Feshbach resonance

A Feshbach resonance is an effect of the physical scattering theory, which is named after the American physicist Herman Feshbach. It occurs when a bound state ( molecular state ) between two atoms is in a scattering potential, which is only slightly above the energy total energy of the incoming particles. In this case, the two atoms can occupy this bound state. As a result, "feel" it the potential for a long time and the change of their wave function is increasing. Thus, the scattering length of the system changes.

Feshbach resonances can be artificially induced: In the case of the magnetic Feshbach resonance, use is made of the Zeeman effect, so as to shift the scattering potentials to each other that this resonance occurs. Wherein the optical contrast is coupled Feshbach resonance with the atoms of the molecular state, by irradiating laser light with the exact energy gap. Both methods can be as the scattering length change almost arbitrarily.

The molecular states are normally not actually occupied: This is impossible, since it would indeed energy gained or lost. Only when the energy of the state of the molecules exactly coincides with the total energy of the incoming atoms applies energy conservation and the atoms forming molecules. In a magnetic Feshbach resonance can cause this case artificial and so are able to generate molecules. You only need the magnetic field be set so that the formation of molecules. Then you can lower the molecular potential, and persist the molecules.

Ultra-cold atoms and molecules

The most important applications of the Feshbach resonance are expected in the field of ultracold atoms lie ( 10-7 Kelvin or less). They are popular as a research object, despite experimental challenges, but notably if bosons to heavy losses because during tuning of the magnetic fields the scattering length changes suddenly, the atoms can come by and one by collision processes loses the atoms from the trap.

For fermions this is what is welcome: the Feshbach resonance allows not only an interaction, but even one that can be changed as described above will. With fermions could therefore also for the first time, as mentioned above, are able to generate molecules. As these in turn represented as Fermionenpaare bosons, could this in the form of a Bose -Einstein condensate ( in the literature: BEC) condense.

But not only that: the molecular state is lower in energy than the total energy of the incoming atoms ( the interaction operator in the Hamiltonian must be negative ), the interaction is attractive. If there is an attractive interaction between fermions, so one of the requirements of the BCS theory is fulfilled. So you could create a BCS state of ultracold atoms, which is similar to the electron ground state of a superconductor. Is obtained in this way by the Feshbach resonance a so-called BEC -BCS transition, a currently (2007) very active field of research.