In the 1970s, Vitaly Efimov found a remarkable thing. He calculated that the neutrons in tritium could form three-body conglomerations even if any two neutrons had no interest in each other. These Efimov states remained merely a theoretical idea until years later when researchers found experimental evidence for them, not with nucleons, but with ultracold atoms. But questions remain about how universal this phenomenon is—do these states depend on special properties of the atoms involved or other microscopic details? In a paper in Physical Review Letters, Sanjukta Roy at the University of Florence, Italy, and colleagues present experimental evidence for Efimov states in potassium-39, which has very different atomic character than atoms used previously.
Efimov states are purely quantum mechanical beasts. The force that holds them together can be fine tuned with an effect known as the Feshbach resonance—a rare situation in which colliding atoms have just the right kinetic energy to reach a bound state. Typically, atoms like cesium have very broad ranges for the resonance, but potassium resonances are extremely narrow, limiting the chances for Efimov threesomes to form. Theory said that the only way it could happen was if some special interactions could glue the atoms together.
In their experiments, Roy et al. tested the properties of Efimov states formed using seven different ultranarrow Feshbach resonances in potassium and found that they indeed exhibit the same universal properties as Efimov states of other atoms. According to Roy et al., it is now the theorists’ turn to figure out why potassium does this and whether it opens a new range of atomic species that can form Efimov triplets. – David Voss