Wigner-von Neumann type perturbations of periodic Schr\"odinger Operators

TL;DR

This paper investigates how oscillatory, decaying perturbations affect the spectrum of periodic Schrödinger operators, showing preservation of the absolutely continuous spectrum and characterizing the embedded singular spectrum.

Contribution

It provides new bounds on the Hausdorff dimension of the singular spectrum and identifies an explicit countable set where embedded eigenvalues can occur, demonstrating optimality.

Findings

Absolutely continuous spectrum is preserved under certain perturbations.

Explicit bounds on the Hausdorff dimension of the singular spectrum.

Existence of an optimal countable set where embedded eigenvalues can occur.

Abstract

We consider decaying oscillatory perturbations of periodic Schr\"odinger operators on the half line. More precisely, the perturbations we study satisfy a generalized bounded variation condition at infinity and an $L^p$ decay condition. We show that the absolutely continuous spectrum is preserved, and give bounds on the Hausdorff dimension of the singular part of the resulting perturbed measure. Under additional assumptions, we instead show that the singular part embedded in the essential spectrum is contained in an explicit countable set. Finally, we demonstrate that this explicit countable set is optimal. That is, for every point in this set there is an open and dense class of periodic Schr\"odinger operators for which an appropriate perturbation will result in the spectrum having an embedded eigenvalue at that point.