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arXiv:0812.4335 · Physical Review A

Direct evaporative cooling of K41 into a Bose-Einstein condensate

T. Kishimoto, Jun Kobayashi, K. Noda, K. Aikawa, Masahito Ueda, S. Inouye

Published Mar 30, 2009• 29 citations
arXivHTMLPDF

Abstract

We have investigated the collisional properties of $^{41}\text{K}$ atoms at ultracold temperatures. To demonstrate the possibility of using $^{41}\text{K}$ as a coolant, a Bose-Einstein condensate of $^{41}\text{K}$ atoms in the stretched state $(F=2,{m}_{F}=2)$ was created by direct evaporative cooling in a magnetic trap. An upper bound of the three-body loss coefficient for atoms in the condensate was determined to be $4(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}29}\text{ }{\text{cm}}^{\ensuremath{-}6}\text{ }{\text{s}}^{\ensuremath{-}1}$. A Feshbach resonance in the $F=1,\text{ }{m}_{F}=\ensuremath{-}1$, state was observed at 51.35(10) G, which is in good agreement with theoretical prediction.

DOI
10.1103/physreva.79.031602

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We report on experiments with ultracold chromium atoms. Preparing a cloud of 52Cr atoms in a crossed optical dipole trap (CODT) and applying magnetic fields between 4 and 600 G, we observe 14 Feshbach resonances by the occurrence of increased atom loss at distinct magnetic field values. A comparison with theory taking only dipole–dipole coupling into account shows very good agreement between experimental and theoretical resonance positions and allows us to extract the s-wave scattering lengths a 6=112(14) a 0, a 4= 58(6) a 0, a 2=−7(20) a 0 of the involved molecular potentials as well as the dispersion coefficients C 6= 733(70) au and au. The strongest resonance at 589 G has a calculated width of 1.7 G and reveals a three-body loss coefficient below L 3, max∼3×10−36 m6 s−1. Further evaporative cooling within the CODT leads to the formation of a Bose–Einstein condensate (BEC) with up to 100 000 condensed atoms. The magnetic dipole–dipole interaction between the Cr atoms is so strong that, as a first mechanical manifestation of dipole–dipole interaction, we observe a modification of the condensate expansion which depends on the alignment of the atomic magnetic dipoles with respect to the axis of the CODT. This magnetostrictive effect is in very good agreement with the theory of dipolar quantum gases and shows that a Cr-BEC is an excellent model system to study dipole–dipole interactions in degenerate quantum gases.