Controllable friction of dark solitons in Bose-Fermi mixtures

We study controllable friction in a system consisting of a dark soliton in a one-dimensional Bose gas and a non-interacting, degenerate Fermi gas. The fermions act as impurity atoms, not part of the original condensate, that scatter off of the soliton. We study semi-classical dynamics of the dark soliton by treating it as a particle with negative mass, and calculate its friction coefficient. Surprisingly, the amount of friction depends on the ratio of interspecies (impurity-condensate) to intraspecies (condensate-condensate) interaction strengths. By tuning this ratio, one can access a regime where the friction coefficient vanishes. We compare our results to experimental regimes and conclude that tunable friction has measurable physical consquences in experiments with Bose-Fermi mixtures.