P. C. Hendry

Decay of quantized vorticity in superfluid 4He at mK temperatures

Abstract An experimental investigation of the free decay of quantized turbulence in isotopically pure superfluid 4He at mK temperatures is discussed. Vortices are created by a vibrating grid, and detected by their trapping of negative ions. Preliminary results suggest the existence of a temperature-independent vortex decay mechanism below T∼70 mK .

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.

Continuous flow apparatus for preparing isotopically pure 4He

A He-4 isotopic purification cryostat has been developed, capable of sustained operation in continuous flow. Starting from a feedstock of helium of the natural isotopic ratio, He-3/He-4 = X3 = 10^-7, it yields a purified product for which X3 < 5 X 10^-13 at a production rate of 3.3 STP m3/h. The isotopically purified He-4 is being used for a variety of applications, including quantum evaporation experiments, studies of ion motion at the He II/vacuum interface, down scattering and containment of ultra-cold neutrons, and investigations of the breakdown of superfluidity in He-4.

Flow of He II due to an Oscillating Grid in the Low-Temperature Limit

The macroscopic flow properties of pure He II are probed in the limit of zero temperature using an oscillating grid. As the oscillation amplitude passes a first critical threshold, the resonant frequency starts decreasing but the flow remains nondissipative. Beyond a second critical amplitude, the flow undergoes a transition to turbulence and becomes dissipative. Nonlinearity and hysteresis observed between the thresholds are attributed to a boundary layer of quantized vortices.

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.

4He isotopic purification in continuous flow

We report the successful operation of a 4He isotopic purifier capable of producing helium with a 3He/4He ratio of < 10^-12 on a continuous basis.

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.