Bands of pure a.c. spectrum for lattice Schr{\"o}dinger operators with a more general long range condition. Part I

TL;DR

This paper uses commutator methods to establish limiting absorption principles and the presence of absolutely continuous spectrum for certain lattice Schrödinger operators with long-range potentials, especially for specific decay conditions and dimensions.

Contribution

It extends the understanding of spectral properties of lattice Schrödinger operators with long-range potentials under generalized decay conditions, focusing on the existence of a.c. spectrum bands.

Findings

Limiting absorption principles hold in specific spectral bands.

Absolutely continuous spectrum exists for certain long-range decay conditions.

Results are particularly detailed for small values of the decay parameter , , , .

Abstract

Commutator methods are applied to get limiting absorption principles for the discrete standard and Molchanov-Vainberg Schr\"odinger operators $H_{\mathrm{std}}= \Delta+V$ and $H_{\mathrm{MV}} = D+V$ on $\ell^2(\mathbb{Z}^d)$, with emphasis on $d=1,2,3$. Considered are electric potentials $V$ satisfying a long range condition of the type: $V-\tau_j ^{\kappa}V$ decays appropriately for some $\kappa \in \mathbb{N}$ and all $1 \leq j \leq d$, where $\tau_j ^{\kappa} V$ is the potential shifted by $\kappa$ units on the $j^{\text{th}}$ coordinate. More comprehensive results are obtained for specific small values of $\kappa$, such as $\kappa =1,2,3,4$. In this article, we work in a simplified framework in which the main takeaway appears to be the existence of bands where a limiting absorption principle holds, and hence absolutely continuous (a.c.) spectrum, for $\kappa>1$ and $\Delta$ (resp.\ $\kappa>2$ and $D$). Other decay conditions for $V$ arise from an isomorphism between $\Delta$ and $D$ in dimension 2. Oscillating potentials are natural examples in application.