Bose-Einstein condensation ofSr88through sympathetic cooling withSr87

Abstract

We report Bose-Einstein condensation of 88 Sr, which has a small, negative s-wave scattering length (a 88 = −2a 0 ). We overcome the poor evaporative cooling characteristics of this isotope by sympathetic cooling with 87 Sr atoms. 87 Sr is effective in this role despite the fact that it is a fermion because of the large ground-state degeneracy arising from a nuclear spin of I = 9/2, which reduces the impact of Pauli blocking of collisions. We observe a limited number of atoms in the condensate (N max ≈ 10 4 ) that is consistent with the value of a 88 and the optical dipole trap parameters.

Collisional properties of cold spin-polarized nitrogen gas: Theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules

“…We also observe trapping of the bosonic isotope 15 N by using isotopically enriched (>98%+) 15 N 2 as the process gas. No differences in trap loss were observed between 15 N and 14 N at a trap temperature of 600 mK.…”

“…At temperatures below 5 mK, inelastic collisions occur in the Wigner s-wave regime, and the rate constants for dipolar relaxation tend to zero for 14 N and approach a constant value of 5.5 × 10 −13 cm 3 /s for 15 N. The ratios of the rate constants for elastic scattering and dipolar relaxation displayed in Fig. 10(b) that because the elastic cross section for 14 N becomes very small at T < 1 mK, 15 N is a more promising candidate for cooling molecules to temperatures below 1 mK, whereas both 14 N and 15 N isotopes appear suitable for sympathetic cooling to temperatures above 1 mK. Figure 11 shows the temperature dependence of stateto-state rate constants for dipolar relaxation in 14 |M S B = 3/2 → |M S A = 1/2 |M S B = 3/2 dominates over the whole range of magnetic fields at both 0.1 and 0.6 K, and the double spin-flip transition |M S A = 3/2 |M S B = 3/2 → |M S A = 1/2 |M S B = 1/2 is the next most efficient.…”

“…10). Thus, neither 14 N nor 15 N appears suitable for sympathetic cooling of molecules below 1 mK. It might be possible to further reduce the temperature of trapped molecules via evaporative cooling at low magnetic fields [69] once N atoms are removed from the trap.…”

“…In the limit of vanishing collision energy, the variation of the cross sections with E C is determined by the Wigner threshold law; the cross sections for elastic scattering vary as as E C tends to zero. The situation for the bosonic isotope 15 N is the opposite: the inelastic cross section diverges and the elastic cross section approaches a constant. At collision energies above ∼0.01 cm −1 , partial waves with > 1 begin to factor in, altering the dependence of the cross sections of collision energy, and leading to the appearance of shape resonances.…”

“…This may be accomplished via sympathetic cooling, a technique based on collisional equilibration of thermal energy, which takes place when a gas of molecules is brought into thermal contact with a cold reservoir of atoms. Because sympathetic cooling is driven by elastic collisions, it is a truly general technique, which has found numerous applications * tshcherb@cfa.harvard.edu in cold atom physics [13][14][15]. Most of the sympathetic cooling experiments performed so far used alkali-metal atoms (typically 87 Rb [13,14]) because of their easy availability via laser cooling and their attractive collisional properties, which allow for sustainable evaporative cooling down to quantum degeneracy.…”