Orbitally stable standing waves of a mixed dispersion nonlinear Schrödinger equation
We study the mixed dispersion fourth order nonlinear Schrödinger equation \begin{equation*} %\tag{\protect{4NLS}}\label{4nls} i \partial_t ψ-γΔ^2 ψ+βΔψ+|ψ|^{2σ} ψ=0\ \text{in}\ \R \times\R^N, \end{equation*} where $γ,σ>0$ and $β\in \R$. We focus on standing wave solutions, namely solutions of the form $ψ(x,t)=e^{iαt}u(x)$, for some $α\in \R$. This ansatz yields the fourth-order elliptic equation \begin{equation*} %\tag{\protect{*}}\label{4nlsstar} γΔ^2 u -βΔu +αu =|u|^{2σ} u. \end{equation*} We consider two associated constrained minimization problems: one with a constraint on the $L^2$-norm and the other on the $L^{2σ+2}$-norm. Under suitable conditions, we establish existence of minimizers and we investigate their qualitative properties, namely their sign, symmetry and decay at infinity as well as their uniqueness, nondegeneracy and orbital stability.