Experiments on Solid Helium

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

****Non-Technical Abstract**** Solid 4He is a quantum solid (i.e. the properties are dominated by quantum effects) with unique properties. For example, 4He atoms are apparently able to flow though solid 4He as they do through a liquid. Prior work suggested that this behavior indicated a "supersolid" state of matter analogous to superfluidity and superconductivity. This is no longer believed to be the case but the mechanism allowing atoms to flow through the solid remains a mystery. In this project a different approach to study the flow of atoms will be used. A pressure difference will be applied across the solid by a unique technique that does not employ pushing on the crystal sides. Using this technique we will study the properties of this fascinating system as a function of flow rate and 3He concentration. Participating undergraduate, graduate and post-graduate (postdocs) students will gain experience in fundamental physics and cutting-edge technology and be prepared for future work in research, teaching or industrial settings. These investigations may lead to advances in materials science that could have significant technological implications, for example in metallurgy. ****Technical Abstract**** This project is centered on an investigation of a fundamental and important question in Condensed Matter Physics: What is the mechanism by which 4He atoms are apparently able to flow through a sample cell that is filled with solid 4He? Prior work by others initially suggested that there may be a bulk "supersolid" state of matter that may exist in solid 4He at very low temperatures. This is an issue that 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. The experimental approach used in this research is to impose a chemical potential gradient on the solid: for example, by application of a pressure difference to liquid helium that interfaces the solid instead of applying mechanical pressure directly to the 4He crystal lattice; or, the application of a temperature difference to utilize the superfluid Fountain Effect to drive the flow. The approach employs the known behavior of 4He to remain a liquid at elevated pressure in the porous material 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) provided further evidence for flow of the liquid-solid melting curve. The specific mechanism for this flow will be explored including an exploration of the mechanism by which the presence of 3He dramatically suppresses the flow at a concentration-dependent temperature. One approach will be to understand how the flow responds when the solid is subjected to calibrated applied stress that deforms the solid. Another approach will be to increase the sensitivity and explore questions about the possibility of a critical flux and a possible onset of measurable dissipation. Yet another approach will be to explore the extent to which pressure suppresses mass injection and the growth of the solid in the presence of flow under various conditions. The students (undergraduate, graduate) and post-graduate (postdocs) involved in these studies will gain experience in fundamental physics and cutting-edge technology. Personnel who leave the group will be poised to contribute to scientific research and technological development in industrial, national laboratory, or academic settings.