Sharp decay thresholds for eigenvalues of discrete Schrödinger operators on $\mathbb{Z}^2$
We study eigenvalues of discrete Schrödinger operators $H=Δ+V$ on $\mathbb{Z}^2$, where $Δ$ is the uncentered Laplacian, i.e., the un-normalized adjacency operator of $\mathbb{Z}^2$ and $V$ decays at infinity. By Weyl's theorem, the essential spectrum of $H$ is $[-4,4]$. We determine the sharp decay thresholds for existence of eigenvalues in three distinct spectral regimes. While it is natural to expect different behavior at the spectral edge $λ=\pm4$, and the bulk, there is a further distinction between the regular energies $0<|λ|<4$ and the interior critical energy $λ=0$ stemming from the reducibility of the corresponding Fermi surface in the latter case. For every $0<|λ|<4$, we construct potentials $V$ satisfying $|V(n)|\leq C|n|^{-1}$ for which $λ$ is an eigenvalue of $Δ+V$, and prove absence of eigenvalues when $|V(n)|\leq C|n|^{-1-\varepsilon}$ for some $\varepsilon>0$. At $λ=0$, the critical power changes and we construct potentials $V$ satisfying $|V(n)|\leq C|n| ^{-2}$ for which $0$ is an eigenvalue of $Δ+V$, as well as prove absence when $|V(n)|\leq C|n|^{-2-\varepsilon}$ for any $\varepsilon>0$. Finally, at each spectral edge, we show that, for every $K\geq3$, an eigenvalue can be created by potentials supported on exactly $K$ sites, whereas a potential supported on at most two sites cannot create an edge eigenvalue. The proofs combine Green-function expansions and moment cancellation, Hilbert-space-valued iterations, discrete Carleman estimates, and a uniform Green-kernel estimate.