Intrinsic localized modes for DNLS equation with competing nonlinearities: bifurcations

Abstract

We study nonlinear excitations described by DNLS-type equations with so-called competing nonlinearities. These are the nonlinearities that consist of two power terms with coefficients of different sign. A key feature of these models is the presence of two governing parameters: $\alpha$, which characterizes the coupling between lattice sites, and $\gamma$, which quantifies the balance between competing nonlinearities. Our study focuses on intrinsic localized modes (ILMs) -- solutions that exhibit spatial localization over a few lattice sites. The basic example for our study is the cubic-quartic equation that recently has been used to describe 3D BEC cloud in the mean field approximation with Lee-Huang-Yang corrections. We employ numerical continuation from the anti-continuum limit (ACL) where the coupling between the lattice sites is neglected (the case $\alpha=0$). We analyze $\alpha$-dependent branches of the basic ILMs and their bifurcations when $\gamma$ varies. Our study shows that all branches of ILMs originated at anti-continuum limit, except a finite number, bifurcate and do not exist for large values of $\alpha$. We present tables of bifurcations for the ILMs that involve not more than 3 excited lattice sites. It is shown that the model supports nonsymmetric ILMs that have no counterparts in ACL. Also we study the branches of ILMs that can be continued unlimitedly when $\alpha\to \infty$ (called here $\infty$-branches). It was found that for any $\gamma$ there are exactly two (up to symmetries) $\infty$-branches. When $\gamma$ grows these branches undergo a sequence of bifurcations. Finally, we compare our results with the results for the quadratic-cubic equation and the cubic-quintic equation and found no qualitative difference in (a) tables of bifurcations, (b) presence of solutions without ACL counterpart and (c) scenario of switching of $\infty$-branches.