N. S. Lawson

Nonappearance of vortices in fast mechanical expansions of liquid He=4 through the lambda transition

A new experiment has been performed to study the formation of topological defects (quantized vortices) during rapid quenches of liquid He-4 through the superfluid transition, with particular care taken to minimize vortex creation via conventional hydrodynamic flow processes. It is found that the generated vortices, if any, are being produced at densities at least 2 orders of magnitude less than might be expected on the basis of the Kibble-Zurek mechanism.

Creation of Quantized Vortices at the Lambda Transition in Liquid Helium-4

A fast (∼ 3 ms) adiabatic expansion of a volume of liquid4He through the lambda transition is being used to study the nature of the transition and to model the cosmological false vacuum to true vacuum phase transition of the early universe. Preliminary results are reported showing that, in accordance with theoretical predictions by W H Zurek (Nature 317, 505; 1985), there is copious production of quantized vortex lines, which represent the superfluid analogue of cosmic strings. The line density after the expansion appears to decay in two distinct stages, with a fast decay being followed by a much slower one, in agreement with earlier work on the decay of quantum turbulence created in thermal counterflow. Extrapolation of the initial fast decay suggests an initial line density, immediately following the expansion, of ∼107 cm−2. Smaller, but still substantial, vortex densities are also found to occur when the system is expanded from below the lambda transition, and the physical implications are discussed.

The breakdown of superfluidity in liquid 4He. VI, Macroscopic quantum tunnelling by vortices in isotopically pure He II

Measurements are reported of the rate v at which negative ions nucleate quantized vortices in isotopically pure superfluid 4He for electric fields E, temperatures T and pressures P within the range 103 ⩽ E ⩽ 106 V m-1, 75 ⩽ T ⩽ 500 mK, 12 ⩽ P ⩽ 23 bar (= 2.3 MPa). The form of v(E, T) differs in unexpected ways from that observed in earlier work at higher P, exhibiting: a pronounced dip in v(T) at ca. 0.3 K whose depth and precise position depends on E and P ; an exponential increase in v(T) at higher T, with an activation energy considerably smaller than the roton energy gap; and distinct structure in v(E). The experimental data are discussed and analysed in terms of the macroscopic quantum tunnelling model proposed by Muirhead et al. (Phil. Trans. R. Soc. Lond. A 311, 433 (1984)). The relatively small barrier heights of ca. 2—3 K deduced from the data on this basis are construed as confirmation that the initial vortex is a loop rather than an encircling ring. The temperature dependence of v at low pressures is interpreted in terms of a phonon-driven vortex nucleation mechanism, and values for its cross section are extracted from the data. The minima in v(T) are ascribed to phonon damping of the tunnelling process, and the kinks observed in some of the low-temperature v(E) curves are attributed to tunnelling of the system into the first excited state of the nascent vortex loop.

Expansion of Liquid 4He Through the Lambda Transition

Zurek suggestedNature 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 topological defects in liquid4He, 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 produced the predicted high vortex-densities. The present paper describes a new experiment incorporating a redesigned expansion cell that minimises vortex creation arising from conventional hydrodynamic flow. The post-quench line-densities of vorticity produced by the new cell are no more than 1010m−2, a value that is at least two orders of magnitude less than the theoretical prediction. We conclude that most of the vortices detected in the original experiment must have been created through conventional flow processes.

Does the Kibble Mechanism Operate in Liquid 4He

A rapid passage of liquid 4He through the lambda (superfluid) transition is expected (W. H. Zurek, Nature 317, 505; 1985) to result in the production of topological defects (quantized vortices) through the Kibble mechanism, the same process that is believed to have produced cosmic strings at the GUT phase transition of the early universe. But recent experiments (Dodd et al, Phys. Rev. Lett 81, 3703; 1998)) show that the density of vortices created, if any, seems to be smaller than predicted by a factor of at least 100. Possible ways of improving the sensitivity of the experiment are discussed.

Cosmological experiments in liquid 4He: status and prospects

A recent experiment, in which a small volume of liquid 4He is expanded rapidly through the lambda transition to model the mechanism of cosmic string creation in the early universe, is reviewed. New data are presented, suggesting that, although most of the quantized vortices generated in the expansion are created (like cosmic strings) by passage through the phase transition at a finite speed, a proportion of them are probably produced through more conventional processes. Possible ways of redesigning the experiment, so as to reduce the latter extraneous vortex creation processes, are discussed.

Experiments on the Kibble mechanism in liquid4He

New experiments are in progress to study the creation of quantised, vortices during a fast adiabatic passage through the lambda transition, thus modelling the production of cosmic strings, via the Kibble mechanism in the early universe. The expansion cell differs from that used in the initial experiments in several respects. In particular: it has been designed to minimise the flow of liquid parallel to surfaces which is believed to have caused some vortex creation via conventional mechanisms; and a hydraulically actuated needle-valve is used to close off the filling capillary where it enters the cell, thus isolating the sample. The results will be presented and discussed.

Evidence for an energy barrier impeding the creation of quantized vortices in He II.

The rate nu at which negative ions create quantized vortex rings in isotopically pure He II has been measured for temperatures 70 < T < 500 mK, pressures 12 < P < 19 bar and electric fields 10^4 < E < 10^S V/m. The result can be taken to imply the existence of an energy barrier impeding the nucleation process of ~3K.

Vortex creation in the expansion of He II from just below the lambda transition.

Quantised vortices are created when HeII is expanded from a point close to, but slightly (≈3mK) below, the lambda transition. It is believed that these probably arise through conventional flow processes, caused by non-idealities in the design of the expansion system. Their density is smaller than that of vortices created via the Kibble mechanism in expansions passing through the transition; nonetheless, their presence inevitably reduces the accuracy of the latter experiments for modeling cosmic string creation. An investigation of the phenomenon with improved bolometry, is described and discussed.

Vortex generation in HeII below 100 mK

Possible ways of creating quantised vortex lines in liquid4He below 100 mK including, in particular, an electrostaticallydriven vibrating grid, are discussed.