****Technical Abstract**** This project is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: What is the mechanism by which 4He atoms are apparently able to flow though solid 4He? Prior controversial work by others has suggested that there may be a "supersolid" state of matter that may exist in solid 4He at very low temperatures. This is an issue that has aroused intense interest in the Condensed Matter community, stimulated a number of experiments and theoretical works, and resulted in a number of possible explanations and some substantial paradoxes. The theoretical debate insists that perfect crystals of solid helium cannot be a "supersolid", and that any mass flux through the solid must be carried by defects. Various defect mechanisms have been proposed. Some believe that a previously unknown plasticity mechanism may be at work. The experimental approach used in this research is to impose a chemical potential gradient on the solid by a unique technique: for example, application of a pressure difference to liquid helium that interfaces the solid instead of applying pressure directly to the 4He crystal lattice; or, the application of a temperature difference to utilize the superfluid Fountain Effect. The approach employs the known behavior of 4He to remain a liquid at elevated pressure in Vycor (a porous glass), at pressures at which bulk 4He would be a solid. Studies as a function of temperature and pressure (and 3He impurity concentration) will provide further evidence for flow and limit the set of possible mechanisms that are responsible for flow. The students (undergraduate, graduate) and post-graduate (postdocs) involved in these studies will gain experience in fundamental physics and cutting-edge technology. Graduating students will be poised to contribute to scientific research and technological development in industrial, national laboratory, and academic settings.
This research is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: What is the mechanism by which 4He atoms are apparently able to flow though solid 4He? Prior controversial work by others has suggested that there may be a "supersolid" state of matter that may exist in solid 4He at very low temperatures. This is an issue that has aroused intense interest in the Condensed Matter community, stimulated a number of experiments and theoretical works, and resulted in a number of possible explanations and some substantial paradoxes. Some believe that a previously unknown plasticity mechanism may be at work. Most agree that disorder in the solid is a crucial ingredient. The approach used in this research is to impose, for example, a pressure difference across the solid by a unique technique that does not employ pushing on the crystal sides of the solid; instead, application of a pressure difference is made to superfluid liquid helium that interfaces the solid on its sides using the unique properties of superfluid helium in a porous material that contains the superfluid. Participating undergraduate, graduate and post-graduate (postdocs) students will gain experience in fundamental physics and cutting-edge technology. These investigations may lead to advances in materials science that could have significant technological implications, for example in metallurgy.
