This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Superfluid helium at very low temperatures is known to flow without friction, but it is known that this superfluid state can be destroyed when the temperature of the liquid is raised above a critical value. It is thought that this transition from a superfluid to a normal liquid is caused by quantum whirlpools that are thermally excited in the liquid helium as the temperature is raised, but the details of this process are still not well understood. This project will study the superfluid phase transition in a novel geometry, in the form of a liquid helium film only a few atomic layers thick that is adsorbed on the surface of a carbon nanotube, which is only a few nanometers in diameter. The superfluid properties (and hence the whirlpools that are excited) will be monitored by propagating a temperature wave in the helium film coating the nanotubes. It is important to unravel the nature of this phase transition because recent experiments in high-temperature superconductors (materials where the electric current flows without friction) show evidence that the whirlpools are also involved in the transition from a superconductor to a normal material, where electrical currents can no longer flow without friction. An understanding of how the whirlpools are involved may lead to insights on how to increase the transition temperature: the "holy grail" of superconductivity research is to raise the transition to room temperature, which would have enormous implications for energy technology. This work will provide the thesis project of a Ph.D. student, giving valuable training in low-temperature techniques, nanomaterial fabrication and characterization, and optical and laser techniques. Undergraduate students will also participate in the experiments, giving them experience in how research is carried out.
Quantized vortices are known to play a crucial role in the properties of superfluid helium. As the temperature of the fluid is raised, more and more vortices are thermally excited, until the critical temperature is reached where the superfluid density is driven to zero by the vortices, defining the phase transition to a normal liquid. This project will study the superfluid phase transition in a novel geometry, in the form of a liquid helium film only a few atomic layers thick that is adsorbed on the surface of a carbon nanotube, which is only a few nanometers in diameter. Initial measurements have shown that it is possible to adsorb a superfluid film on the nanotubes, and that the superfluid properties can be probed by propagating a temperature wave in the liquid helium coating the tubes. The superfluid phase transition will be studied as function of the film thickness, with particular attention to an unusual re-entrant behavior that has been observed close to the onset thickness. A second project that will be undertaken involves the generation of a bubble in cryogenic liquids using a focused laser pulse, and observation of the luminescence pulse that occurs at the end of the bubble collapse phase, where very high temperatures are generated from the compression of the gas in the bubble. Initial studies showed this process occurs in liquid nitrogen, and studies will also be undertaken in alcohols at low temperature. This work will constitute the thesis project of a Ph.D. graduate student, providing training in low-temperature techniques, nanomaterial fabrication and characterization, and optical and laser techniques, and undergraduate students will also participate in the experiments.
