Spontaneous soliton symmetry breaking in two-dimensional coupled Bose-Einstein condensates supported by optical lattices

Abstract

Models of two-dimensional (2D) traps, with the double-well structure in the third direction, for Bose-Einstein condensate (BEC) are introduced, with attractive or repulsive interactions between atoms. The models are based on systems of linearly coupled 2D Gross-Pitaevskii equations (GPEs), where the coupling accounts for tunneling between the wells. Each well carries an optical lattice (OL) (stable 2D solitons cannot exist without OLs). The main subject of the work is spontaneous symmetry breaking (SSB) in two-component 2D solitons and localized vortices. We demonstrate that, in the system with attraction or repulsion, SSB occurs in families of symmetric or antisymmetric solitons (or vortices), respectively. The corresponding bifurcation destabilizes the original solution branch and gives rise to a stable branch of asymmetric solitons or vortices. In the model with attraction, all stable branches eventually terminate due to the onset of collapse. Stable asymmetric solitons in higher finite bandgaps, and vortices with a multiple topological charge are found too. The models also give rise to first examples of \textit{embedded solitons%} and \textit{embedded vortices}(the states located inside Bloch bands) in two dimensions. In the linearly-coupled system with opposite signs of the nonlinearity in the two cores, two distinct types of stable solitons and vortices are found, dominated by either the self-attractive component or the self-repulsive one. In the system with a mismatch between the two OLs, a \textit{pseudo-bifurcation} is found: when the mismatch attains its largest value ($π$), the bifurcation does not happen, as branches of different solutions asymptotically approach each other but fail to merge.