Exotic vortex lattices in two-species Bose-Einstein condensates
  We numerically investigate vortex lattices in rotating two-component Bose-Einstein condensates in which the two components have unequal atomic masses and interact attractively with each other. For sufficiently strong attraction, the system is found to exhibit exotic ground-state structures in a harmonic trap, such as lattices having a square geometry or consisting of two-quantum vortices. The obtained states satisfy the Feynman relation, and they can be realized with current experimental techniques. 
Pekko Kuopanportti, Jukka A. M. Huhtam\"aki, and Mikko M\"ott\"onen
cond-mat.quant-gas