Beyond spin models in orbitally-degenerate open-shell nanographenes

TL;DR

This paper reveals that certain open-shell nanographenes exhibit low-energy excitations involving coupled spin and orbital degrees of freedom due to orbital degeneracy, challenging the traditional spin-only models.

Contribution

It demonstrates the importance of including orbital degrees of freedom in modeling nanographenes, using theoretical methods on nitrogen-doped triangulenes.

Findings

Orbital degeneracy leads to coupled spin-orbital excitations.

Traditional spin-only models are insufficient for some nanographenes.

Rich interplay between spin and orbital degrees of freedom confirmed.

Abstract

The study of open-shell nanographenes has relied on a paradigm where spins are the only low-energy degrees of freedom. Here we show that some nanographenes can host low-energy excitations that include strongly coupled spin and orbital degrees of freedom. The key ingredient is the existence of orbital degeneracy, as a consequence of leaving the benzenoid/half-filling scenario. We analyze the case of nitrogen-doped triangulenes, using both density-functional theory and Hubbard model multiconfigurational and random-phase approximation calculations. We find a rich interplay between orbital and spin degrees of freedom that confirms the need to go beyond the spin-only paradigm, opening a new venue in this field of research.