Dynamics of an impurity in a finite temperature Bose-Einstein condensate

Superfluidity of zero-temperature BECs is the exotic property of dissipationless flows below a critical velocity, also known as the Landau criterion. This implies that any flow obstacle or impurity will experience vanishing drag below this critical velocity. Energy dissipation kicks in at velocities above the critical one, due to the formation of elementary excitations. In this talk we consider the dissipation mechanism and inertial effects of a 2D BEC at finite temperature in the present of an impurity. Analytical expressions are obtained for the forces on the impurity in the limit of a weakly-coupling. These expressions are compared to the analogues classical hydrodynamical forces and verified by numerical simulations. For non-steady flows the force is time dependent and dominated by inertial effects, which is analogues to the inertial force that acts on solid particles in a classical fluid. For steady flows the force is dominated by a self-induced drag, which doesn’t vanish below a critical velocity. At low velocities this is caused by the energy dissipation through interactions of the condensate with the thermal cloud, and it is analogous to the classical Stokes’ drag. There still exists a critical velocity for which the drag is dominated by the acoustic excitations.