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Europhysics Letters (EPL)

Quantum halos

Karsten Riisager, D. V. Fedorov, A. S. Jensen

Published Mar 1, 2000• 75 citations
Publisher

Abstract

Quantum halos are defined as systems with dominating few-body structure and radii large compared to the sizes of the classically allowed regions. Dimensionless, universal scaling plots of radii vs. binding energies of two- and three-body systems are constructed to characterize and classify halo states. Short-range as well as long-range interactions are considered and molecular, atomic and nuclear states appear on the same plot. Specific two- and three-body systems are discussed, including Efimov states.

DOI
10.1209/epl/i2000-00180-y

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Quantum halos are defined as systems with dominating few-body structure and radii large compared to the sizes of the classically allowed regions. Dimensionless, universal scaling plots of radii vs. binding energies of two- and three-body systems are constructed to characterize and classify halo states. Short-range as well as long-range interactions are considered and molecular, atomic and nuclear states appear on the same plot. Specific two- and three-body systems are discussed, including Efimov states.
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The different kinds of behaviour of three-body systems in the weak-binding limit are classified with specific attention to the transition from a true three-body system to an effective two-body system. For weakly bound Borromean systems approaching the limit of binding we show that the size-binding energy relation is an almost universal function of the three s -wave scattering lengths measured in units of a hyperradial scaling parameter defined as a mass weighted average of two-body equivalent square well radii. We explain why three-body halos follow this curve and why systems appearing above reveal two-body substructures. Three-body quantum halos 2-3 times larger than the limit set by zero hypermoment are possible.
A method to study weakly bound three-body quantum systems in two dimensions is formulated in coordinate space for short-range potentials. Occurrences of spatially extended structures (halos) are investigated. Borromean systems are shown to exist in two dimensions for a certain class of potentials. An extensive numerical investigation shows that a weakly bound two-body state gives rise to two weakly bound three-body states, a reminiscence of the Efimov effect in three dimensions. The properties of these two states in the weak binding limit turn out to be universal.
A halo is an intrinsically quantum object defined as a bound state of a spatial size which extends deeply into the classically forbidden region. Previously, halos have been observed in bound states of two and less frequently of three atoms. Here, we propose a realization of halo states containing as many as six atoms. We report the binding energies, pair correlation functions, spatial distributions, and sizes of few-body clusters composed by bosonic dipolar atoms in a bilayer geometry. We find two very distinct halo structures, for large interlayer separation the halo structure is roughly symmetric and we discover an unusual highly anisotropic shape of halo states close to the unbinding threshold. Our results open avenues of using ultracold gases for the experimental realization of halos composed by atoms with dipolar interactions and containing as many as six atoms.
The parametric region in the plane defined by the ratios of the energies of the subsystems and the three-body ground state, in which Efimov states can exist, is determined. We use a renormalizable model that guarantees the general validity of our results in the context of short-range interactions. The experimental data for one- and two-neutron separation energies, implies that among the halo nuclei candidates, only 20C{}^{20}\mathrm{C}20C has a possible Efimov state, with an estimated energy less than 14 KeV below the scattering threshold.
The ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T↓, D↓, ^{3}He, ^{4}He, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.