Realization of BEC-BCS-crossover physics in a compact oven-loaded magneto-optic-trap apparatus

We report on a simple oven-loaded magneto-optic trap (MOT) apparatus for the creation of both molecular Bose-Einstein condensates (mBECs) and degenerate Fermi gases (DFGs) of lithium. The apparatus does not require a Zeeman slower or a two-dimensional MOT, nor does it require any separation or differential pumping between the effusive atom source and the science chamber. The result is an exceedingly simple, inexpensive, and compact vacuum system ideal for miniaturization. We discuss our work in the context of other realizations of quantum degenerate gases by evaporation in optical dipole traps and illustrate that our apparatus meets the key requirements of atom number and trap lifetime. With this system, we also demonstrate the production of a mBEC, and we use it to observe the pairing gap of a strongly interacting two-component DFG in the BEC-BCS crossover regime.

Figure 1

Atom number transferred into the $\mathrm{CODT}$ versus atom number in the MOT. Plotted are the initial atom number in the high power $\mathrm{CODT}$ at 200 W total power (circles), the atom number in the high power $\mathrm{CODT}$ after evaporation to 100 W total with 50 W per beam (squares), and the atom number transferred into the low power $\mathrm{CODT}$ (diamonds) for various initial MOT atom numbers.

Figure 2

Drawing of the vacuum system showing the effusive atomic source (oven), the metal shield used to prevent Li from coating the inside of the quartz cell, and the MOT position. The inset shows the 3D model of the oven.

Figure 3

The atom loading rate (diamonds), the MOT lifetime (circles), the ODT lifetime (triangles), and the steady-state atom number (squares) of the MOT are shown as a function of the oven temperature with parameter values as defined in Table . We typically operate the source in the shaded region (between 590 K and 625 K), where a good compromise is found between a sufficiently large MOT atom number and a long ODT trap lifetime. The trend lines serve only as a guide for the eye.

Figure 4

Geometry and position of the MOT beams, the high- and low-power ODT beams, and the lower magnetic coil relative to the glass cell. Not shown are the third MOT beam along the $z$ axis, the top magnetic coil, and the small “gradient” coil placed inside and concentric to it. The atomic source is attached to the feedthrough on the right side of the image (see Fig. 2). For reference, gravity points in the $-\widehat{z}$ direction and the $x$-$y$ plane is parallel to the optical table surface.

Figure 5

Ensemble temperature (diamonds) and particle number (circles) as a function of the $\mathrm{CODT}$ laser power (i.e., trap depth) in the final forced evaporation stage at (a) $B=800$ G (below the FR) and (b) $B=900$ G (above the FR). The number of “spin-up” atoms (atoms in the $|1\rangle$ state) is detected at these high magnetic fields by absorption imaging. For the evaporation below the FR, when the temperature is well below the binding energy all of the atoms form FR molecules and the number detected in the $|1\rangle$ state corresponds to the total FR molecule number. The solid line indicates the expected particle number for an efficient evaporative cooling. The dashed line indicates the Fermi temperature for the $|1\rangle$ component at each power computed from the particle number and the trap frequencies. In (a) the dotted line shows the critical temperature for BEC for the molecules.

Figure 6

Absorption images and resulting horizontal density profiles of ${}^6$Li${}_2$ Feshbach molecules after a 3-ms TOF expansion at a magnetic field of $B=690\text{G}$ for different ensemble temperatures: (a) $T=710$ nK ($T/T_{\mathrm{c}}\sim 1.2$), (b) $T=230$ nK ($T/T_{\mathrm{c}}\sim 0.9$), (c) $T=110$ nK ($T/T_{\mathrm{c}}\sim 0.8$), (d) $T=65$ nK ($T/T_{\mathrm{c}}\sim 0.6$). The wings of the profiles are fit to a Gaussian function and the bimodal character of the density profile is evident for temperatures well below the critical temperature. For the coldest molecular cloud (d) we show in (e) the absorption images after different free expansion times. The in situ shape of the molecular cloud (image taken at 0 ms) has the anisotropic shape of the $\mathrm{CODT}$, and the cloud anisotropy reverses during the TOF expansion.

Figure 7

Radio frequency (rf) spectroscopy of a strongly interacting two-component Fermi gas at $B=839.2$ G. The number of atoms lost from the $|2\rangle$ state is plotted after the application of an rf field as a function of its frequency offset from the unpaired atom resonance. In (a) and (b), where the temperature is above the pair dissociation temperature, $T^{*}$, the rf spectrum shows a single resonance. This corresponds to the energy required to flip the spin of an unpaired atom. In (c) and (d), the ensemble temperature is $T<T^{*}$, and a second maximum appears at higher energy due to pair formation. Here the temperature is slightly above the critical temperature for pair condensation, and these so-called preformed pairs exist in what is known as the “pseudogap” region [56, 57]. In (d) where the temperature is lower than in (c), the ratio of pairs to free atoms is larger, as expected; however, the gap energy is less because the Fermi temperature in (d) is smaller than in (c). The lines show a fit of the data to two Lorentzians and are meant only as a guide for the eye.