Mixed spectral regimes for square Fibonacci Hamiltonians

TL;DR

This paper demonstrates the existence of parameter regimes in square Fibonacci Hamiltonians that produce mixed spectra and density of states, revealing new spectral phenomena in aperiodic quantum systems.

Contribution

It introduces methods to identify parameter regimes with mixed spectra and density of states in square Fibonacci Hamiltonians and continuum models, a novel phenomenon in aperiodic Hamiltonians.

Findings

Existence of parameter sets with mixed interval-Cantor spectra.

Presence of mixed absolutely continuous and singular continuous density of states.

Applicability of methods to other substitution-based models.

Abstract

For the square tridiagonal Fibonacci Hamiltonian, we prove existence of an open set of parameters which yield mixed interval-Cantor spectra (i.e. spectra containing an interval as well as a Cantor set), as well as mixed density of states measure (i.e. one whose absolutely continuous and singular continuous components are both nonzero). Using the methods developed in this paper, we also show existence of parameter regimes for the square continuum Fibonacci Schrodinger operator yielding mixed interval-Cantor spectra. These examples provide the first explicit examples of an interesting phenomenon that has not hitherto been observed in aperiodic Hamiltonians. Moreover, while we focus only on the Fibonacci model, our techniques are equally applicable to models based on any two-letter primitive invertible substitution.