Measurements of attenuation of third sound: Evidence of trapped vorticity in thick films of superfluid ⁴He

We present results of a study of third sound in thick ⁴He films in circular resonator geometry. Frequency and line shapes of third sound resonances are measured for temperatures between 0.3 and 2.1 K in saturated films approximately 30 nm thick. From these measurements we calculate the attenuation of the sound. We find that the attenuation at a given temperature is a function of history of the film, strongly affected by such events as large, sudden (more than milli-Kelvin per second) temperature spikes. We also observe variable frequency splitting of resonances, indicating trapped circulation. Our measurements, taken together with other reported attenuation experiments, are incompatible with dissipation mechanisms dependent on thermodynamic properties alone. Measurements indicate a linear dissipation mechanism, inconsistent with vortex drag and re-connection models. We conclude that high attenuation values, evidence of trapped circulation, and variation in attenuation values support the hypothesis that thick films of ⁴He contain high densities of remnant quantized vortices. The vortex populations suggested by trapped circulation are consistent with proposed linear dissipation mechanisms due to vortex-normal fluid interactions and vortex dimple drag.