Expansion of liquid He-4 through the lambda transition

Zurek suggested (Nature 317, 505; 1985) that the Kibble mechanism, through which topological defects such as cosmic strings are believed to have been created in the early Universe, can also result in the formation of topologi cal defects in liquid He-4, i.e. quantised vortices, during rapid quenches through the superfluid transition. Preliminary experiments (Hendry et al., Nature 368, 315; 1994) seemed to support this idea in that the quenches pro duced the predicted high vortex-densities. The present paper describes a ne w experiment incorporating a redesigned expansion cell that minimises vor t ex creation arising fi om conventional hydrodynamic flow. The post-quench l ine-densities of vorticity produced by the new cell are no more than 10(10) m(-2), a value that is at least two orders of magnitude less than the theo retical prediction. We conclude that most of the vortices detected in the o riginal experiment must have been created through conventional flow process es.