, where the hyperfine structure is inverted, is inappropriate. Introducing a three-state Fermi model, where a spin state is shared by the open and closed channel states, we show that effects of ``Pauli blocking'' appear in the internal structure of the condensate wave function.

DOI: 10.1103/physrevlett.94.240402

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^{6}\mathrm{Li}

system, we argue that its application to

^{40}\mathrm{K}

Fermion-mediated BCS-BEC crossover in ultracold

{}^{40}

K gases

^{6}\mathrm{Li}

system, it has been argued recently that its application to the

^{40}\mathrm{K}

system, where the hyperfine structure is inverted, is inappropriate. Here, the formation of a Feshbach resonance involves the formation of a Feshbach molecule which shares a spin state with the open channel. In such an arrangement, the mechanism of condensate formation cannot escape the effects of Pauli exclusion. Building on a model three-state fermion Hamiltonian recently introduced by Parish et al. to describe the

^{40}\mathrm{K}

system, we elucidate the mean-field structure of the model, investigate the internal structure of the condensate wave function in the crossover region, and develop a field theory of the transition.

{E}_{F}

is the Fermi energy of the gas in the absence of interactions and $\ensuremath{\eta}$ is the Zeeman energy difference between the two channels. We also consider the related corrections to the fermionic excitation spectrum, and briefly discuss the collective modes of the system.

{T}_{c}

and non-BCS characteristics below. We discuss how these effects influence the experimental signatures of superfluidity.