Classical analogies for the force acting on an impurity in a Bose-Einstein condensate

Rønning, Jonas; Skaugen, Audun; Hernández-García, Emilio; López, Cristóbal; Angheluta, Luiza
Submitted (2020)

We study the hydrodynamic forces acting on a finite-size impurity moving in a two-dimensional Bose-Einstein condensate at non-zero temperature. The condensate is modeled by the damped-Gross Pitaevskii (dGPE) equation and the impurity by a Gaussian repulsive potential giving the coupling to the condensate. The width of the Gaussian potential is equal to the coherence length, thus the impurity can only emit waves. Using linear perturbation analysis, we obtain analytical expressions corresponding to different hydrodynamic regimes which are then compared with direct numerical simulations of the dGPE equation and with the corresponding expressions for classical forces. For a non-steady flow, the impurity experiences a time-dependent force that, for small coupling, is dominated by the inertial effects from the condensate and can be expressed in terms of the local material derivative of the fluid velocity, in direct correspondence with the Maxey and Riley theory for the motion of a solid particle in a classical fluid. In the steady-state regime, the force is dominated by a self-induced drag. Unlike at zero temperature, where the drag force vanishes below a critical velocity, at finite temperatures, the drag force has a net contribution from the energy dissipated in the condensate through the thermal drag at all velocities of the impurity. At low velocities this term is similar to the Stokes' drag in classical fluids. There is still a critical velocity above which the main drag pertains to energy dissipation by acoustic emissions. Above this speed, the drag behaves non-monotonically with impurity speed, reflecting the reorganization of fringes and wake around the particle.

Movies (density of the condensate in the situation described in Sect. IV) available at https://cloud.ifisc.uib-csic.es/nextcloud/index.php/s/AFzw6JxNW77DT6d


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