Generation of Large Vortex-Free Superfluid Helium Nanodroplets

Superfluidity of quasi-2D 4He droplets on graphite

The superfluid response of nanoscale size quasi-2D 4He droplets adsorbed on a graphite substrate is investigated by computer simulations. It is found that clusters comprising as few as 7 atoms are stable at temperatures of ≲0.15 K. Clusters of ∼20 atoms or less are liquid-like and ∼100% superfluid. As the size is increased, the central region crystallizes, forming the commensurate C1/3 phase, with a quasi-1D surface layer of superfluidity possibly evolving into a Luttinger liquid or a transverse quantum superfluid with increasing cluster size. The relevance to quasi-2D molecular spectroscopy is discussed.

Formation of magnesium clusters in superfluid helium nanodroplets

Magnesium atoms in liquid helium have been hypothesized to form a metastable foam structure, in which a layer of helium atoms surrounds each magnesium atom, inhibiting their coalescence into a compact cluster. This conjecture is based on the weak interaction between the magnesium atoms themselves and with the helium atoms and was used to explain observations in femtosecond two-photon ionization experiments by different groups. However, this theory is incongruent with previous infrared spectroscopic observations, indicating the formation of tightly bound clusters when different atoms and molecules combine inside liquid helium. In this paper, we report the spectra (from 210 to 2210 nm) of magnesium-doped superfluid helium nanodroplets at different averaged droplet sizes and number of dopants. The measured spectra in this study are consistent with the formation of compact magnesium clusters rather than the metastable foam structure.

Electron diffraction of foam-like clusters between xenon and helium in superfluid helium droplets

We report electron diffraction results of xenon clusters formed in superfluid helium droplets, with droplet sizes in the range of 105-106 atoms/droplet and xenon clusters from a few to a few hundred atoms. Under four different experimental conditions, the diffraction profiles can be fitted using four atom pairs of Xe. For the two experiments performed with higher helium contributions, the fittings with one pair of Xe-He and three pairs of Xe-Xe distances are statistically preferred compared with four pairs of Xe-Xe distances, while the other two experiments exhibit the opposite preference. In addition to the shortest pair distances corresponding to the van der Waals distances of Xe-He and Xe-Xe, the longer distances are in the range of the different arrangements of Xe-He-Xe and Xe-He-He-Xe. The number of independent atom pairs is too many for the small xenon clusters and too few for the large clusters. We consider these results evidence of xenon foam structures, with helium atoms stuck between Xe atoms. This possibility is confirmed by helium time-dependent density functional calculations. When the impact parameter of the second xenon atom is a few Angstroms or longer, the second xenon atom fails to penetrate the solvation shell of the first atom, resulting in a dimer with a few He atoms in between the two Xe atoms. In addition, our results for larger droplets point toward a multi-center growth process of dopant atoms or molecules, which is in agreement with previous proposals from theoretical calculations and experimental results.

Two-dimensional Brownian motion of active particle on superfluid helium surface

We report an experimental study of the 2D dynamics of active particles driven by quantum vortices on the free surface of superfluid helium at T = 1.45 К. The particle motion at short times (< 25 ms) relates to anomalous diffusion mode typical for active particles, while for longer times it corresponds to normal diffusion mode. The values of the rotational and translational kinetic energies of the particle allow to determine for the first time the intensity of the particle-vortex interaction and the dissipation rate of the vortex bundle energy. Strong bonding between a particle and a vortex is explained by coupling of normal and superfluid components.