Ultracold atom superfluidity induced by the Feshbach resonance

We discuss the possible signatures of superfluidity induced by the Feshbach resonance in the ultracold gas of fermion atoms. Optically or magnetically trapped atoms such as 6 Li or 40 K are used in two hyperfine states where part of them is converted into the diatomic molecules. These fermion and boson entities get coupled in a presence of the external magnetic field. Eventually, at critical T c , they simultaneously undergo transition to the superfluid state. Approaching this transition from above there appear various signatures manifesting a gradually emerging order parameter, but yet the long range coherence is not established due to the strong quantum fluctuations. Fermion atoms are characterized by the gapped excitation spectrum ( pseudogap ) up to temperature T p (larger than T c ) while boson molecules exhibit collective features such as first sound showing up above a certain critical momentum q . crit ( T ) Upon lowering temperature to T c this critical value shifts to zero and hence there appears the Goldstone mode signaling the symmetry broken superfluid state. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)