### Abstract

We study a gas of fermions undergoing a wide resonance s -wave BCS-Bose-Einstein-condensate (BEC) crossover, in the BEC regime at zero temperature. We calculate the chemical potential and the speed of sound of this Bose-Einstein-condensed gas, as well as the condensate depletion, in the low-density approximation. We discuss how higher-order terms in the low-density expansion can be constructed. We demonstrate that the standard BCS-BEC gap equation is invalid in the BEC regime and is inconsistent with the results obtained here. The low-density approximation we employ breaks down in the intermediate BCS-BEC crossover region. Hence our theory is unable to predict how the chemical potential and the speed of sound evolve once the interactions are tuned towards the BCS regime. As a part of our theory, we derive the well-known result for the bosonic scattering length diagrammatically and check that there are no bound states of two bosons. © 2006 The American Physical Society.

Original language | English |
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Journal | Physical Review A |

Volume | 73 |

Issue number | 5 |

DOIs | |

Publication status | Published - 18 May 2006 |

## Cite this

Levinsen, J., & Gurarie, V. (2006). Properties of strongly paired fermionic condensates.

*Physical Review A*,*73*(5). https://doi.org/10.1103/PhysRevA.73.053607