Existence of absolutely continuous spectrum for Galton-Watson random trees

TL;DR

This paper provides a criterion for the presence of absolutely continuous spectrum in operators on Galton-Watson trees, linking offspring distribution variance to spectral properties, with applications to various models.

Contribution

It introduces a quantitative criterion for absolutely continuous spectrum on Galton-Watson trees, including conditions on offspring variance and average degree, with applications to the Anderson model.

Findings

Supercritical Poisson Galton-Watson trees have non-trivial absolutely continuous spectrum at high average degree.

The Karp and Sipser core exhibits purely absolutely continuous spectrum under certain conditions.

A quantitative version of Klein's Theorem relates disorder size to spectrum type in the Anderson model.

Abstract

We establish a quantitative criterion for an operator defined on a Galton-Watson random tree for having an absolutely continuous spectrum. For the adjacency operator, this criterion requires that the offspring distribution has a relative variance below a threshold. As a byproduct, we prove that the adjacency operator of a supercritical Poisson Galton-Watson tree has a non-trivial absolutely continuous part if the average degree is large enough. We also prove that its Karp and Sipser core has purely absolutely spectrum on an interval if the average degree is large enough. We finally illustrate our criterion on the Anderson model on a d-regular infinite tree with d $\ge$ 3 and give a quantitative version of Klein's Theorem on the existence of absolutely continuous spectrum at disorder smaller that C $\sqrt$ d for some absolute constant C.