Coexistence of Ferromagnetic and Overscreened Kondo Effects in a Single Molecule

Abstract

The quest for quantum ground states beyond the Fermi-liquid paradigm remains a central challenge in many-body physics. The ferromagnetic Kondo effect is an intriguing variant of the Kondo effect in which an asymptotically free spin gives rise to singular Fermi-liquid behavior. Despite its theoretical importance, this regime has eluded experimental observation owing to subtle spectroscopic signatures, small energy scales, and symmetry constraints in nanostructures. Here, we demonstrate the coexistence of ferromagnetic and overscreened Kondo effects within a 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 signatures of singular Fermi-liquid behavior, supported by many-body calculations. The molecular design provides control over spin configuration and coupling asymmetry, allowing distinct regimes within the same platform. Our results establish a strategy for realizing non-Fermi-liquid physics at the atomic scale and demonstrate that ferromagnetic Kondo behavior can be deliberately engineered in molecular systems.

arXiv comment26 pages, 3 figures, plus supplemental material (12 pages, 8 figures)