Detecting A Phonon Flux In Superfluid He-4 By A Nanomechanical Resonator

Nanoscale mechanical resonators are widely utilized to provide high sensitivity force detectors. Here we demonstrate that such high-quality-factor resonators immersed in superfluid He-4 can be excited by a modulated flux of phonons. A nanosized heater immersed in superfluid He-4 acts as a source of ballistic phonons in the liquid-"phonon wind". When the modulation frequency of the phonon flux matches the resonance frequency of the mechanical resonator, the motion of the latter can be excited. This ballistic thermomechanical effect can potentially open up new types of experiments in quantum fluids.