Evidence of Bose-Einstein Condensation in an Atomic Gas with Attractive Interactions

TLDR

For spin‑polarized ${}^{7}$Li, the negative s‑wave scattering length indicates attractive interactions, and Bose‑Einstein condensation was previously predicted to be impossible in such a system. The experiment used a permanent‑magnet trap to laser‑cool ${}^{7}$Li atoms to 200 µK, then evaporatively cooled them, and modeled probe‑laser diffraction as a sensitive indicator of a spatially localized condensate. Evidence of Bose‑Einstein condensation was found, with phase‑space densities of 100–400 nK, diffraction of a probe laser beam, and indications that most of the up to 2 × 10⁵ atoms reside in the condensate.

Abstract

Evidence for Bose-Einstein condensation of a gas of spin-polarized ${}^{7}$Li atoms is reported. Atoms confined to a permanent-magnet trap are laser cooled to 200 \ensuremath{\mu}K and are then evaporatively cooled to lower temperatures. Phase-space densities consistent with quantum degeneracy are measured for temperatures in the range of 100 to 400 nK. At these high phase-space densities, diffraction of a probe laser beam is observed. Modeling shows that this diffraction is a sensitive indicator of the presence of a spatially localized condensate. Although measurements of the number of condensate atoms have not been performed, the measured phase-space densities are consistent with a majority of the atoms being in the condensate, for total trap numbers as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms. For ${}^{7}$Li, the spin-triplet $s$-wave scattering length is known to be negative, corresponding to an attractive interatomic interaction. Previously, Bose-Einstein condensation was predicted not to occur in such a system.