Physical Origin of the Universal Three-body Parameter in Atomic Efimov Physics

TL;DR

This paper explains the universal three-body parameter in atomic Efimov physics as arising from van der Waals interactions, which induce a deformation that prevents atoms from coming too close, thus determining the Efimov spectrum.

Contribution

It identifies the microscopic origin of the universal three-body parameter as the van der Waals correlation causing three-body deformation.

Findings

Universal ratio of scattering length to van der Waals length explained

Deformation mechanism accounts for experimental observations

Numerical calculations confirm the theoretical model

Abstract

We address the microscopic origin of the universal three-body parameter that fixes the spectrum of three-atom systems in the Efimov regime. We identify it with the van der Waals two-body correlation, which causes the three-atom system to deform when the three atoms come within the distance of the van der Waals length, effectively preventing them from coming closer due to the kinetic-energy cost associated with this three-body deformation. This deformation mechanism explains the universal ratio of the scattering length at the triatomic resonance to the van der Waals length observed in several experiments and confirmed by numerical calculations.