Single-particle excitations in a trapped gas of Fermi atoms in the BCS-BEC crossover region

Abstract

We investigate the single-particle properties at T=0 of a trapped superfluid gas of Fermi atoms with a Feshbach resonance. A tunable pairing interaction associated with the Feshbach resonance leads to the BCS-BEC crossover, where the character of superfluidity continuously changes from the BCS-type to a BEC of composite bosons. In this paper, we extend our previous work for a uniform superfluid Fermi gas [Y. Ohashi and A. Griffin, Phys. Rev. A {\bf 67}, 063612 (2003)] to include the effect of a harmonic trap. We directly solve the Bogoliubov-de Gennes coupled equations, and find self-consistent values for the spatially-dependent local density $n({\bf r})$ as well as the composite BCS order parameter ${\tilde \Delta}({\bf r})$. Using these results, we calculate the single-particle density of states in the crossover region, and from this determine the true single-particle energy gap ($E_{\rm g}$) of the trapped Fermi superfluid at T=0. This is associated with the in-gap (or Andreev) states in the low density region at the edge of the trap. We calculate the laser-induced current $I(\omega)$, as measured in recent rf-spectroscopy experiments. We show how the high-energy part of $I(\omega)$ gives information about ${\tilde \Delta}(r=0)$ at the center of the trap. We emphasize that the narrow "unpaired atom" peak in the rf-data gives information about $E_g$ and the low-energy in-gap states of a Fermi superfluid. While our calculations are limited at T=0, we use them to discuss the recent Innsbruck data and the LDA calculations of T\"orm\"a and co-workers.