Spin and Charge Control of Topological End States in Chiral Graphene Nanoribbons on a 2D Ferromagnet

TL;DR

This study demonstrates the reversible control of spin states in topological graphene nanoribbons on a ferromagnetic substrate, revealing how electrostatic and magnetic interactions influence their charge and spin configurations.

Contribution

It introduces a method to reversibly switch spin states in topological nanoribbons via atomic manipulation and models these effects with an effective Hubbard dimer.

Findings

Pristine chiral graphene nanoribbons can maintain charge-neutral or singly charged spin states.

Moiré-modulated work function and exchange field enable control of spin multiplicities.

Effective Hubbard model describes the phase diagram of accessible spin states.

Abstract

Tailor-made graphene nanostructures can exhibit symmetry-protected topological boundary states that host localized spin-$1/2$ moments. However, one frequently observes charge transfer on coinage metal substrates, which results in spinless closed-shell configurations. Using low temperature scanning tunneling spectroscopy, we demonstrate here that pristine topologically nontrivial chiral graphene nanoribbons synthesized directly on the ferromagnet $\textrm{GdAu}_2$ can either maintain a charge-neutral diradical singlet or triplet configuration, or exist in a singly anionic doublet state. As an underlying mechanism, we identify a moir\'{e}-modulated work function and exchange field, as corroborated by Kelvin-probe force microscopy and spin-flip spectroscopy. The joint electrostatic and magnetic interactions allow reversibly switching between the three spin multiplicities by atomic manipulation. We introduce an effective Hubbard dimer model that unifies the effects of local electrostatic gating, electron-electron-correlation, hybridization and exchange field to outline the phase diagram of accessible spin states. Our results establish a platform for the local control of $\pi$-radicals adsorbed on metallic substrates.