Bose-Einstein Condensation of Chromium

We report on the generation of a Bose-Einstein condensate in a gas of chromium atoms, which have an exceptionally large magnetic dipole moment and therefore underlie anisotropic long-range interactions. The preparation of the chromium condensate requires novel cooling strategies that are adapted to its special electronic and magnetic properties. The final step to reach quantum degeneracy is forced evaporative cooling of ${}^{52}\mathrm{Cr}$ atoms within a crossed optical dipole trap. At a critical temperature of $T_c\approx 700\mathrm{nK}$, we observe Bose-Einstein condensation by the appearance of a two-component velocity distribution. We are able to produce almost pure condensates with more than 50 000 condensed ${}^{52}\mathrm{Cr}$ atoms.

Figure 1

Comparison of trap lifetimes before (crosses) and after (circles) pumping the atoms to the lowest Zeeman substate. The inset shows the change of the axial size of the expanded cloud in time of flight after different holding times.

Figure 2

Density profiles from absorption images of atom clouds taken after 5 ms of ballistic expansion: (a) thermal cloud at $1.1\mu \mathrm{K}$; (b) two-component distribution at 625 nK, slightly below $T_C$; (c) nearly pure condensate with 47 000 atoms.

Figure 3

Time of flight series of absorption images with expansion times from 1 to 9 ms. (a) BEC released from an almost isotropic trap; (b) BEC released from an anisotropic trap.

Figure 4

Condensate fraction ($N_0/N$) dependence on temperature relative to the transition temperature of an ideal gas ($T/T_C^0$) given by Eq. (1). Triangles represent our measured data. Black circles represent the predicted fraction following Eq. (3) including corrections due to finite size effects and the contact interaction The dashed curve shows the dependence for the ideal gas. Error bars are mainly due to uncertainties in the measurement of the trap frequencies, the temperature, and the number of atoms.