Spontaneous symmetry breaking in a split potential box

TL;DR

This paper analyzes spontaneous symmetry breaking in a one-dimensional model with a split potential box, using analytical and numerical methods to explore bifurcations, stability, and state transformations relevant to Bose-Einstein condensates and nonlinear optics.

Contribution

It provides the first comprehensive analysis of SSB in a simple split potential box model, combining analytical predictions with systematic numerical validation.

Findings

The ground state always undergoes supercritical SSB bifurcation.

Variational approximation accurately predicts bifurcation for moderate barrier strengths.

Unstable modes tend to evolve into the asymmetric ground state.

Abstract

We report results of the analysis of the spontaneous symmetry breaking (SSB) in the basic (actually, simplest) model which is capable to produce the SSB phenomenology in the one-dimensional setting. It is based on the Gross-Pitaevskii - nonlinear Schroedinger equation with the cubic self-attractive term and a double-well-potential built as an infinitely deep potential box split by a narrow (delta-functional) barrier. The barrier's strength, epsilon, is the single free parameter of the scaled form of the model. It may be implemented in atomic Bose-Einstein condensates and nonlinear optics. The SSB bifurcation of the symmetric ground state (GS) is predicted analytically in two limit cases, viz., for deep or weak splitting of the potential box by the barrier. For the generic case, a variational approximation (VA) is elaborated. The analytical findings are presented along with systematic numerical results. Stability of stationary states is studied through the calculation of eigenvalues for small perturbations, and by means of direct simulations. The GS always undergoes the SSB bifurcation of the supercritical type, as predicted by the VA at moderate values of epsilon, although the VA fails at small epsilon, due to inapplicability of the underlying ansatz in that case. However, the latter case is correctly treated by the approximation based on a soliton ansatz. On top of the GS, the first and second excited states are studied too. The antisymmetric mode (the first excited state) is destabilized at a critical value of its norm. The second excited state undergoes the SSB bifurcation, like the GS, but, unlike it, the bifurcation produces an unstable asymmetric mode. All unstable modes tend to spontaneously reshape into the asymmetric GS.