Critical Velocities for Energy Dissipation from Periodic Motions of   Impurity in Bose-Einstein Condensates
  A phenomenon of energy dissipation in Bose-Einstein condensates is studied based on a microscopic model for the motion of impurity. Critical velocities for onset of energy dissipation are obtained for periodic motions, such as a dipole-like oscillation and a circular motion. The first example is similar to a series of MIT group experiments settings where the critical velocity was observed much below the Landau critical velocity. The appearance of the smaller values for the critical velocity is also found in our model, even in the homogeneous condensate in the thermodynamic limit. This suggests that the landau criterion be reexamined in the absence of quantized vortices in the bulk limit. 
Jun Suzuki