Observation of the Ferromagnetic Kondo Effect

TL;DR

This study reports the experimental observation of the ferromagnetic Kondo effect in a molecular system, revealing singular Fermi-liquid behavior and demonstrating control over many-body quantum states at the atomic scale.

Contribution

It provides the first experimental evidence of the ferromagnetic Kondo effect in a molecular system and shows how to engineer non-Fermi-liquid physics using molecular design.

Findings

Signatures of singular Fermi-liquid behavior observed via scanning tunneling spectroscopy.

Coexistence of ferromagnetic and overscreened Kondo effects demonstrated in a single molecular system.

Molecular design enables control over spin states and coupling asymmetry, accessing different many-body regimes.

Abstract

The quest for quantum ground states beyond the conventional Fermi-liquid paradigm remains a central challenge in many-body physics. The ferromagnetic Kondo effect represents a particularly intriguing case: an exotic 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 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$\unicode{x2014}$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 can not only be observed but also deliberately engineered in molecular systems.