****NON-TECHNICAL ABSTRACT**** This research project is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: Is it possible (and if so, what is the mechanism) for 4He atoms to flow though solid 4He. Prior work by others has suggested that this strange phenomenon might be possible and that there may be a ?supersolid? state of matter that may exist in solid 4He at very low temperatures. A ?supersolid? would represent a new quantum state of matter, therefore the question of its existence 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. To determine whether it is possible for 4He atoms to flow through solid 4He, this project will impose 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 liquid helium that interfaces the solid on its sides. The approach employs the known behavior of 4He to remain a liquid at elevated pressure in Vycor (a porous glass). The pressure is applied to the liquid helium in the Vycor and atoms are fed into the solid helium through the interface where the Vycor meets the solid. The students involved in these studies will gain experience in fundamental physics and cutting-edge technology. They will work toward a Ph.D. degree and at graduation will be poised to contribute to scientific research and technological development in industrial, national laboratory, and academic settings.
This research project is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: Is it possible for 4He atoms to flow though solid 4He and if so, what is the mechanism. Prior work by others has suggested such flow might be possible and more specifically that there may be a ?supersolid? state of matter that may exist in solid 4He at very low temperatures. A ?supersolid? would represent a new quantum state of matter, therefore the question of its existence has aroused intense interest in the Condensed Matter community, stimulated a number of experiments and theoretical debate, 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. To investigate the possibility of mass flux through solid 4He, this project will impose a chemical potential gradient on the solid by a unique technique: application of a pressure difference to liquid helium that interfaces the solid instead of applying pressure directly to the 4He crystal lattice. This 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. Application of a chemical potential difference across solid helium flanked by Vycor will allow atoms to be injected into the solid without application of a mechanical pressure difference to the lattice. Studies as a function of temperature and pressure will provide evidence (or not) for flow and limit the set of possible mechanisms that are responsible for flow. The students involved in these studies will gain experience in fundamental physics and cutting-edge technology. Upon completion of Ph.D. dissertations they will be poised to contribute to scientific research and development in industrial, national laboratory, and academic settings.
Most folks think of solids as well behaved things, e.g. objects that retain their shape and don’t allow anything to pass through them. Helium, one of the rare gasses and the gas that makes a blimp buoyant (4He), is rather remarkable and does not become a solid, even at the lowest temperatures achievable in a laboratory, unless pressure in excess of twenty five times atmospheric pressure is applied. Then helium will solidify. But, it is an unusual solid. Recent work in the field of experimental low temperature physics suggested that solid helium at very low temperatures and elevated pressure may in fact demonstrate rather extraordinary behavior, behavior that the community at times referred to as supersolid. The first experiments to demonstrate something unusual were rotational experiments and these were interpreted to indicate that when the solid helium was rotated, part of it remained in place and did not participate in the rotation. This would suggest that it might be possible to cause solid helium to flow through itself, a notion that is totally foreign to everyday experience. Other more recent work suggests that when it is cooled the solid becomes stiffer and this may be related to the earlier observations. Indeed recent work makes it clear that the original experimental evidence for a possible supersoild was in fact caused by this stiffening effect and not due to supersolid behavior. This has become a very controversial issue and it is now believed by many, including those who use rotational methods, that the original interpretations were in fact not correct and such experiments mimicked supersolid behavior because of the unexpected stiffening. The work represented by NSF grant DMR-08-55954 is an effort to answer the question of whether or not it might be possible to cause helium to flow though solid helium by some mechanism. By use of an innovative technique, this work allows a chemical potential difference (e.g. a pressure difference or a temperature difference) to be applied directly to solid helium at the interface between superfluid liquid helium and the solid, at a pressure when ordinarily all of the helium would be solid. This is very different from squeezing the solid; e.g. if you have a porous rock with liquid in it, if you squeeze the rock nothing will come out, but if you feed liquid in one side, it will come out the other. What has been discovered here is that under some conditions is it indeed possible to cause helium to flow through the solid helium by the imposition of a pressure difference, or indeed, only a temperature difference. In the case of superfluid helium, the imposition of a temperature difference across a superleak results in mass flux – the well-known "fountain effect." The detailed reason why the flow takes place in our new solid experiments is not yet known, but flow is clearly present and it appears to have a rapidly increasing dissipation as a limiting velocity is approached. Such a "critical velocity" is a characteristic of superfluid behavior. Dissipation is encountered when such a special velocity is approached. Indeed, our flux measurements indicate that the flow we observe is dissipative and this behavior is consistent with what is called Bosonic Luttinger liquid behavior – a special behavior characteristic of one-dimensional flow (flow along a very narrow pathway). It is now believed that our results can be explained by the presence of true superfluid-like behavior along the cores of imperfections called dislocations that are present in the solid. The most recent results, carried out in the presence of various concentrations of the rare helium isotope 3He, show that the 3He can poison the flow paths in a reversible way at a sharply defined temperature that depends on the concentration of the 3He. Newly understood behavior of dislocations may in time have ramifications for materials science in a broader context. Metals have dislocations and their behavior influences the ultimate properties of the metal. This subject is one of substantial current activity, with considerable attention from theorists. Whether this is the true explanation if our flow results will be determined by future work. The work has provided opportunities for an undergraduate student, a graduate student and a postdoctoral research associate. The undergraduate participated in aspects of the work for two years and then moved on to graduate study for a Ph.D. in Physics. The graduate student completed work on a Ph.D. moved to a postdoctoral position and is now in a teaching position at a liberal arts college. The postdoctoral research associate continues to work on the project with the support of NSF grant DMR 12-05271.
