Period-doubling Instability of Bose-Einstein Condensates Induced in   Periodically Translated Optical Lattices
  We observe the formation of momentum distributions indicative of spatial period-doubling of superfluid Bose-Einstein condensates in periodically translated optical lattices. The effect is attributed to dynamic instability of the condensate wavefunction caused by modulation-induced coupling of ground and excited bands. 
Nathan Gemelke, Edina Sarajlic, Yannick Bidel, Seokchan Hong, and   Steven Chu