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arXiv:2508.21778 · Physical Review A

Magnetic soliton molecules in binary condensates

R. M. V. Röhrs, Chunlei Qu, R. N. Bisset

Published Nov 13, 2025• 2 citations
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Abstract

Two-component Bose-Einstein condensates in the miscible phase can support polarization solitary waves, known as magnetic solitons. By calculating the interaction potential between two magnetic solitons, we elucidate the mechanisms and conditions for the formation of bound states—or —and support these predictions with dynamical simulations. We analytically determine the of bound states consisting of two oppositely polarized solitons and find good agreement with full numerical simulations. Collisions between bound states—either with other bound states or with individual solitons—produce intriguing dynamics. Notably, collisions between a pair of bound states exhibit a dipole-like behavior. We anticipate that such bound states, along with their rich collision dynamics, are within reach of current experimental capabilities.

DOI
10.1103/863l-45kn

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Two-component Bose-Einstein condensates in the miscible phase can support polarization solitary waves, known as magnetic solitons. By calculating the interaction potential between two magnetic solitons, we elucidate the mechanisms and conditions for the formation of bound states—or —and support these predictions with dynamical simulations. We analytically determine the of bound states consisting of two oppositely polarized solitons and find good agreement with full numerical simulations. Collisions between bound states—either with other bound states or with individual solitons—produce intriguing dynamics. Notably, collisions between a pair of bound states exhibit a dipole-like behavior. We anticipate that such bound states, along with their rich collision dynamics, are within reach of current experimental capabilities.
In this thesis, we investigate polarised solitary waves, known as magnetic solitons and dark-antidark solitons, in binary (two-component) Bose-Einstein condensates in the miscible phase. These solitons have been recently realised in experiments and are predicted to form bound states. We calculate the interaction potentials between magnetic solitons with the same and opposite polarisation, uncovering the mechanisms underlying bound state formation. By considering two superimposed counter-propagating magnetic solitons with opposite polarisation, we analytically calculate the dissociation energy of the bound states and find good agreement with full numerical simulations. Furthermore, we show that the oscillation period of bound states depends on the maximum soliton separation. We subsequently extend these results to dark-antidark solitons. For binary dipolar Bose- Einstein condensates, we derive the dispersion relation of elementary excitations and find that, in the presence of a spin-roton, spin-density modulations form around dark-antidark solitons. These modulations lead to additional local maxima and minima in the inter-soliton potentials, enabling the formation of additional bound state regions. Additionally, we compare the collision characteristics of magnetic solitons in non-dipolar systems with dark-antidark solitons in dipolar–nondipolar condensate mixtures and explore the collisions between bound states in the non-dipolar case.
We experimentally investigate the dynamics of spin solitary waves (magnetic solitons) in a harmonically trapped, binary superfluid mixture. We measure the in situ density of each pseudospin component and their relative local phase via an interferometric technique we developed and as such, fully characterize the magnetic solitons while they undergo oscillatory motion in the trap. Magnetic solitons exhibit nondispersive, dissipationless longtime dynamics. By imprinting multiple magnetic solitons in our ultracold gas sample, we engineer binary collisions between solitons of either the same or opposite magnetization and map out their trajectories.
We study solitary waves of polarization (magnetic solitons) in a two-component Bose gas with slightly unequal repulsive intra- and interspin interactions. In experimentally relevant conditions we obtain an analytical solution which reveals that the width and the velocity of magnetic solitons are explicitly related to the spin healing length and the spin sound velocity of the Bose mixture, respectively. We calculate the profiles, the energy, and the effective mass of the solitons in the absence of external fields and investigate their oscillation in a harmonic trap where the oscillation period is calculated as a function of the oscillation amplitude. The stability of magnetic solitons in two dimensions and the conditions for their experimental observation are also briefly discussed.
The interaction between two bright solitons in a one-dimensional dipolar Bose-Einstein condensate (BEC) is investigated, with the aim of finding the regimes where they form a stable bound state, known as the soliton molecule. To study soliton interactions in BECs we employed a method similar to that used in experimental investigation of the interaction between solitons in optical fibers. The idea consists in creating two solitons at some spatial separation from each other at initial time t0{t}_{0}t0​ and then measuring the distance between them at a later time t1>t0{t}_{1}>{t}_{0}t1​>t0​. Depending on whether the distance between solitons has increased, decreased, or remained unchanged, compared to its initial value at t0{t}_{0}t0​, we conclude that the soliton interaction was repulsive, attractive, or neutral, respectively. We propose an experimentally viable method for estimating the binding energy of a soliton molecule, based on its dissociation at critical soliton velocity. Our theoretical analysis is based on the variational approach, which appears to be quite accurate in describing the properties of soliton molecules in dipolar BECs, as reflected in the good agreement between the analytical and the numerical results.
We study magnetic solitons, solitary waves of spin polarization (i.e., magnetization), in binary Bose-Einstein condensates in the presence of Rabi coupling. We show that the system exhibits two types of magnetic solitons, called $2\ensuremath{\pi}$ and $0\ensuremath{\pi}$ solitons, characterized by a different behavior of the relative phase between the two spin components. $2\ensuremath{\pi}$ solitons exhibit a $2\ensuremath{\pi}$ jump of the relative phase, independent of their velocity, the static domain wall explored by Son and Stephanov being an example of such $2\ensuremath{\pi}$ solitons with vanishing velocity and magnetization. $0\ensuremath{\pi}$ solitons instead do not exhibit any asymptotic jump in the relative phase. Systematic results are provided for both types of solitons in uniform matter. Numerical calculations in the presence of a one-dimensional harmonic trap reveal that a $2\ensuremath{\pi}$ soliton evolves in time into a $0\ensuremath{\pi}$ soliton, and vice versa, oscillating around the center of the trap. Results for the effective mass, the Landau critical velocity, and the role of the transverse confinement are also discussed.
Spin mixing induced dynamics of spinor solitons in F=1F=1F=1 Bose Einstein condensates
We explore soliton interactions in a homogeneous spinor F=1F=1F=1 Bose Einstein Condensate (BEC) in the presence of a magnetic field, focusing on dark bright dark and bright dark bright configurations. We investigate how these interactions depend on the phase differences among bright solitons and their influence during the dynamics. Our findings align with prior non spinor results, i.e., repulsion among in phase bright solitons and attraction among out of phase pairs in self repulsive atomic BECs. The potential bright soliton attraction, added to the short range repulsion of dark dark soliton interactions, can lead to bound states. However, we find that these bound states break in the presence of spinor interactions due to the particle exchange dynamics between the hyperfine states of the components. Additonally, we develop an effective classical model to describe the soliton dynamics, using a Lagrangian approach. The accuracy of the model is tested by comparing it against numerical simulations. Our results suggest that the proposed model captures the essential features of soliton behavior in the presence of spin interactions, and provides congruent soliton trajectories and interspecies particle exchange dynamics in most of the cases.