Artificially built Kondo chains with organic radicals on metallic surfaces: new model system of heavy fermion quantum criticality

Heavy fermion quantum criticality is an extremely rich domain of research which represents a framework to understand strange metals as a consequence of a Kondo breakdown transition. Here we provide an experimental realization of such systems in terms of organic radicals on a metallic surface. The ground state of organic radicals is a Kramer’s doublet that can be modeled by a spin 1/2 degree of freedom. Using on-surface synthesis and scanning tunneling microscopy (STM) tip manipulation, one can controllably engineer and characterize chains of organic radicals on a Au(111) surface. The spatial-resolved differential conductance reveals site-dependent low-energy excitations, which support the picture of emergent many-body Kondo physics. Using quantum Monte Carlo simulations, we show that a Kondo lattice model of spin chains on a metallic surface reproduces accurately the experimental results. This allows us to interpret the experimental results in terms of a heavy fermion metal, below the coherence temperature. We foresee that the tunability of these systems will pave the way to realize quantum simulators of heavy fermion criticality.

💡 Research Summary

The authors present a comprehensive experimental and theoretical study of artificially constructed Kondo chains built from organic π‑radicals on a Au(111) surface, establishing a new platform for heavy‑fermion quantum criticality. By depositing a brominated precursor onto Au(111) at 473 K and employing on‑surface synthesis together with STM tip manipulation, they convert closed‑shell molecules into open‑shell radicals (spin‑½ Kramers doublets) and assemble them into linear chains of controllable length and geometry (trans/cis isomers, varying hydrogen saturation). High‑resolution STM imaging confirms the structural integrity of monomers, dimers, trimers, tetramers, pentamers, and hexamers, while DFT calculations reveal that each radical hosts an unpaired π‑electron, giving rise to a localized S = ½ moment that hybridizes with the Au surface state electrons.

Site‑resolved scanning tunneling spectroscopy (STS) uncovers a striking site dependence of low‑energy excitations. Isolated radicals exhibit a pronounced zero‑bias Fano resonance, which fits to a Kondo temperature TK≈64 K. When radicals are linked into chains, TK varies between 34 K and 118 K, reflecting changes in the Ruderman‑Kittel‑Kasuya‑Yosida (RKKY) exchange mediated by the metallic substrate. Even‑length chains (dimers, trimers, tetramers) lose the zero‑bias peak and instead display two asymmetric side peaks at ≈ ±8–12 mV, indicative of a competition between Kondo screening and antiferromagnetic RKKY coupling that produces collective spin‑flip excitations. Odd‑length chains retain a zero‑bias Kondo resonance on the terminal radicals while the central sites develop a “U‑shaped” suppression, reproducing the even‑odd alternation known from 1D Kondo lattice theory.

To interpret these observations, the authors construct a Kondo‑lattice Hamiltonian that couples a one‑dimensional array of spin‑½ moments to a two‑dimensional conduction electron sea. Large‑scale quantum Monte Carlo (QMC) simulations of this model reproduce the experimental dI/dV spectra quantitatively: the position, width, and intensity of the zero‑bias and side peaks, as well as the TK dependence on chain length, are all captured. The simulations demonstrate that the radicals and substrate electrons form a heavy‑fermion band with a dramatically enhanced effective mass, confirming that the system behaves as a heavy‑fermion metal below a coherence temperature of order 60 K—substantially higher than in previously studied organic Kondo systems.

The work highlights the unprecedented tunability of this platform. By varying chain length, the trans/cis ratio, and the degree of hydrogen saturation, one can continuously adjust the ratio of RKKY exchange to Kondo screening without external pressure or magnetic field. This provides a route to drive the system across a quantum critical point (QCP) and to explore Kondo‑breakdown transitions in a controlled, atom‑by‑atom fashion. The authors therefore propose that organic‑radical Kondo chains on metallic surfaces constitute a versatile quantum simulator for heavy‑fermion quantum criticality, opening avenues for systematic studies of non‑Fermi‑liquid behavior, emergent coherence, and the interplay of magnetism and Kondo physics in low dimensions.