Bragg Spectroscopy of a Strongly Interacting Fermi Gas

We present a comprehensive study of the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer (BEC-BCS) crossover in fermionic ${}^6\mathrm{Li}$ using Bragg spectroscopy. A smooth transition from molecular to atomic spectra is observed with a clear signature of pairing at and above unitarity. These spectra probe the dynamic and static structure factors of the gas and provide a direct link to two-body correlations. We have characterized these correlations and measured their density dependence across the broad Feshbach resonance at 834 G.

Figure 1

(a) Schematic of Bragg scattering experiments. Bragg scattering of molecules from (b) a pre-expanded molecular BEC and (c) a trapped molecular BEC, both at 750 G. The field of view for images (b) and (c) is $650\mu \mathrm{m}$ by $485\mu \mathrm{m}$. Elastic collisions between scattered and unscattered particles distort the resultant cloud in the trapped case.

Figure 2

Bragg spectra showing $X(q,\delta )$ for trapped Fermi gases across the BEC-BCS crossover. Magnetic field and ($1/k_{F}a$) for each spectrum are given in the legend. The inset shows the 750 and 991 G spectra along with the calculated $X(q,\delta )$ for an ideal Fermi gas and molecular BEC at 750 G.

Figure 3

Experimental static structure factor, $S_{\mathrm{expt}}(q=5k_F)$, as a function of $1/k_{F}a$ and magnetic field (top).

Figure 4

Response of trapped gases to a Bragg pulse resonant with pairs ($\delta /2\pi =145\mathrm{kHz}$) as a function of cloud density. Magnetic field values and ($1/k_{F}a$) are listed in the legend.