A current line of research is experimental and theoretical investigations
into the superfluid phase transition of liquid helium. When cooled to temperatures
below 2.17 Kelvin (-460 F), liquid helium loses all viscosity and can move
without friction, known as the superfluid state. The results in our lab indicate
that the mechanism underlying this phase transition is quantized vorticity. At
low temperatures there is not enough energy in the liquid to excite any vortex
lines, and the helium can flow freely. As the temperature is raised, however,
vortex loops (similar to smoke rings in air) begin to be thermally created, and
they oppose the flow of the liquid. Finally at temperatures above 2.17 K (called
the l-point) the liquid becomes filled with a tangle of vorticity, bringing the
frictionless flow of the helium to a halt, turning it back into a normal liquid.
Ducks in a classical vortex
Selected recent publications:
G.A. Williams, Vortex Rings
and the Superfluid l-Transition,
J. Low Temp. Phys. 89, 91 (1992).
G.A. Williams, Vortex Dynamics
and Superfluid Relaxation Near the 4He l-Transition,
Phys. Rev. Lett. 71, 392 (1993).
G. A. Williams, Vortex
Dynamics at the Superfluid l-Transition,
J. Low Temp. Phys. 93, 1079 (1993).
H. Cho and G. A. Williams, Vortex
Core Size in Submonolayer Superfluid 4He Films,
Phys. Rev. Lett. 75, 1562 (1995). Download the PDF file (103 Kb)
G. A. Williams, Specific Heat
and Superfluid Density of Bulk and Confined 4He Near the l-Transition,
J. Low Temp. Phys. 101, 415 (1995).
G. A. Williams, Dimensionality
Crossover of the 4He Superfluid Transition in a Slab Geometry,
J. Low Temp. Phys. 101, 463 (1995).
H. Cho and G. A. Williams, Superfluid
Phase Transition of 3He-4He Mixture Films Adsorbed on
Alumina Powder ,
J. Low Temp. Phys. 110, 533 (1998). Download the PDF file (190 Kb)
G. A. Williams, Vortex Loop Phase
Transitions in Liquid Helium, Cosmic Strings,and High-T_c
Phys. Rev. Lett. 82, 1201 (1998). Download the PDF file (130 Kb)
Web Powerpoint slides comparing superfluid transitions in porous materials: Bose condensation or broadened Kosterlitz-Thouless ?