Listening" to the flow of solid 4He"
Although solid He does not exhibit bulk supersolidity, there is evidence that grains of solid can move inside a solid matrix.
To observe a flow of solid directly, we constructed a "microphone" embedded in solid He contained inside a torsional oscillator.
The microphone can detect vibrations down a few percent of a lattice constant.
Our idea was that if the solid He flows past the microphone, the atomic corrugation of its surface should generate vibrations at a
frequency f = flow speed/lattice constant. We indeed found that solid He can flow while maintaining its solid structure.
At our lowest temperature of 0.5K, a frictionless flow of solid was observed, in agreement with the prediction that grain boundaries can be superfluid.