Ground-state characteristics of helium–alkali tetramers

Abstract

Li, 23 Na, 39,40,41 K, 85,87 Rb or 133 Cs, were studied using diffusion Monte Carlo method. Besides ground-state energies, exact distribution functions were evaluated. Spatial arrangements of atoms in clusters qualify He-A tetramers as transitional systems, which reveal halo-like spatial extension. On the one hand, swapping 4 He by 3 He atom in a He-Li tetramer, one can trace how quasi-halo structure transforms into halo structure. On the other hand, swapping 3 He by 4 He in He-A tetramer reveals behavior previously noticed in helium droplets doped with an alkali atom. Helium atoms start grouping on one side of an alkali atom, which is clearer in He-A clusters with more 4 He atoms that have stronger self-binding. The effect is the more pronounced the bigger alkali atom is, since it is then harder for the remaining He atoms to surround an alkali.

Universality of size-energy ratio in four-body systems

“…Constituents and tetramer types are distinguished with different symbols (see the legends). Available published data were taken from helium 20 , helium-alkali 21 , and helium-tritium tetramers 31 . Necessary additional features, like the structure properties 〈 r 2 〉, and new data for all other clusters included in the Figure were calculated.…”

“…Additionally, when a system is partially constrained by asymmetrical potential barriers, a mixed system can exhibit also small deviations below the universal line. For instance, an alkali atom in He-alkali tetramers 21 prevents symmetric configurations: the helium atoms group together in one side of the alkali atom instead of surrounding it, limiting the size of the cluster.…”

“…A stochastic approach was applied by means of the second-order diffusion Monte Carlo method 33 in which pure estimators 34 were implemented. Masses and trial wave-functions were taken from our previous works 15,20,21,31 , while the same bias removal approach was applied.…”

“…Extensions of the imaging approach applied in the case of helium trimers was qualified as feasible for the four-body sector 8 . Recently, four-body halo and quasi-halo states were predicted to exist in helium tetramers 20 and in tetramers obtained by swapping one He atom by an alkali one 21 . Moreover, a thorough analysis for a large set of pure and mixed weakly-bound atomic dimers and trimers showed that universal size-energy scaling extends even below the halo area 15 , in the so called quasi-halo region.…”

Van der Waals five-body size-energy universality

“…A thorough analysis for a large set of pure and mixed weakly-bound atomic dimers, trimers, and tetramers showed that universal size-energy scaling extends even below the halo area 21 , in the so called quasi-halo region. Four-body systems with large size were found in Helium and Helium-alkali tetramers 27 , 28 , but also in pentamers 29 . It is therefore interesting to explore the existence of a universal relation between energy and size of five-body clusters, in a wide range extending from weakly-bound quantum halo systems to classical ones.…”