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

Multimode probing of superfluid 4He by tuning forks

Flexural mode vibrations of miniature piezoelectric tuning forks (TF) are known to be highly sensitive to superfluid excitations and quantum turbulence in 3He and 4He quantum fluids, as well as to the elastic properties of solid 4He, complementing studies by large scale torsional resonators. Here we explore the sensitivity of a TF, capable of simultaneously operating in both the flexural and torsional modes, to excitations in the normal and superfluid 4He. The torsional mode is predominantly sensitive to shear forces at the sensor - fluid interface and much less sensitive to changes in the density of the surrounding fluid when compared to the flexural mode. Although we did not reach the critical velocity for quantum turbulence onset in the torsional mode, due to its order of magnitude higher frequency and increased acoustic damping, the torsional mode was directly sensitive to fluid excitations, linked to quantum turbulence created by the flexural mode. The combination of two dissimilar modes in a single TF sensor can provide a means to study the details of elementary excitations in quantum liquids, and at interfaces between solids and quantum fluid

Driving nanomechanical resonators by phonon flux in superfluid 4He\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.Comment: 6 pages, 4 figure

Thermal conductivity and torsional oscillations of solid He-4

Polycrystalline samples of hcp He-4 of molar volume V-m = 19.5 cm(3) with small amount of He-3 impurities were grown in an annular container by the blocked-capillary method. Three concentrations of He-3, x(3), were studied: isotopically purified He-4 with the estimated x(3) <10(-10), commercial 'well-grade' helium with x(3) similar to 3.10(-7) and a mixture with x(3) = 2.5.10(-6). Torsional oscillations at two frequencies, 132.5 and 853.6 Hz, and thermal conductivity were investigated before and after annealing. The solid helium under investigation was located not only in the annular container but also in the axial fill line inside two torsion rods and dummy bob of the double-frequency torsional oscillator. The analysis of the frequency shifts upon loading with helium and changing temperatures of different parts of the oscillator suggests that the three techniques probe the properties of solid helium in three different locations: the two different torsion modes respond to the changes of the shear modulus of solid helium in either of the two torsion rods while the thermal conductivity probes the phonon mean free path in solid helium inside the annular container. The temperature and width of the torsional anomaly increase with increasing frequency and x(3). The phonon mean free path increases with increasing x(3). Annealing typically resulted in an increased phonon mean free path but often in little change in the torsional oscillator response. While the magnitude of the torsional anomaly and phonon mean free path can be very different in different samples, no correlation was found between them. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765093