as much as 100 times more than the internal dissipation of the oscillator at certain values of pressure at millikelvin temperatures in superfluid He-4. These features smeared out at temperatures above 20 mK, although practically no thermal excitations in He-4 are yet present. Another fork somewhat larger showed indication of similar anomalies as well, but at greatly lesser scale.