An ultracold, optically trapped mixture of 87Rb and metastable 4He atoms Adonis Silva Flores, Hari Prasad Mishra, Wim Vassen and Steven Knoop Eur. Phys. J. D, 71 3 (2017) 49 Published online: 02 March 2017 DOI: 10.1140/epjd/e2017-70675-y