The number and location of two particle Schr\"odinger operators on a lattice

Abstract

We study the Schr\"odinger operators ${H}_{\lambda\mu}(K)$ with $K\in\mathbb{T}^2$ being the fixed quasimomentum of a pair of particles, associated with a system of two arbitrary particles on a two-dimensional lattice $\mathbb{Z}^2$ with on-site and nearest-neighbor interactions of strengths $\lambda\in\mathbb{R}$ and $\mu\in\mathbb{R}$, respectively. We divide the $(\lambda,\mu)$-plane of parameters $\lambda$ and $\mu$ into connected components, such that in each component, the Schr\"{o}dinger operator $H_{\lambda\mu}(0)$ has a fixed number of eigenvalues. These eigenvalues are located both below the bottom of the essential spectrum and above its top. Additionally, we establish a sharp lower bound for the number of isolated eigenvalues of $H_{\lambda\mu}(K)$ within each connected component.