Dynamics of finite momentum Bose polarons

Abstract

We study the behavior of a finite-momentum impurity immersed in a weakly interacting Bose-Einstein condensate (BEC) of ultra-cold atoms near an interspecies Feshbach resonance. Using the time-dependent variational approach, we study both ground state properties and quench dynamics of the system after a sudden immersion of the impurity into the BEC. We find evidence of a ground state phase transition when the impurity has a velocity greater than that of the sound velocity (Landau critical velocity) associated with Bogoliubov quasiparticle excitations of the BEC. As we cross from the subsonic regime to the supersonic regime, we get a breakdown of the polaron quasiparticle description of the system, emission of Cherenkov phonons, and a sound-like dispersion of the system. This phase transition manifests in several ways during real-time dynamics of the system and showcases a rich interplay between polaronic physics and Cherenkov physics. One key signature, dissipation in the the supersonic regime, can be seen in experimental protocols where the impurity and BEC are made to move relative to each other through an external force. We suggest one such experimental protocol to measure the polaron's effective mass as long as the impurities are subsonic. While the measurement scheme becomes error-prone due to dissipation when the impurities are allowed to become supersonic, this sensitivity suggests a way to experimentally probe the Cherenkov physics of supersonic impurities immersed in a BEC.