Communication: Nucleation of quantized vortex rings in 4He nanodroplets

Abstract

Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba+ ion created by photoionization of neutral Ba at the surface of a 4He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba+ ion. The process is expected to be quite general and very efficient under standard experimental conditions

Picosecond solvation dynamics of alkali cations in superfluidHe4nanodroplets

“…These steps have been identified as the filling of the first solvation shell around the cation, somewhat defining the snowball. In addition, these two systems complement our work on Ba + [28]. Since compared to Ba + , the interaction of an alkali cation with helium is much stronger, the results will provide additional insight into the snowball formation and the nucleation of vortices in small helium droplets.…”

“…For completeness, we also show in Fig. 3 the He-Ba + pair potential [28]. The potential is much less attractive (−30.1 K) as compared to the Rb + -He and + -He pair potential [28].…”

“…The 6p 2 P ← 6s 2 S absorption spectrum of the Ba + cation was found to be identical to that obtained in bulk liquid He, indicating that the ion becomes indeed solvated by the droplet within the 100 ns time scale of the experiment [33]. The solvation dynamics of the Ba + has also been addressed theoretically within time-dependent density functional theory (TDDFT) for the Ba@He 1000 system [28]. It was found that due to the relatively strong ion-helium interaction, the velocity of the Ba + cation during the solvation process temporarily exceeds v L , leading to the nucleation of a quantized ring vortex.…”

“…It was found that due to the relatively strong ion-helium interaction, the velocity of the Ba + cation during the solvation process temporarily exceeds v L , leading to the nucleation of a quantized ring vortex. The calculated 6p 2 P ← 6s 2 S absorption spectrum of Ba + revealed a clear time dependence due to the solvation of the ion but was not noticeably affected by the presence of the ring vortex [28].…”

“…It is expected that they could be detected by spectroscopic techniques [23,24] as their presence affects the spectrum of the dopants they contain [25,26]. However, for the systems considered the width of the absorption spectrum is larger than the shift introduced by the presence of the vortex, thereby impeding the possible detection of vortices [27,28].…”