Abstract
We present a computational study, based on time-dependent Density Functional theory, of the real-time interaction and trapping of Ar and Xe atoms in superfluid 4He nanodroplets either pure or hosting quantized vortex lines. We investigate the phase-space trajectories of the impurities for different initial conditions and describe in detail the complex dynamics of the droplets during the capture of the impurities. We show that the interaction of the incoming atom with the vortex core induces large bending and twisting excitations of the vortex core lines, including the generation of helical Kelvin waves propagating along the vortex core. We have also calculated the stationary configurations of a 4He droplet hosting a 6-vortex array whose cores are filled with Ar atoms. As observed in recent experiments, we find that doping adds substantial rigidity to the system, such that the doped vortex array remains stable, even at low values of the angular velocities where the undoped vortices would otherwise be pushed towards the droplet surface and be expelled.
Highlights
IntroductionThe pickup of Ar, Kr and Xe atoms in the gas phase by 4HeN droplets with N 4 103 atoms produced by nozzle beam expansions was described about twenty years ago by Toennies and coworkers.[9]
08028 Barcelona, Spain f Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, Barcelona, Spain † Electronic supplementary information (ESI) available
For a self-bound 4He droplet, deformation comes from ‘‘rotation’’ itself and it turns out to be minute even for angular frequencies close to the critical frequency for onevortex nucleation; the conclusion is that the droplet ‘‘does not rotate’’; in other words, it is unable to store an appreciable amount of angular momentum before vortex nucleation
Summary
The pickup of Ar, Kr and Xe atoms in the gas phase by 4HeN droplets with N 4 103 atoms produced by nozzle beam expansions was described about twenty years ago by Toennies and coworkers.[9]. Special attention is paid to the time-dependent interaction of Xe and Ar atoms with helium nanodroplets hosting vortex lines, and to the effect of multiply-doped vortex arrays in large helium droplets. We study how a Xe atom dynamically interacts with a droplet hosting a vortex line, under different initial conditions resulting in different velocity regimes of the impurity as it collides with the vortex core: (i) a Xe atom initially at rest on the droplet surface and sinking under the effect of solvation forces; and (ii) a head-on collision of a moving Xe or Ar atom against the 4He nanodroplet. We study the stationary state of a large 4He15000 droplet hosting a ring of six vortex lines, doped with Ar atoms completely filling all six vortex cores. This is the simplest system that mimics those experimentally described in ref. They constitute an important part of this work, since often it is only by viewing how a complex microscopic process unfolds in real time that one can catch important physical details which would otherwise escape in a written account
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