Bound Electron Pairs in a Degenerate Fermi Gas

A.S. Tyapin

D. S. Jin

Y. M. Malozovsky, J. D. Fan

The binding of two fermions in a degenerate Fermi gas is examined in terms of the T-matrix. We deal with effective T-matrix of two fermions in the degenerate Fermi gas in terms of the scattering amplitude. This makes approach valid for the retarded as well as non-retarded interactions. We show that for the interaction of a pair of fermions in a quiescent Fermi sea there are two different series of the ladder diagrams-Exchange and Hartree diagrams, describing a bound state and producing two different T-scattering matrices. The exchange series of the ladder diagrams corresponds to the T-matrix in the triplet-state. This T-matrix contains the pole solution corresponding to the bound state for the attractive interaction only. In contrast, the Hartree diagrams lead to the T-matrix of a pair of fermions in the singlet-state, which have a pole solution corresponding to the bound state of a pair of fermions for the repulsive interaction. Thus, we are led to assert that the bound state of a pair of fermions in the singlet-state in the quiescent Fermi sea can form for the repulsive interaction only and there is no bound state in the singlet-state from the attractive interaction.

S. R. Granade, Michael E. Gehm, Kenneth M. O’Hara +1 more

We achieve degeneracy in a mixture of the two lowest hyperfine states of 6Li by direct evaporation in a CO2 laser trap, yielding the first all optically produced degenerate Fermi gas. More than 10(5) atoms are confined at temperatures below 4 microK at full trap depth, where the Fermi temperature for each state is 8 microK. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose-Einstein condensation of strongly bound pairs.

Michael G. Bowler

S. J. J. M. F. Kokkelmans, Murray Holland, R. Walser +1 more

We consider the superfluid phase transition that arises when a Feshbach resonance pairing occurs in a dilute Fermi gas. This is related to the phenomenon of superconductivity described by the seminal Bardeen-Cooper-Schrieffer (BCS) theory. In superconductivity, the phase transition is caused by a coupling between pairs of electrons within the medium. This coupling is perturbative and leads to a critical temperature Tc which is small compared to the Fermi temperature TF. Even high-Tc superconductors typically have a critical temperature which is two orders of magnitude below TF. Here we describe a resonance pairing mechanism in a quantum degenerate gas of potassium (40K) atoms which leads to superfluidity in a novel regime--a regime that promises the unique opportunity to experimentally study the crossover from the BCS phase of weakly-coupled fermions to the Bose Einstein condensate of strongly-bound composite bosons. We find that the transition to a superfluid phase is possible at the high critical temperature of about 0.5TF. It should be straightforward to verify this prediction, since these temperatures can already be achieved experimentally.