Beyond superfluidity in driven non-equilibrium Bose-Einstein condensates

The phenomenon of superfluidity in open Bose-Einstein condensates (BEC) is analysed numerically and analytically. It is found that a superfluid phase is feasible even above the speed of sound, when forces due to inhomogeneous non-equilibrium processes oppose the contributions of homogeneous processes. Furthermore a regime of accelerating impurities can be observed for particular pumping/decay strategies. All findings are derived within the complex Gross-Pitaevskii (GP) theory with creation and annihilation terms. Utilising this framework the effective force acting on an impurity as it moves with velocity v through the open condensate can be calculated. The result shows that the force is continuously increasing/decreasing with increasing velocity starting from the state of zero motion at v = 0, a property that can be traced down to the additional homogeneous annihilation/creation term in the extended GP model. Our findings stand in stark contrast to the concept of a topological phase transition to frictionless flow below a critical velocity as observed for equilibrium Bose-Einstein condensates analytically, numerically and for trapped atoms experimentally.