| Abstract: | Superfluidity, a frictionless flow, is the most exotic property of condensed 4He. Its proposed solid state analogue, supersolidity, is a state in which solid 4He can flow like liquid while maintaining its solid structure. Supersolidity was predicted both for bulk solid and for the solid analogue of a thin film – a grain boundary. Currently, the search for supersolidity in the bulk yielded a null result. Despite this, there are strong indications that individual crystallites can move in relation to the surrounding solid. In this work, we search for flow of solid He crystal grains past each other along the grain boundary separating them. We developed a novel in-situ flow detection method using a sensitive “microphone” embedded on the inner wall of the cell containing the solid. Flow is detected by counting the vibrations induced by individual rows of 4He atoms moving past the microphone. We find that at speeds less than 7m/sec, the solid grains flow without detectable friction. At larger speeds, friction sets in, in a way characteristic of a critical velocity of a Superfluid. Our findings also indicate the flow is perpendicular to the crystal’s c direction. |
