Bose-Einstein Condensation of Erbium

We report on the achievement of Bose-Einstein condensation of erbium atoms and on the observation of magnetic Feshbach resonances at low magnetic fields. By means of evaporative cooling in an optical dipole trap, we produce pure condensates of ${}^{168}\mathrm{Er}$, containing up to $7\times {10}^4$ atoms. Feshbach spectroscopy reveals an extraordinary rich loss spectrum with six loss resonances already in a narrow magnetic-field range up to 3 G. Finally, we demonstrate the application of a low-field Feshbach resonance to produce a tunable dipolar Bose-Einstein condensate and we observe its characteristic $d$-wave collapse.

Figure 1

Absorption images and integrated density profiles showing the BEC phase transition for different evaporation times. The absorption images are an average of four images taken after 24 ms of expansion. The color bar shows the optical density. The solid lines are fits to the data using Gaussian (a), bimodal (b) and (c), and Thomas-Fermi (d) distribution. The dotted lines represent the Gaussian part of the bimodal fit, describing the thermal atoms. From the fit we extract: $N=3.9\times {10}^5$, $T=1100\mathrm{nK}$ (a), $N=2.1\times {10}^5$, $T=408\mathrm{nK}$ (b), $N=1.6\times {10}^5$, $T=222\mathrm{nK}$ (c), $N=6.8\times {10}^4$ (d), where $N$ is the total atom number. For (b) and (c), we extract a condensate fraction of 5% and 20%, respectively.

Figure 2

Observation of Feshbach resonances in Er-Er collisions. The measured temperature (a) and atom number (b) are plotted as a function of the magnetic field. The minima in the atom number indicate the Feshbach resonance poles, marked by the thin vertical lines. The maxima in the temperatures to the right of the three stronger loss features (arrows) are attributed to the respective zero crossings of the scattering length. The varying background in the atom number is presumably due to ramping effects caused by the sweep of the magnetic field across the resonances.

Figure 3

Absorption images showing the $d$-wave collapse of the BEC. The field of view is $290\mu \mathrm{m}\times 290\mu \mathrm{m}$. The images are an average of eight pictures. The color bar shows the optical density. The images are taken for different target values of the magnetic field: 1.208 G (a), 0.963 G (b), 0.947 (c), 0.942 G (d), 0.939 G (e), and 0.934 (f). We note that the actual magnetic-field value might be slightly higher (10 mG) because of ramping issues and eddy currents [7].