Standing waves for 4-superlinear Schrödinger-Poisson systems with indefinite potentials

In this paper we consider 4-superlinear Schrödinger–Poisson systems. In contrast to most studies, we consider the case where the potential V is indefinite so that the Schrödinger operator − Δ + V possesses a finite-dimensional negative space. We obtain nontrivial solutions for the problem via Morse theory.