Prethermal ripplons riding on a 1D superfluid interface

Fluctuations at the interface between immiscible superfluids are described in terms of "ripplon'" excitations, analogous to capillary waves in classical fluids. Here, we study the equilibration of ripplons at such an interface in a phase-separated two-dimensional Bose-Einstein condensate. We directly measure the interface's height profile as it evolves over time, and find that each individual mode exhibits a thermal distribution of excitations. This allows us to assign a temperature to each mode, and we find that only high-momentum modes have equilibrated with the bulk. By contrast, low-momentum ripplon modes are at an elevated temperature, indicating the system as a whole is in a long-lived prethermal state. We attribute this to an absence of decay channels for the lowest energy ripplons, which have mode energies below that of any phonon. We calibrate the bulk temperature by measuring thermally induced spin mixing.