A Molecular Magnet with a Dual Personality express
A tiny magnet, made from just two molecules, was placed on a metal surface, and it behaved like a temperamental conductor. Normally, electrons in a metal move in a coordinated way, like a crowd on an escalator. But when cooled significantly, this order breaks down, and the motion becomes unpredictable. It's precisely this kind of behavior that was captured in the new experiment.
Stochastic fluctuations of the spins in the molecule cause the metal's electrons to rearrange. Instead of a single simple regime, two emerge at once: one resembles an enhancement of magnetism, the other a partial suppression of it. Their combination gives rise to a special state that theorists call a singular Fermi liquid. It's as if an orchestra were playing two scores simultaneously, and both could be heard.
The setup of two triangular molecules, spin units with different magnetic moment magnitudes, provides physicists with a precise tool. By changing the distance between the molecules or the composition of the substrate, one can switch the system between different quantum regimes. This is a step toward controlling matter at the atomic level, where the familiar laws of electricity cease to apply.
Original abstract
as written by the authors arXiv:2604.07174
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.
The triangular molecules used in the work are called triangulenes. Their shape is no accident: it forces the electron spins to align in a particular way, creating a clean magnetic moment without unwanted impurities.
In Greg Egan's novel 'Schild's Ladder', materials with exotic quantum properties are described, created by precise molecular assembly. The new work shows that such constructs are becoming a reality.