Collision of impurities with Bose-Einstein condensates

TL;DR

This paper uses numerical simulations to study how a bosonic impurity interacts with a Bose-Einstein condensate in a quasi-one-dimensional setup, revealing different behaviors like oscillation, trapping, and soliton formation depending on interaction strength.

Contribution

It provides detailed numerical analysis of impurity-BEC collisions, highlighting new phenomena such as impurity trapping and soliton creation in a quasi-1D system.

Findings

Impurity oscillates around BEC for weak interactions.

Impurity gets trapped and alters BEC density at intermediate couplings.

Strong interactions lead to soliton formation and reflection phenomena.

Abstract

Quantum dynamics of impurities in a bath of bosons is a long-standing problem of solid-state, plasma, and atomic physics. Recent experimental and theoretical investigations with ultracold atoms focused on this problem, studying atomic impurities immersed in a atomic Bose-Einstein condensate (BEC) and for various relative coupling strengths tuned by the Fano-Feshbach resonance technique. Here we report extensive numerical simulations on a closely related problem: the collision between a bosonic impurity made of few $^{41}$K atoms and a BEC made of $^{87}$Rb atoms in a quasi one-dimensional configuration and under a weak harmonic axial confinement. For small values of the interspecies interaction strength (no matter the sign of it), we find that the impurity, which starts from outside the BEC, simply oscillates back and forth the BEC cloud, but the frequency of oscillation depends on the interaction strength. For intermediate couplings, after a few cycles of oscillation the impurity is captured by the BEC and strongly changes its amplitude of oscillation. In the strong interaction regime, if the interspecies interaction is attractive, a local maximum (bright soliton) in the density of BEC occurs where the impurity is trapped; instead, if the interspecies interaction is repulsive, the impurity is not able to enter in the BEC cloud and the reflection coefficient is close to one. On the other hand, if the initial displacement of the impurity is increased, the impurity is able to penetrate in the cloud leading to the appearance of a moving hole (dark soliton) in the BEC.