Introduction

Our group has recently developed a technique to use solid hydrogen particles to visualize the flow in superfluid helium. The ability for bosons to condense into the ground state of a system at low temperatures can cause phase transitions which produce phenomena such as superconductivity, Bose-Einstein condensation, and superfluidity. Similar to other phase transitions, phase defects occur in these otherwise ordered states and have a dramatic impact on these systems. Magnetic line defects occur in type II superconductors. In the case of superfluidity, quantized vortices of the superflow form along filaments (line defects). We seek to understand the dynamics of these vortex defects and their impact on the phase transition.

Technique

Our visualization technique begins with the injection of hydrogen gas into the liquid helium above the superfluid transition temperature. The hydrogen forms solid particles of sizes of order 1 micron. Evaporative cooling is used to lower the temperature of the liquid helium below the superfluid transition. The solid hydrogen is attracted to and trapped by the filaments of the vortices and may then be used to directly visualize the formation and dynamics of the line vortices in the bulk of the superfluid.

This work was published in Nature 1 June 2006 and is available here.

Images taken from our experiments are shown below. The first pair shows the phase transition, with liquid-phase helium on the left and superfluid helium on the right. 

Publications