A condition for purely absolutely continuous spectrum for CMV operators using the density of states

TL;DR

This paper establishes a criterion for when an ergodic family of CMV matrices has purely absolutely continuous spectrum, linking the density of states and Lyapunov exponent, extending Kotani's theorems to CMV operators.

Contribution

It provides an averaging formula for the derivative of the absolutely continuous part of the density of states measure for CMV matrices and characterizes the spectral type using this formula.

Findings

Spectral type is almost surely purely absolutely continuous if and only if the density of states is absolutely continuous and the Lyapunov exponent vanishes almost everywhere.

Derived an averaging formula for the derivative of the absolutely continuous part of the density of states measure.

Extended Kotani's theorems from Schrödinger operators to CMV matrices.

Abstract

We prove an averaging formula for the derivative of the absolutely continuous part of the density of states measure for an ergodic family of CMV matrices. As a consequence, we show that the spectral type of such a family is almost surely purely absolutely continuous if and only if the density of states is absolutely continuous and the Lyapunov exponent vanishes almost everywhere with respect to the same. Both of these results are CMV operator analogues of theorems obtained by Kotani for Schr\"odinger operators.