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Physics World

Superfluid vortex nucleation displayed

Eric Varoquaux

Published Jan 1, 1991• 0 citations
Publisher

Abstract

Vortex lines are known to exist in superfluid helium – photographs of them have even been taken by Dick Packard and his students at Berkeley in 1982 – but how they are formed is still situation has now been reported by P C Hendry and colleagues at Lancaster University, UK, who have studied in detail how high-velocity icons nucleate vortex loops – a process hitherto very much the preserve of theorists.

DOI
10.1088/2058-7058/4/1/22

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Vortex lines are known to exist in superfluid helium – photographs of them have even been taken by Dick Packard and his students at Berkeley in 1982 – but how they are formed is still situation has now been reported by P C Hendry and colleagues at Lancaster University, UK, who have studied in detail how high-velocity icons nucleate vortex loops – a process hitherto very much the preserve of theorists.
Take a good look at these photographs; eventually they will probably be in many textbooks. These photographs represent visual evidence for macroscopic quantization in rotating supertluid helium and are the latest in a series of experiments by Richard Packard and his collaborators at the University of California, Berkeley. E. J. Yarmchuk, M. J. V. Gordon and Packard recently published these pictures in Phys. Rev. Lett. 43, 214 (1979). These patterns display the model structure of the stationary states of a single macroscopic wave function describing superfluidity.
Though vortices are an unavoidable presence in bulk superfluid helium, they have not been observed to date in helium nanodroplets. The energy and angular momentum of both straight and curved vortex states of a helium nanodroplet are examined as a function of droplet size. For droplets in the size range of many experiments, it is found that during the pickup of heavy solutes, a significant fraction of events deposit sufficient energy and angular momentum to form a straight vortex line. Curved vortex lines exist down to nearly zero angular momentum and energy, and thus could in principle form in almost any collision. Further, the coalescence of smaller droplets during the cooling by expansion could also deposit sufficient angular momentum to form vortex lines. Despite their high energy, most vortices are predicted to be stable at the final temperature (0.38 K) of helium nanodroplets due to lack of decay channels that conserve both energy and angular momentum.
In superconductors, as in Helium II (He II), the holes round which quantisation occurs can be either physical boundaries, or the cores of quantised vortex lines. For convenience we summarise here the resemblances and differences between vortex lines and vortex states in He II and in superconductors. In contrast, states in which a whole vortex is attached to the wire should be comparatively stable. The rotation of the superfluid can be satisfactorily explained by assuming that it is threaded by a series of parallel straight vortex lines. Consider a single straight vortex line in the superfluid. The energy associated with a single vortex line is mainly the kinetic energy of the circulating superfluid. The number and separation of vortex lines present for a given angular velocity clearly depends on their individual circulations. Two rival models for the ion-vortex transition have been proposed, generally known as the peeling model and the quantum transition model.
Within density functional theory, we have investigated the coalescence dynamics of two superfluid helium nanodroplets hosting vortex lines in different relative orientations, which are drawn towards each other by the Van der Waals mutual attraction. We have found a rich phenomenology depending on how the vortex lines are oriented. In particular, when a vortex and antivortex lines are present in the merging droplets, a dark soliton develops at the droplet contact region, which eventually decays into vortex rings. Reconnection events are observed between the vortex lines or rings, leading to the creation of more vortices. Our simulations show the interplay between vortex creation and reconnections, as well as the effect of the droplet surface which pins the vortex ends and, by reflecting short-wavelength excitations produced by the interactions between vortices, strongly affects the droplet final state. Additional vorticity is nucleated in the proximity of surface indentations produced in the course of the dynamics, which in turn interact with other vortices present in the droplets. These effects, obviously absent in the case of bulk liquid helium, show that the droplet surface may act as a multiplier of vortex reconnections. The analysis of the energy spectrum shows that vortex-antivortex ring annihilation, as well as vortex-antivortex reconnections, yields roton bursts of different intensity.
Free superfluid helium droplets constitute a versatile medium for a diverse range of experiments in physics and chemistry that extend from studies of the fundamental laws of superfluid motion to the synthesis of novel nanomaterials. In particular, the emergence of quantum vortices in rotating helium droplets is one of the most dramatic hallmarks of superfluidity and gives detailed access to the wave function describing the quantum liquid. This review provides an introduction to quantum vorticity in helium droplets, followed by a historical account of experiments on vortex visualization in bulk superfluid helium and a more detailed discussion of recent advances in the study of the rotational motion of isolated, nano- to micrometer-scale superfluid helium droplets. Ultrafast X-ray and extreme ultraviolet scattering techniques enabled by X-ray free-electron lasers and high-order harmonic generation in particular have facilitated the in situ detection of droplet shapes and the imaging of vortex structures inside individual, isolated droplets. New applications of helium droplets ranging from studies of quantum phase separations to mechanisms of low-temperature aggregation are discussed.
We have produced and observed quantized vortex rings emitted from orifices when superfluid helium is forced through channels at critical velocity. The vortices are identified by measuring their velocity and size, and are found to be in the n=1n=1n=1 quantum state.