Introduction

Abstract

In recent years, a new type of quantum matter has emerged in the field of condensed matters physics, which adds a fundamental new class of materials to the so far basic distinction between conductors and insulators. However, the classification into systems which are able to conduct an electrical current and systems which are not, is not complete. The discovery of the integer and fractional quantum Hall effect in the 1980’s revealed that there exist systems which are insulating in the bulk, but necessarily exhibit a gapless edge state at the boundaries with the important characteristic that the boundary conductance is a requirement of the bulk properties of the system [1, 2].