equations for the linear stability. A good agreement is found between them.'
author:
- 'I. Morera'
- 'A. Muñoz Mateo'
- 'A. Polls'
- 'B. Juliá-Díaz'
title: 'Dark-dark-soliton dynamics in two density-coupled Bose-Einstein condensates'
---

Introduction
============

Over the last two decades, Bose-Einstein condensation (BEC) has enabled the study of numerous physical concepts [@Pitaevskii2003; @Pethick2008]. Among them, an interesting scenario is the connection between the non-linear waves and atomic systems, that leads to the so-called matter-wave solitons [@Kevrekidis2008; @Kevrekidis2015]. These structures emerge from the balance between linear dispersion and interactions, which are accounted for from non-linear terms in the equations of motion. Specifically, mean field descriptions of BECs, as provided by the Gross-Pitaevskii equation (GP), incorporate a non-linear term proportional to the interatomic interaction strength $g$, which is measured by the s-wave scattering length $a$. Depending on the sign and magnitude of the latter and the dimensionality of the system, a large number of structures can be found: dark [@Frantzeskakis2010] and bright [@Strecker2003] solitons, vortices [@Fetter2001], etc.

Dark solitons in BECs are localized non-linear excitations that present a notch in the condensate density and a phase step across its center. They have been observed in numerous experiments using different techniques 