Excitations in the BCS-BEC crossover regime of a superfluid Fermi gas with a Feshbach resonance

We investigate single-particle excitations at T = 0 in the BCS-BEC crossover regime of a superfluid Fermi gas. We solve the Bogoliubov-de Gennes equations in a trap, including a tunable pairing interaction associated with a Feshbach resonance. We show that the single-particle energy gap E-g is dominated by the lowest-energy Andreev bound state localized at the surface of the gas, which is much smaller than the superfluid order parameter Delta(r = 0) at the center of the trap. We also calculate the rf-tunneling current spectrum I-F(omega). In a broad Feshbach resonance, the peak energy in I-F(omega) is shown to give, not E-g, but Delta(r = 0). In a narrow resonance, significant spectral weight coming from the low-energy Andreev states hides the appearance of the peak in I-F(omega) at omega similar or equal to Delta(r = 0). Our results for a broad resonance is in good agreement with the recent rf-tunneling measurement in a superfluid Li-6. (c) 2006 Elsevier B.V. All rights reserved.