Experiments on solid Helium-4

This thesis presents data on experiments done with hcp 4He. The main focus is on the possibility of superfluidity inside solid 4He. The presence of superfluidity inside the solid should be manifested by a flow through the solid. The aim of these experiments was to search for a D.C. flow, characterize it and determine if the flow is due to superfluidity. A cell was designed that exploits the properties of helium in confined geometries to provide an interface between superfluid liquid helium, and hcp solid helium at pressures greater than the bulk melting curve. This interface allowed for a direct injection of atoms into the solid from the superfluid. With this cell, two different methods can be used to induce flow. In the first method, mass is injected directly into one side of the solid, while the pressure of the other side is monitored for a response. In the second method, flow is induced by changing the temperature of liquid reservoirs connected to the solid through porous Vycor glass. In addition to observing flow, it is also seen that a fountain pressure exists between the two liquid reservoirs connected to each other through the Vycor, and solid helium. Using this same design, the growth of solid helium was also studied at pressure higher than the bulk melting curve. Here, the solid cannot grow along a free surface, since off the melting curve the cell is supposed to be completely filled with solid. Near the melting curve, transients of 1–5 mK are seen in the temperature along with pressure drops [special characters omitted] 160 mbar. These transients are shown to probably be due to the solidification of metastable liquid regions imbedded in the solid. Off the melting curve, growth of the solid continues, and this growth is studied in the context of, “isochoric compressibility.