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arXiv:2003.03306 · Physical review. B./Physical review. B

Rotating mixed He3−He4 nanodroplets

Martí Pi, Francesco Ancilotto, José María Escartín, R. Mayol, M. Barranco

Published Aug 7, 2020• 7 citations
arXivHTMLPDF

Abstract

Mixed $^{3}\mathrm{He}\ensuremath{-}^{4}\mathrm{He}$ droplets may acquire angular momentum during their passage through the nozzle of the experimental apparatus, cooling down and undergoing isotopic segregation, developing a $^{3}\mathrm{He}$ crust surrounding a superfluid $^{4}\mathrm{He}$ core. Within density functional theory, we investigate their stability and the relations between their angular momenta and their shapes. We uncover a variety of behaviors where the interplay between the superfluid $^{4}\mathrm{He}$ core and the normal-fluid $^{3}\mathrm{He}$ coating leads to a scenario that both bears analogies with viscous rotating drops and displays new features such as configurations with a fissioned or three-lobed $^{4}\mathrm{He}$ core, or with multiply charged vortices.

DOI
10.1103/physrevb.102.060502

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Mixed 3He\ensuremath−4He^{3}\mathrm{He}\ensuremath{-}^{4}\mathrm{He}3He\ensuremath−4He droplets may acquire angular momentum during their passage through the nozzle of the experimental apparatus, cooling down and undergoing isotopic segregation, developing a 3He^{3}\mathrm{He}3He crust surrounding a superfluid 4He^{4}\mathrm{He}4He core. Within density functional theory, we investigate their stability and the relations between their angular momenta and their shapes. We uncover a variety of behaviors where the interplay between the superfluid 4He^{4}\mathrm{He}4He core and the normal-fluid 3He^{3}\mathrm{He}3He coating leads to a scenario that both bears analogies with viscous rotating drops and displays new features such as configurations with a fissioned or three-lobed 4He^{4}\mathrm{He}4He core, or with multiply charged vortices.
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Within density functional theory, we have studied the interplay between vortex arrays and capillary waves in spinning prolate 4He^{4}\mathrm{He}4He droplets made of several thousand helium atoms. Surface capillary waves are ubiquitous in prolate superfluid 4He^{4}\mathrm{He}4He droplets, and depending on the size and angular momentum of the droplet, they may coexist with vortex arrays. We have found that the equilibrium configuration of small prolate droplets is vortex free, evolving towards vortex hosting as the droplet size increases. This result is in agreement with a recent experiment [O'Connell et al., Phys. Rev. Lett. 124, 215301 (2020)] that disclosed that vortex arrays and capillary waves coexist in the equilibrium configuration of very large drops. In contrast to viscous droplets executing rigid-body rotation, the stability phase diagram of spinning 4He^{4}\mathrm{He}4He droplets cannot be universally described in terms of dimensionless angular momentum and angular velocity variables: Instead, the rotational properties of superfluid helium droplets display a clear dependence on the droplet size and the number of vortices they host.
Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, exploring the dynamic properties of individual droplets is experimentally challenging. In this work, we used single-shot femtosecond x-ray coherent diffractive imaging to investigate the rotation of single, isolated superfluid helium-4 droplets containing ~10(8) to 10(11) atoms. The formation of quantum vortex lattices inside the droplets is confirmed by observing characteristic Bragg patterns from xenon clusters trapped in the vortex cores. The vortex densities are up to five orders of magnitude larger than those observed in bulk liquid helium. The droplets exhibit large centrifugal deformations but retain axially symmetric shapes at angular velocities well beyond the stability range of viscous classical droplets.