Double-degenerate Bose-Fermi mixture of strontium

We report on the attainment of a spin-polarized Fermi sea of ${}^{87}\mathrm{Sr}$ in thermal contact with a Bose-Einstein condensate (BEC) of ${}^{84}\mathrm{Sr}$. Interisotope collisions thermalize the fermions with the bosons during evaporative cooling. A degeneracy with $T/T_F=0.30(5)$ is reached for a ${}^{87}\mathrm{Sr}$ Fermi sea of $2\times {10}^4$ atoms together with an almost pure ${}^{84}\mathrm{Sr}$ BEC of ${10}^5$ atoms.

Figure 1

Optical pumping and state detection of ${}^{87}\mathrm{Sr}$. (a) Internal states and transitions of ${}^{87}\mathrm{Sr}$ used for optical pumping and detection of spin polarization. The optical pumping beam (dashed arrow) is scanned in frequency to address all $m_F$ states. Internal state detection is performed by levitating only the $m_F=9/2$ state atoms against gravity using a ${\sigma }^{+}$-polarized laser beam on a cycling transition (solid arrow). (b) Levitated $m_F=9/2$ state atoms and atoms in all other $m_F$ states, 15 ms after switching off the dipole trap.

Figure 2

Temperature and atom number as a function of the trap depth during evaporative cooling. (top) Temperature of ${}^{84}\mathrm{Sr}$ (black circles) and temperature (red triangles) and Fermi temperature (blue squares) of ${}^{87}\mathrm{Sr}$. (bottom) Atom numbers of ${}^{84}\mathrm{Sr}$ (black circles) and ${}^{87}\mathrm{Sr}$ (red triangles). A BEC forms below a trap depth of 2.8 $\mu$K (arrow).

Figure 3

Azimuthally averaged density distributions of a degenerate Fermi gas at $T/T_F=0.3$ and a thermal fermionic cloud at $T/T_F=1$ (inset) after 10 ms of free expansion. Fermi-Dirac distributions (solid red lines) fit both measurements well. The quantum degenerate sample deviates in shape from a Gaussian (dotted blue line). The difference between the degenerate and thermal samples becomes even more evident when fitting a Gaussian to only the outer wings, outside the disk with radius $\sqrt{2}w$ (dashed line), where $w$ is the $1/e$ width of a Gaussian fit to the full distribution. For $T/T_F=1$ (inset), the difference between the three fits is barely visible.

Figure 4

$T/T_F$ as a function of the ${}^{87}\mathrm{Sr}$ atom number during evaporative cooling. $T/T_F$ is determined from Fermi fits to absorption images. Two independent fit parameters are used to calculate $T/T_F$: the width of the fitted function (filled circles) and the fugacity (triangles). The statistical error visible in the scatter of the data is larger than systematic uncertainties.

Figure 5

Time-of-flight absorption images of a nearly pure ${}^{84}\mathrm{Sr}$ BEC and a ${}^{87}\mathrm{Sr}$ Fermi gas at $T/T_F=0.45$ after 15 ms of expansion.