In the vicinity of a Feshbach resonance only a hand full of length scales, such as the scattering length and the effective range, determine the observed physics. Few-body observables, such as recombination loss maxima and minima, are related to the underlying length scales via universal theories. In particular, the Efimov-van-der-Waals universality relates the position of the first three-body recombination resonance on the negative scattering length side to the van-der-Waals length according to the Feshbach resonance strength. While numerous experiments in several species seem to verify this universality, Lithium was found to defy it.
Here we use coherent few-body spectroscopy (the DITRIS interferometer) to explore the positive scattering length side in Lithium. By creating a superposition of two different loosely bound states, Feshbach dimers and Efimov trimers, we measure the Efimov binding energy relative to that of the dimer. It is applied to the theoretically controversial and experimentally demanding regime, where the first excited trimer supposedly merges with the dimer-atom continuum. Contrary to the universal expectation we discover that it crosses into the continuum and remains a long-lived bound state.
In an attempt to explain this observation, we build minimal models including the dominant multi-channel contributions to the long-range physics. In two separate models we add either an additional closed or open channel – both of which are present in Lithium. The failure of these models to replicate the experiment indicates that, in addition to the multi-channel character of the interactions, the short-range details of the interaction potential are necessary. This is in stark contrast to the assumption of universality.
|Presenter name||Yaakov Yudkin|
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