Feshbach resonance

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In physics, a Feshbach resonance can occur upon collision of two slow atoms, when they temporarily stick together forming an unstable compound with short lifetime (so-called resonance).^[1] It is a feature of many-body systems in which a bound state is achieved if the coupling(s) between at least one internal degree of freedom and the reaction coordinates, which lead to dissociation, vanish. The opposite situation, when a bound state is not formed, is a shape resonance. It is named after Herman Feshbach, a physicist at MIT.

Feshbach resonances have become important in the study of cold atoms systems, including Fermi gases and Bose–Einstein condensates (BECs).^[2] In the context of scattering processes in many-body systems, the Feshbach resonance occurs when the energy of a bound state of an interatomic potential is equal to the kinetic energy of a colliding pair of atoms. In experimental settings, the Feshbach resonances provide a way to vary interaction strength between atoms in the cloud by changing scattering length, a_sc, of elastic collisions. For atomic species that possess these resonances (like K³⁹ and K⁴⁰), it is possible to vary the interaction strength by applying a uniform magnetic field. Among many uses, this tool has served to explore the transition from a BEC of fermionic molecules to weakly interacting fermion-pairs the BCS in Fermi clouds. For the BECs, Feshbach resonances have been used to study a spectrum of systems from the non-interacting ideal Bose gases to the unitary regime of interactions.