Roton creation and vortex nucleation in superfluids

P. I. Soininen, Nikolai B. Kopnin, M.M. Salomaa

Igor S. Aranson, Victor Steinberg

洋 山本

Sergei V. Iordanskii

José María Escartín, Francesco Ancilotto, M. Barranco +1 more

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.

A. F. Borghesani

Abstract This chapter examines the wealth of experiments on the nucleation of vortex rings in the vicinity of the giant discontinuity. The nucleation in pure superfluid and in dilute isotopic mixtures has been investigated and three main models have been theoretically developed to interpret the experimental data: the peeling, the girdling, and the fluctuation models. The effects of isotopic impurities, high fields, and high pressures have been investigated and the results are described. The more recent experiments have elucidated the mechanisms of vortex nucleation, including the roton emission at velocities exceeding the Landau critical velocity and the possibility of macroscopic quantum tunnelling over a barrier. The enormous influence of isotopic impurities is also described.