Reshaped three-body interactions and the observation of an Efimov state in the continuum

Efimov trimers are three-body quantum states that appear near two-atom Feshbach resonances. As interaction strength is reduced to a critical value, an Efimov state reaches the atom-dimer threshold and, in conventional scenarios, is expected to dissociate. Using coherent few-body spectroscopy on 7Li across a narrow Feshbach resonance, we find instead that the Efimov trimer persists above the atom-dimer threshold as a metastable state embedded in the continuum. We identify this behavior as a universal effect for narrow resonances arising from a repulsive interaction in the atom-dimer channel that reshapes three-body interactions, creating a barrier and a double-well structure. Our observations clarify the nature of 7Li Efimov states and provide a framework to understand puzzling phenomena associated with narrow resonances.

Yaakov Yudkin, Roy Elbaz, José P. D'Incao, Paul S. Julienne, Lev Khaykovich