Although solid He does not exhibit supersolidity, there are nevertheless strong indications that individual grains of solid show mobility inside the solid matrix[1,2]. To try and observe a flow of solid directly, we have constructed a sensitive "microphone" embedded in solid He contained inside a torsional oscillator. The microphone can detect vibrations down to the 10^-11 m range, a few percent of a lattice constant. The idea was that as the solid He flows past the microphone, the atomic corrugation its surface should generate vibrations at a frequency f = flow speed/lattice constant. For a speed of 10 µm/sec, f comes out in the 40-80 kHz range, depending on the crystal orientation.  The actual frequency was extracted from the time dependent signal using FFT.   We indeed found that solid He can flow while maintaining its solid structure. From the data, we determined the direction of the flow relative to the crystal orientation of the solid.  In addition, our results reveal the nature of the friction force between moving and static solid He.

[1] O. Pelleg, M. Shay, S. Lipson, E. Polturak, J. Bossy, J. C. Marmeggi, H. Kentaro,  E. Farhi, and A. Stunault,  Phys. Rev. B 73, 024301 (2006).

[2] A.Eyal, O. Pelleg, L. Embon, and E. Polturak,  Phys. Rev. Lett. 105, 025301 (2010).