Jacob Johansen

Geometric scaling of Efimov states in a ⁶Li-¹³³Cs mixture.

In few-body physics, Efimov states are an infinite series of three-body bound states that obey universal discrete scaling symmetry when pairwise interactions are resonantly enhanced. Despite abundant reports of Efimov states in recent cold atom experiments, direct observation of the discrete scaling symmetry remains an elusive goal. Here we report the observation of three consecutive Efimov resonances in a heteronuclear Li-Cs mixture near a broad interspecies Feshbach resonance. The positions of the resonances closely follow a geometric series 1, λ, λ². The observed scaling constant λ(exp)=4.9(4) is in good agreement with the predicted value of 4.88.

Testing universality of Efimov physics across broad and narrow Feshbach resonances

The emergence of Efimov states in ultracold atomic systems is expected to have a universal behaviour, but a new experimental study defies this expectation, reporting a clear deviation around a narrow Feshbach resonance. Efimov physics is a universal phenomenon in quantum three-body systems. For systems with resonant two-body interactions, Efimov predicted an infinite series of three-body bound states with geometric scaling symmetry1. These Efimov states, first observed in cold caesium atoms2, have been recently reported in a variety of other atomic systems3,4,5,6,7,8,9,10,11,12,13. The intriguing prospect of a universal absolute Efimov resonance position across Feshbach resonances remains an open question. Theories predict a strong dependence on the resonance strength for closed-channel-dominated Feshbach resonances, whereas experimental results have so far been consistent with the universal prediction. Here we directly compare the Efimov spectra in a 6Li–133Cs mixture near two Feshbach resonances which are very different in their resonance strengths, but otherwise almost identical. Our result shows a clear dependence of the absolute Efimov resonance position on Feshbach resonance strength and a clear departure from the universal prediction for the narrow Feshbach resonance.

Observation of a Degenerate Fermi Gas Trapped by a Bose-Einstein Condensate

We report on the formation of a stable quantum degenerate mixture of fermionic ^{6}Li and bosonic ^{133}Cs in an optical trap by sympathetic cooling near an interspecies Feshbach resonance. New regimes of quantum degenerate Bose-Fermi mixtures are identified. With moderate attractive interspecies interactions, we show that a degenerate Fermi gas of Li can be fully confined in a Cs Bose-Einstein condensate without external potentials. For stronger attraction where mean-field collapse is expected, no such instability is observed. Potential mechanisms to explain this phenomenon are discussed.