Driving nanomechanical resonators by phonon flux in superfluid $\mathbf{^4He}$

We report on nanomechanical resonators with very high-quality factors operated as mechanical probes in liquid helium \(^4\mathrm{He}\), with special attention to the superfluid regime down to millikelvin temperatures. Such resonators have been used to map out the full range of damping mechanisms in the liquid on the nanometer scale from \(10\,\mathrm{mK}\) up to \(\sim3\,\mathrm{K}\). The high sensitivity of these doubly-clamped beams to thermal excitations in the superfluid \(^4\mathrm{He}\) makes it possible to drive them using the momentum transfer from phonons generated by a nearby heater. This so-called "\textit{phonon wind}" is an inverse thermomechanical effect that until now has never been demonstrated, and provides the possibility to perform a new type of optomechanical experiments in quantum fluids.