# Topological characterization of Landau levels for $2$D massless Dirac fermions in $3$D layered systems

## TL;DR

This paper explores the topological properties of Landau levels associated with 2D massless Dirac fermions in 3D layered systems, emphasizing the robustness of zero-mode Landau levels and their topological protection.

## Contribution

It provides a topological analysis of Landau levels for 2D massless Dirac fermions in layered 3D systems, extending understanding beyond graphene.

## Key findings

- Zero-mode Landau level is topologically robust.
- Protection of zero-mode LL depends on local chiral symmetry.
- Topological characterization applies to various 2D Dirac systems.

## Abstract

A topological concern is addressed in view of the extensively and intensively studied topological phases of condensed matter. In this realm, the phases with topological order cannot be characterized by symmetry alone. Moreover, the relevant phase transitions do occur without spontaneous symmetry breaking, beyond the scope of Landau's theory. The first realization of such phases is the discovery of the integer quantum Hall effect (QHE), which was followed soon by a topological interpretation. Later on, a distinct, half-integer QHE was also found from graphene, which has almost spin degeneracy described by $SU(2)$ symmetry. The previous theoretical predictions were realized in this finding. It has been well understood that the anomaly of this half-integer QHE originates from the presence of $2$D massless Dirac fermions around the zero energy with respect to the original Dirac points (DPs). The very characteristic lies in that there exists a topologically robust zero-mode LL that is a constant function of the perpendicular magnetic field. More deeply, this zero-mode LL is protected by the local chiral symmetry (CS), against CS preserving perturbations provided that intervalley scattering between the double DPs is inhibited, where the CS arises from the global sublattice symmetry in spinless graphene. Since massless Dirac particles are broadly present in condensed matter with various symmetries, not to mention Dirac bosonic systems, it is of interest to see how about the situations in other systems with $2$D massless Dirac fermions. We address several notes in a topological viewpoint on the presence of $2$D massless Dirac fermions in $3$D layered systems.In particular, we focus on the zero-mode LL since this LL signifies $2$D massless Dirac fermions.