Feshbach resonance

In the field of physics, a Feshbach resonance, named after Herman Feshbach, 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. In the field of physics, a Feshbach resonance, named after Herman Feshbach, 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. Feshbach resonances have become important in the study of the cold atoms systems, both the Fermi gases as well as the Bose–Einstein condensates (BECs). 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, which have hyperfine structure coupled via Coulomb or exchange interactions. In experimental settings, the Feshbach resonances provide a way to vary interaction strength between atoms in the cloud by changing scattering length, asc, 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 region of the BEC (of fermionic molecules) to the BCS (of weakly interacting fermion-pairs) transition 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. Consider a general quantum scattering event between two particles. In this reaction, there are two reactant particles denoted by A and B, and two product particles denoted by A' and B' . For the case of a reaction (such as a nuclear reaction), we may denote this scattering event by The combination of the species and quantum states of the two reactant particles before or after the scattering event is referred to as a reaction channel. Specifically, the species and states of A and B constitute the entrance channel, while the types and states of A' and B' constitute the exit channel. An energetically accessible reaction channel is referred to as an open channel, whereas a reaction channel forbidden by energy conservation is referred to as a closed channel. Consider the interaction of two particles A and B in an entrance channel C. The positions of these two particles are given by r → A {displaystyle {vec {r}}_{A}} and r → B {displaystyle {vec {r}}_{B}} , respectively. The interaction energy of the two particles will usually depend only on the magnitude of the separation R ≡ | r → A − r → B | {displaystyle Requiv |{vec {r}}_{A}-{vec {r}}_{B}|} , and this function, sometimes referred to as a potential energy curve, is denoted by V c ( R ) {displaystyle V_{c}(R)} . Often, this potential will have a pronounced minimum and thus admit bound states.