Quadratic fractional solitons

TL;DR

This paper introduces a new model combining quadratic and fractional diffraction nonlinearities to study solitons in quantum gases, deriving analytical and numerical solutions, and exploring stability and lattice effects.

Contribution

It presents the first detailed analysis of quadratic fractional solitons, including analytical norm relations, variational approximation of quasi-Townes solitons, and stability of multi-peak gap solitons.

Findings

Fundamental solitons are numerically constructed with exact norm-chemical potential relation.

Quasi-Townes solitons are analyzed via variational approximation at =1/2.

Families of multi-peak gap solitons exhibit stability regions that shrink with more peaks.

Abstract

We introduce a system combining the quadratic self-attractive or composite quadratic-cubic nonlinearity, acting in the combination with the fractional diffraction, which is characterized by its L\'{e}vy index $\alpha $. The model applies to a gas of quantum particles moving by L\'{e}vy flights, with the quadratic term representing the Lee-Huang-Yang correction to the mean-field interactions. A family of fundamental solitons is constructed in a numerical form, while the dependence of its norm on the chemical potential characteristic is obtained in an exact analytical form. The family of \textit{quasi-Townes solitons}, appearing in the limit case of $\alpha =1/2$, is investigated by means of a variational approximation. A nonlinear lattice, represented by spatially periodical modulation of the quadratic term, is briefly addressed too. The consideration of the interplay of competing quadratic (attractive) and cubic (repulsive) terms with a lattice potential reveals families of single-, double-, and triple-peak gap solitons (GSs) in two finite bandgaps. The competing nonlinearity gives rise to alternating regions of stability and instability of the GS, the stability intervals shrinking with the increase of the number of peaks in the GS.