Contributed talk

Intermolecular exchange in coupled spin-1/2 nanographenes
David Jacob^1,2 and J. Fernández-Rossier³

1 Dpto. de Polímeros y Materiales Avanzados: Física, Química y Tecnología, Universidad del País Vasco UPV/EHU, Av. Tolosa 72, 20018 San Sebastián, Spain
2 IKERBASQUE, Basque Foundation for Science, Pl. Euskadi 5, 48009 Bilbao, Spain
3 International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal

Open-shell nanographenes can be covalently bonded and still preserve their local moments, forming interacting spin lattices. In the case of benzenoid nanographenes, the Ovchinnikov-Lieb rules anticipate the spin of the ground state of the superstructure and thereby the sign of the intermolecular exchange. Here we address the underlying microscopic mechanisms for intermolecular exchange in this type of system. We find that, in general, three different mechanisms contribute. First, Hund's ferromagnetic exchange that promotes ferromagnetic interactions of electrons in overlapping orbitals. Second, superexchange driven by intermolecular hybridization, identical to Anderson kinetic exchange, which is a decreasing function of the Hubbard-U energy scale and is always antiferromagnetic. Third, a Coulomb-driven superexchange, that increases as a function of U and involves virtual excitation of excited molecular orbitals that are extended over the entire structure. We find that Coulomb-driven superexchange can be either ferro- or antiferromagnetic, accounting for Ovchinnikov-Lieb rules. We compute these exchange energies for the case of coupled S=1/2 phenalenyl triangulenes, using multiconfigurational methods both with Hubbard and extended Hubbard models, thereby addressing the influence of long-range Coulomb interactions on the exchange interactions [1].

Figure 1: Left: Coupled Spin-1/2 nanographenes. Right: Coulomb-driven superexchange mechanism.

[1] D. Jacob and J. Fernández-Rossier, Phys. Rev. B 106, 205405 (2022).