Spectra of definite type in waveguide models

TL;DR

This paper introduces an abstract method to identify spectral points of definite type in waveguide operators, providing new insights into realness of bound states and spectral behavior in non-self-adjoint, PT-symmetric waveguides.

Contribution

It develops a novel abstract approach for spectral analysis of tensor product operators, especially useful for non-self-adjoint waveguide models with symmetries, and derives new spectral properties.

Findings

Realness of weakly coupled bound states established

Low lying essential spectrum shown to be real in PT-symmetric waveguides

Pseudospectrum exhibits normal behavior near low lying essential spectrum

Abstract

We develop an abstract method to identify spectral points of definite type in the spectrum of the operator $T_1\otimes I_2 + I_1\otimes T_2$. The method is applicable in particular for non-self-adjoint waveguide type operators with symmetries. Using the remarkable properties of the spectral points of definite type, we obtain new results on realness of weakly coupled bound states and of low lying essential spectrum in the $\mathcal{P}\mathcal{T}$-symmetric waveguide. Moreover, we show that the pseudospectrum has a normal tame behavior near the low lying essential spectrum and exclude the accumulation of non-real eigenvalues to this part of the essential spectrum. The advantage of our approach is particularly visible when the resolvent of the unperturbed operator cannot be explicitly expressed and most of the mentioned spectral conclusions are extremely hard to prove using direct methods.