Particle-hole symmetry broken solutions in graphene nanoribbons: a multi-orbital, mean-field perspective

TL;DR

This paper explores non-magnetic, particle-hole symmetry broken solutions in graphene nanoribbons using a multi-orbital mean-field approach, explaining the lack of experimental magnetic edge states.

Contribution

It introduces particle-hole symmetry broken solutions as a non-magnetic alternative to magnetic edge states in graphene nanoribbons, considering multi-orbital effects and doping regimes.

Findings

Finite hole doping favors non-magnetic solutions over magnetic phases.

Non-magnetic solutions are topologically non-trivial with zero local magnetization.

Doping and thermal fluctuations explain the absence of magnetic edge signatures.

Abstract

Mean-field theories have since long predicted edge magnetism in graphene nanoribbons, where the order parameter is given by the local magnetization. However, signatures of edge magnetism appears elusive in the experiments, suggesting another class of solutions. We employ a self-consistent mean field approximation within a multi-orbital tight-binding model and obtain particle-hole symmetry broken solutions, where the local filling plays the role of the order parameter. Unlike the magnetic edge solutions, these are topologically non-trivial and show zero local magnetization. A small and a large doping regime are studied, and a free energy minimum for finite hole doping is encountered, which may serve as an explanation for the absence of experimental evidence for magnetic edge states in zigzag graphene nanoribbons. The electronic interaction may increase the finite \(d\)-orbital occupation, which leads to a change of the effective Coulomb interaction of the dominant \(p_z\)-orbitals. Our findings indicate that the non-magnetic solution for finite hole doping becomes energetically preferred, compared to the magnetic phases at half-filling, once thermal fluctuations or unintentional doping from the substrate are considered. This result persists even in the presence of the \(d\)-orbitals and the Coulomb interaction therein.