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arXiv:1809.08491 · Journal of Physics B Atomic Molecular and Optical Physics

Modulation instability associated nonlinear dynamics of spin–orbit coupled Bose–Einstein condensates *

Thudiyangal Mithun, Kenichi Kasamatsu

Published Jan 3, 2019• 36 citations
arXivHTMLPDF

Abstract

Abstract We study pattern-forming nonlinear dynamics starting from a continuous wave state of quasi-one-dimensional two-component Bose–Einstein condensates with synthetic spin–orbit coupling induced by Raman lasers. Modulation instability (MI) can occur even when the miscibility condition due to the interatomic interactions is satisfied. We find that the initial stage of the nonlinear development is consistent with the prediction of MI, where the two primary and secondary instability bands lead to the spontaneous growth of the modulation and the subsequent complicated dynamics of pattern formation. At later stages of the evolution, the wave functions undergo clear separation in the momentum space, reflected in the dispersion of the single particle Hamiltonian.

DOI
10.1088/1361-6455/aafbdd

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Abstract We study pattern-forming nonlinear dynamics starting from a continuous wave state of quasi-one-dimensional two-component Bose–Einstein condensates with synthetic spin–orbit coupling induced by Raman lasers. Modulation instability (MI) can occur even when the miscibility condition due to the interatomic interactions is satisfied. We find that the initial stage of the nonlinear development is consistent with the prediction of MI, where the two primary and secondary instability bands lead to the spontaneous growth of the modulation and the subsequent complicated dynamics of pattern formation. At later stages of the evolution, the wave functions undergo clear separation in the momentum space, reflected in the dispersion of the single particle Hamiltonian.
We theoretically investigate the dynamics of modulation instability (MI) in two-dimensional spin–orbit coupled Bose–Einstein condensates (BECs) with Rabi coupling. The analysis is performed for equal densities of pseudo-spin components. Different combination of the signs of intra- and intercomponent interaction strengths are considered, with a particular emphasis on repulsive interactions. We observe that the unstable modulation builds from originally miscible condensates, depending on the combination of the signs of the intra- and intercomponent interaction strengths. The repulsive intra- and intercomponent interactions admit instability and the MI immiscibility condition is no longer significant. The influence of interaction parameters such as spin–orbit and Rabi coupling on MI are also investigated. The spin–orbit coupling (SOC) inevitably contributes to instability regardless of the nature of the interaction. In the case of attractive interaction, SOC manifests in enhancing the MI. Thus, a comprehensive study of MI in two-dimensional spin–orbit coupled BECs of pseudo-spin components is presented.
We present a comprehensive study of modulational instability (MI) in a binary Bose-Einstein condensate with spin-orbit coupling, confined to a deep optical lattice. The system is modeled by a set of discrete Gross-Pitaevskii equations. Using linear stability analysis, we derive the explicit MI conditions for the system, elucidating the critical and distinct roles played by spin-orbit coupling, inter-species nonlinearity, and intra-species nonlinearity. Our analysis, conducted for both unstaggered and staggered fundamental modes, reveals markedly different instability landscapes for these two configurations. The analytical predictions are confirmed by extensive numerical simulations of the full nonlinear dynamics, which vividly illustrate the spatiotemporal evolution of wave amplitudes, phase coherence, and energy localization during the instability process. The numerical results, obtained via a fourth-order Runge-Kutta method, show excellent agreement with the linear stability theory and provide a complete picture of the MI-induced pattern formation.
Abstract We investigate the modulation instability (MI) of plane-wave states in a spin–orbit-coupled spin-1 Bose–Einstein condensate (BEC). The MI of the system can be adjusted by the intensities of the spin–orbit coupling (SOC), Raman coupling and atom–atom interactions. Our analysis shows that the MI of a system without the SOC and Raman coupling mainly occurs for repulsive density–density and spin–exchange interactions due to the combined effects coming from both of them. But when considering the SOC, a multi-peak structure appears in the momentum distribution for the instability, which may be thought of as a precursor to the generation of the stripe-type nonlinear excitations in such systems. The MI becomes more remarkable with increasing intensity of Raman coupling. In addition, the SOC inevitably contributes to the instability for the ternary superfluid regardless of the nature of the density–density and spin–exchange interactions.
We investigate nonlinear dynamics induced by the modulation instability of a two-component mixture in an atomic Bose-Einstein condensate. The nonlinear dynamics is examined using numerical simulations of the time-dependent coupled Gross-Pitaevskii equations. The unstable modulation grows from initially miscible condensates into various types of vector solitary waves, depending on the combinations of the sign of the coupling constants (intracomponent and intercomponent). We discuss the detailed features of the modulation instability, dynamics of solitary wave formation, and an analogy with the collapsing dynamics in a single-component condensate with attractive interactions.
The nonlinear dynamics induced by the modulation instability (MI) of a binary mixture in an atomic Bose-Einstein condensate (BEC) is investigated theoretically under the joint effects of higher-order residual nonlinearities and helicoidal spin-orbit (SO) coupling in a regime of unbalanced chemical potential. The analysis relies on a system of modified coupled Gross-Pitaevskii equations on which the linear stability analysis of plane-wave solutions is performed, from which an expression of the MI gain is obtained. A parametric analysis of regions of instability is carried out, where effects originating from the higher-order interactions and the helicoidal spin-orbit coupling are confronted under different combinations of the signs of the intra- and intercomponent interaction strengths. Direct numerical calculations on the generic model support our analytical predictions and show that the higher-order interspecies interaction and the SO coupling can balance each other suitably for stability to take place. Mainly, it is found that the residual nonlinearity preserves and reinforces the stability of miscible pairs of condensates with SO coupling. Additionally, when a miscible binary mixture of condensates with SO coupling is modulationally unstable, the presence of residual nonlinearity may help soften such instability. Our results finally suggest that MI-induced formation of stable solitons in mixtures of BECs with two-body attraction may be preserved by the residual nonlinearity even though the latter enhances the instability.
Abstract In this work, we investigate the modulational instability of plane wave solutions within a modified Gross–Pitaevskii equation framework. The equation features cubic and quartic nonlinearity. It models the behaviour of quasi-one-dimensional Bose–Einstein condensates in symmetric Bose-Bose mixtures of ultra-dilute cold atoms. The cases of equal amplitudes in both species, as well as of unequal amplitudes are studied. Our study demonstrates the pivotal role of the competition between mean-field attractions and quantum fluctuation-induced repulsions. This competition significantly affects the emergence and evolution of modulational instability. By employing linear stability analysis, we identify the essential conditions that lead to modulational instability. We find that the stability of plane wave solutions significantly depends on the interaction among system parameters. Further development of the instability leads to the fragmentation of the BEC into a chain of quantum droplets. We calculated the number of quantum droplets generated during the nonlinear stage of the instability. Our analytical results are corroborated by numerical simulations of the modified quasi-1D Gross–Pitaevskii equation. These simulations vividly depict the formation, interaction, and coalescence of droplets during the nonlinear phase of modulational instability. The investigation shows that the linear stability analysis of the modified Gross–Pitaevskii equation, considering quantum fluctuations, precisely predicts the initial stage of modulational instability phenomena across different domains of parameter space. In contrast, the later nonlinear stage, studied through numerical simulations, reveals the formation and nonintegrable dynamics of quantum droplets.