Efimov Spectrum takes a Turn

The interaction strength-dependence of a molecule's binding energy discloses crucial information about the system's interaction potential. In general, a bound state features a plethora of bendings, turns and avoided crossings. Contrarily, universal few-body bound states are insensitive to short-range details of the potential, given that their wave function extends far beyond the characteristic interaction range. Here we use refined, recently proposed few-body spectroscopy [1] to create a coherent superposition of two such universal bound states: Feshbach dimers and Efimov trimers. In an interferometer-like experiment we measure the Efimov binding energy relative to that of the dimer in the theoretically controversial and experimentally demanding regime where the first excited trimer supposedly merges with the dimer-atom continuum. Our results show that the Efimov trimer does not live up to its universal promise [2]. Instead we identify three distinct regimes, only one of which is captured by a universal theory, albeit accounting for finite range effects. As the interaction strength is weakened, upon entering the second regime, the energy level takes a sudden turn towards the dimer-atom continuum. It then goes through an avoided crossing such that in the third regime the binding energy increases with decreasing interaction strength. These observations come as a surprise and can serve a guide for realistic multi-channel few-body theories.

[1] Phys. Rev. Lett. 122, 200402 (2019)

[2] arXiv:2004.02723