Exchange interaction, disorder, and stacking faults in rhombohedral graphene multilayers

TL;DR

This study investigates electronic states in multilayer rhombohedral graphene, analyzing effects of disorder and stacking faults on band gaps, order parameters, and topological properties, revealing layer-dependent behaviors and robustness of certain topological features.

Contribution

It introduces a detailed mean-field analysis of disorder and stacking faults in multilayer rhombohedral graphene, highlighting their impact on electronic and topological properties.

Findings

Band gap saturates with increasing layers

Chiral symmetry breaking disorder reduces the band gap

Stacking faults create low-energy flat bands and affect topological invariants

Abstract

We apply the mean-field Hartree Fock theory of gapped electronic states at charge neutrality in bilayer graphene to thin films of rhombohedral graphite with up to thirty layers. For the ground state, the order parameter (the separation of bands at the valley center) saturates to a constant non-zero value as the layer number increases, whereas the band gap decreases with layer number. We consider chiral symmetry breaking disorder in the form of random layer potentials and chiral preserving disorder in the form of random values of the interlayer coupling. The former reduces the magnitude of the mean band gap whereas the latter has a negligible effect, which is due to self-averaging within a film with a large number of layers. We determine the ground state in the presence of an individual stacking fault which results in two pairs of low-energy bands and we identify two separate order parameters. One of them determines the band gap at zero temperature, the other determines the critical temperature leading, overall, to a temperature dependence of the band gap that is distinct to that of pristine rhombohedral graphite. In the presence of stacking faults, each individual rhombohedral section with $m$ layers contributes a pair of low-energy flat bands producing a peak in the Berry curvature located at a characteristic $m$-dependent wave vector. The Chern number per spin-valley flavor for the filled valence bands in the ground state is equal in magnitude to the total number of layers divided by two, the same value as for pristine rhombohedral graphite.