Spontaneous symmetry breaking of self-trapped and leaky modes in quasi-double-well potentials

TL;DR

This paper explores how nonlinear interactions in a double-well potential lead to spontaneous symmetry breaking and self-trapping of modes, with implications for optics and Bose-Einstein condensates, revealing the order of phase transitions depends on barrier height.

Contribution

It introduces a model combining cubic nonlinearity with a double-well potential supporting leaky modes, analyzing the interplay and order of symmetry breaking and self-trapping transitions.

Findings

Symmetry breaking occurs first at high barrier heights.

Leaky modes exhibit asymmetry in radiation tails during SSB.

Collision dynamics can trap solitons, causing persistent shuttle motion.

Abstract

We investigate competition between two phase transitions of the second kind induced by the self-attractive nonlinearity, viz., self-trapping of the leaky modes, and spontaneous symmetry breaking (SSB) of both fully trapped and leaky states. We use a one-dimensional mean-field model, which combines the cubic nonlinearity and a double-well-potential (DWP) structure with an elevated floor, which supports leaky modes (quasi-bound states) in the linear limit. The setting can be implemented in nonlinear optics and BEC. The order in which the SSB and self-trapping transitions take place with the growth of the nonlinearity strength depends on the height of the central barrier of the DWP: the SSB happens first if the barrier is relatively high, while self-trapping comes first if the barrier is lower. The SSB of the leaky modes is characterized by specific asymmetry of their radiation tails, which, in addition, feature a resonant dependence on the relation between the total size of the system and radiation wavelength. As a result of the SSB, the instability of symmetric modes initiates spontaneous Josephson oscillations. Collisions of freely moving solitons with the DWP structure admit trapping of an incident soliton into a state of persistent shuttle motion, due to emission of radiation. The study is carried out numerically, and basic results are explained by means of analytical considerations.