Superfluidity of Bose—Einstein condensates in ultracold atomic gases*
Zhu Qi-Zhonga), Wu Biaoa),b),c)†
       
(a) A macroscopic obstacle moves with a velocity of v inside a superfluid residing in a periodic potential. The superfluid and periodic potential are “locked” together and there is no relative motion between them. (b) The lattice where a superfluid resides is slowly accelerated to a velocity of v .