Ferromagnetic and multichannel Kondo effects in coupled magnetic nanographenes

The ferromagnetic and overscreened Kondo effects are exotic variants of the Kondo effect. Both evade complete Kondo screening and thus lead to unconventional Fermi liquid behavior, but fall into distinct universality classes. While the ferromagnetic Kondo effect results in an asymptotically free spin that gives rise to singular Fermi-liquid behavior, frustration between screening channels leads to non-Fermi liquid behavior characterized by fractional residual entropy for the overscreened case. Despite their theoretical importance both the overscreened and the ferromagnetic Kondo effect remain experimentally difficult to access. Especially the latter has long eluded experimental observation owing to its subtle spectroscopic signatures, vanishingly small energy scales, and strict symmetry constraints in conventional nanostructures.

Here, we demonstrate the coexistence of the ferromagnetic and overscreened Kondo effects within a single molecular spin system, a triangulene dimer comprising spin-1 and spin-1/2 units adsorbed on a metal surface. Low-temperature scanning tunneling spectroscopy reveals characteristic signatures of singular Fermi-liquid behavior, which are fully supported by many-body calculations. The unique molecular design provides intrinsic control over spin configuration and coupling asymmetry, allowing distinct many-body regimes to be accessed within the same platform. Our results establish a robust strategy for realizing non-Fermi-liquid physics at the atomic scale and demonstrate that ferromagnetic Kondo behavior cannot only be observed but also deliberately engineered in molecular systems.

Preprint: E. Turco et al., arXiv:2604.07174 (2026)