Matter-wave solitons in the counterflow of two immiscible superfluids

TL;DR

This paper investigates the formation and dynamics of matter-wave solitons in a binary Bose-Einstein condensate with immiscible components under counterflow conditions, revealing different soliton types and their analytical descriptions.

Contribution

It introduces analytical models for soliton formation in immiscible superfluids, including a reduction to the Mel'nikov system and an effective potential approach for high velocities.

Findings

Dark-bright solitons form at intermediate velocities

Breakdown of superfluidity leads to dark soliton trains

Analytical models accurately predict critical velocities

Abstract

We study formation of solitons induced by counterflows of immiscible superfluids. Our setting is based on a quasi-one-dimensional binary Bose-Einstein condensate (BEC), composed of two immiscible components with large and small numbers of atoms in them. Assuming that the "small" component moves with constant velocity, either by itself, or being dragged by a moving trap, and intrudes into the "large" counterpart, the following results are obtained. Depending on the velocity, and on whether the small component moves in the absence or in the presence of the trap, two-component dark-bright solitons, scalar dark solitons, or multiple dark solitons may emerge, the latter outcome taking place due to breakdown of the superfluidity. We present two sets of analytical results to describe this phenomenology. In an intermediate velocity regime, where dark-bright solitons form, a reduction of the two-component Gross-Pitaevskii system to an integrable Mel'nikov system is developed, demonstrating that solitary waves of the former are very accurately described by analytically available solitons of the latter. In the high-velocity regime, where the breakdown of the superfluidity induces the formation of dark solitons and multi-soliton trains, an effective single-component description, in which a strongly localized wave packet of the "small" component acts as an effective potential for the "large" one, allows us to estimate the critical velocity beyond which the coherent structures emerge in good agreement with the numerical results.