A direct interrogation of superfluidity on molecular scales

Abstract

Time-resolved, pump–probe measurements are used to directly interrogate dissipative fluid dynamics in bulk He-II, on molecular scales, as a function of temperature and pressure. The Rydberg transitions of the triplet ${\mathrm{He}}_2^{*}$ excimers, which solvate in bubble states in liquid helium, are used as nanoscale transducers to initiate and to directly monitor the motion of the fluid in the form of damped oscillations of a 13 Å spherical bubble. The oscillations are damped out after one period, with a temperature-dependent period that directly tracks the normal fraction. As such, the bubble oscillator acts as a nanoviscosimeter. Through simulations of the observed signals, it is established that the coherent response of the bath obeys hydrodynamic equations of motion of a continuum subject to two-fluid flow. Dissipation occurs through two distinct channels: (a) Radiation of sound in the farfield, driven by the acceleration of volume in the compressible fluid; (b) temperature-dependent drag in the near-field. The drag can be considered to be strictly viscous in origin, or due to ballistic scattering of rotons from the bubble edge. The experiments do not distinguish between these two microscopic models. With this caveat in mind, it can be concluded that for these breathing modes of bubble states, the macroscopic concepts of superfluidity scale down to molecular dimensions. The simulations also yield effective potentials that describe the coupling between the compressible Rydberg electron and the compressible fluid.