Beyond superfluidity in 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 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 extended to include 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 drag force is continuously increasing 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. For very large velocities however we observe a reversion of the drag force. 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 BEC analytically (Astrakharchik and Pitaevskii 2004 Phys. Rev. A70 013608; Pinsker 2017 Physica B 521 36-42), numerically (Winiecki et al 1999 Phys. Rev. Lett. 82 26) and for trapped atoms experimentally (Desbuquois et al 2012 Nat. Phys. 8 645; Zwierlein et al 2006 Nature 442 54-58).