Superfluid Studies in Quantum Systems at Low Temperatures

Cornell University, Ithaca NY

Investigators

Abstract

This low temperature physics project involves the phase behavior of quantum fluids, 3He and 4He, which are confined in highly porous low-density 'solids', vycor and aerogel, or adsorbed on high-surface-area solids such as graphite. The first subject will be a study of the Bose Einstein Transition (BEC) in low-density helium systems. In this work, porous Vycor glass is used to provide a " trapping" potential that confines a low-density gas of helium atoms in three-dimensional space. Under normal circumstances, a gas of helium atoms would condense into a liquid phase long before the temperature could be lowered enough for BEC to take place, however for mobile helium atoms adsorbed on the surface of the porous glass the gas phase is stable to the lowest temperatures and the BEC temperature can be reached in practice. The ability to continuously vary the particle density in this system, from the dilute Bose-gas regime to almost the full liquid density, provides a unique opportunity for the study of the evolution of the properties of a Bose-condensed system over a wide range of interaction parameter. The second topic is a study the superfluid transition in the 3He-aerogel system. This system is currently receiving much attention from the low temperature community because its unusual properties. This project will contribute a high-resolution heat capacity and superfluid density measurement. The third experiment seeks to provide a clearer understanding of the reentrant superfluid phase that is found for a certain range of partial coverage of the second layer for 4He adsorbed on hexagonal basal plane graphite. A tentative interpretation is that the superfluid signal arises through percolation of 2-D droplets in the second layer. The very unusual, almost logarithmic, temperature dependence of the superfluid signal is not understood at this time and so remains as fascinating open question. This research program and the previous program provide an excellent training ground for both undergraduate and graduate students inclined toward industrial or academic careers. This training develops experimental management and decision-making skills that are transferable to diverse areas, including genomics or management consulting where former students are currently pursuing successful careers. Phase transitions, the change of one matter form into another, are ubiquitous phenomena in nature. One of the goals of physics has been to achieve a more complete understanding of the phase transition process. It is natural to turn low temperature examples for study for these present several of the least complex systems in which to study the phase transition process. Research supported in this grant will study three contrasting examples of essentially the same phase transition, one that is important from a number of fundamental viewpoints. It is the transition from the superfluid to non-superfluid state. The first example is a system of low-density gas of helium atoms, which undergoes a superfluid or Bose Einstein Condensation (BEC) transition at very low temperatures. If interest is the evolution of this transition as the strength of the interaction between the particles by is increased as the density of the atoms is increased in controlled fashion. The second example is superfluid 3He entrained in low-density aerogel glass. This is a model system for the exploration of the influence of quenched impurities, the complex aerogel network, on the nature of the superfluid transition in liquid 3He. The final example explores the 4He superfluid transition in the world of two dimensions where things are quite different from the 3-D examples mentioned before. The 4He-graphite system has been known for many years to display a large number of different 2-D phase transitions. Most of these involve changes in crystallographic structure of the adsorbed helium, however the superfluid transition is also observed at various coverages in the graphite. One of the least understood of these transitions, and the one that will be investigated, is the so-called reentrant (a property appears and then disappears as a function of experimental variable) superfluid transition observed for partial coverage of the second layer of adsorbed 4He. The superfluid phase appears with increasing second layer coverage and then vanishes as the layer is completed. This interesting behavior is complemented by a yet to be understood temperature dependence for the superfluid signal. Students, both undergrads and graduate, are provided the opportunity to participate in leading-edge research, in projects of modest size. The scientific management and decision-making skills acquired in this type of research have launched successful careers for several graduates who are now working in areas outside of low temperature physics such as genome studies and management consulting. Still others have pursued notably successful physics careers in academe and government.