Theory of weakly bound quantum states: testing the universality of three-body interactions near narrow Feshbach resonances

We study properties of Efimov states and explore their dependence on the strength of the underlying Feshbach resonance. Within a two-channel model for the interatomic interactions, we calculate the energies of the Efimov ground and exited states to test the van der Waals universality expected for strong (broad) Feshbach resonances. By varying the position and strength of the two-body coupling between the channels, we modify the strength/width of the Feshbach resonance, as measured by the 'sres' parameter [1]. Investigating across a range of Feshbach resonance widths, we showcase the appearance of non-universal behavior of the Efimov trimer in the ground and excited states, likely emergent from a repulsive interaction in the atom-dimer channel, which appears for narrow resonances [2]. The interplay between the length-scales describing the short-range, exchange, and the van der Waals interaction across a series of alkali-metal, homonuclear samples is likely also an important factor determining the universal behavior of the Efimov spectrum. Therefore, we estimate the strength of three-body forces in relevant few-body systems using ab initio quantum chemistry methods. We then compare the ground Efimov resonance location dependence on the 'sres' parameter for systems with varying strength of the Feshbach resonance [3].