Matthews M.R., D.S. Hall, D.S. Jin, J.R. Ensher, C.E. Wieman, E.A. Cornell, F. Dalfovo, C. Minniti, and S. Stringari, Physical Review Letters 81, 243-247 (1998).
Obrecht J.M., R.J. Wild, M. Antezza, L. Pitaevskii, S. Stringari, and E.A. Cornell, Physical Review Letters 98, (2007).
and 
and 