Spontaneous symmetry breaking of dual-layer solitons in spin-orbit-coupled Bose-Einstein condensates

TL;DR

This paper investigates how symmetric dual-layer spin-orbit-coupled Bose-Einstein condensate solitons undergo spontaneous symmetry breaking, leading to asymmetric states and complex dynamics, including collapse and delocalization, depending on system parameters.

Contribution

It introduces a double-layer system with linearly coupled binary condensates and analyzes the spontaneous symmetry breaking of 2D solitons using numerical methods.

Findings

Symmetric 2D solitons undergo subcritical SSB bifurcation.

Asymmetric solitons exist up to the Townes soliton norm, then collapse occurs.

Moving solitons are supported up to a critical velocity, beyond which they delocalize.

Abstract

It is known that stable 2D solitons of the semi-vortex (SV) and mixed-mode (MM) types are maintained by the interplay of the cubic attractive nonlinearity and spin-orbit coupling (SOC) in binary Bose-Einstein condensates. We introduce a double-layer system, in which two binary condensates, stabilized by the SOC, are linearly coupled by tunneling. By means of the numerical methods, it is found that symmetric two-layer solitons undergo the spontaneous-symmetry-breaking (SSB) bifurcation of the subcritical type. The bifurcation produces families of asymmetric 2D solitons, which exist up to the value of the total norm equal to the norm of the Townes solitons, above which the collapse occurs. This situation terminates at a critical value of the inter-layer coupling, beyond which the SSB bifurcation is absent, as the collapse sets in earlier. Symmetric 2D solitons that are destabilized by the SSB demonstrate dynamical symmetry breaking, in combination with intrinsic oscillations of the solitons, or transition to the collapse, if the soliton's norm is sufficiently large. Asymmetric MMs produced by the SSB instability start spontaneous drift, in addition to the intrinsic vibrations. Consideration of moving 2D solitons is a nontrivial problem because SOC breaks the Galilean invariance. It is found that the system supports moving MMs up to a critical value of the velocity, beyond which they undergo delocalization.