The data set for the paper "Probing superfluid He-4 with high-frequency nanomechanical resonators down to mK temperatures" accepted for publication in PRB.

Abstract:
Superfluids, such as superfluid He-3 and He-4, exhibit a broad range of quantum phenomena and excitations which are unique to these systems. Nanoscale mechanical resonators are sensitive and versatile force detectors with the ability to operate over many orders of magnitude in damping. Using nanomechanical-doubly clamped beams of extremely high-quality factors Q>1e6, we probe superfluid He-4 from the superfluid transition temperature down to mK temperatures at frequencies up to 11.6 MHz. Our studies show that nanobeam damping is dominated by hydrodynamic viscosity of the normal component of He-4 above 1K. In the temperature range 0.3-0.8K, the ballistic quasiparticles (phonons and rotons) determine the beams' behavior. At lower temperatures, damping saturates and is determined either by magnetomotive losses or acoustic emission into helium. It is remarkable that all these distinct regimes can be extracted with just a single device, despite damping changing over six orders of magnitude.