Uncovering the Triplet Ground State of Triangular Graphene Nanoflakes Engineered with Atomic Precision on a Metal Surface

TL;DR

This study demonstrates that triangular graphene nanoflakes can have a stable spin $S=1$ ground state on surfaces, confirmed by spectroscopy and simulations, and can be manipulated to lower spin states by chemical modification.

Contribution

It provides the first experimental evidence of a stable $S=1$ ground state in engineered graphene nanoflakes on surfaces, and shows how to control their spin states.

Findings

Triangular graphene nanoflakes exhibit an $S=1$ ground state.

Scanning tunnelling spectroscopy reveals an underscreened $S=1$ Kondo state.

Adding hydrogen atoms can switch the spin state to $S=1/2$.

Abstract

Graphene can develop large magnetic moments in custom crafted open-shell nanostructures such as triangulene, a triangular piece of graphene with zigzag edges. Current methods of engineering graphene nano-systems on surfaces succeeded in producing atomically precise open-shell structures, but demonstration of their net spin remains elusive to date. Here, we fabricate triangulene-like graphene systems and demonstrate that they possess a spin $S=1$ ground state. Scanning tunnelling spectroscopy identifies the fingerprint of an underscreened $S=1$ Kondo state on \rev{these} flakes at low temperatures, signaling the dominant ferromagnetic interactions between two spins. Combined with simulations based on the meanfield Hubbard model, we show that this $S=1$ $\pi$-paramagnetism is robust, and can be manipulated to a $S=1/2$ state by adding additional H-atoms to the radical sites. \rev{Our results demonstrate that $\pi$-paramagnetism of high-spin graphene flakes can survive on surfaces, opening the door to study the quantum behaviour of interacting $\pi$-spins in graphene systems.