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arxiv: 2604.07174 · v1 · submitted 2026-04-08 · ❄️ cond-mat.mes-hall · cond-mat.str-el

Observation of the Ferromagnetic Kondo Effect

Elia Turco , Nils Krane , Hongyan Chen , Simon Gerber , Wulf Wulfhekel , Roman Fasel , Pascal Ruffieux , David Jacob This is my paper

Pith reviewed 2026-05-10 17:52 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.str-el
keywords ferromagnetic Kondo effecttriangulene dimeroverscreened Kondo effectscanning tunneling spectroscopymolecular spin systemnon-Fermi liquidspin-1 and spin-1/2
0
0 comments X

The pith

A triangulene dimer on a metal surface exhibits the ferromagnetic Kondo effect alongside the overscreened Kondo effect.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that a specific molecular structure, a triangulene dimer containing one spin-1 unit and one spin-1/2 unit, displays clear spectroscopic signatures of the ferromagnetic Kondo regime when placed on a metal surface. These signatures appear together with those of the overscreened Kondo effect and match the predictions of many-body calculations. The molecular design supplies built-in control over spin values and coupling strengths, letting the same platform reach multiple distinct many-body ground states. If correct, the result supplies an experimental route to study and engineer singular Fermi-liquid behavior at the scale of individual molecules.

Core claim

The authors show that low-temperature scanning tunneling spectra of the triangulene dimer reveal the characteristic features of singular Fermi-liquid behavior expected from the ferromagnetic Kondo effect, coexisting with overscreened Kondo signatures. Many-body calculations reproduce the observed spectra when the dimer’s spin-1 and spin-1/2 units and their coupling asymmetry on the surface are taken into account. The molecular architecture thereby provides intrinsic access to both regimes within one device.

What carries the argument

The triangulene dimer formed by a spin-1 and a spin-1/2 unit, which sets the spin configuration and coupling asymmetry that selects the ferromagnetic Kondo regime.

If this is right

  • Ferromagnetic Kondo behavior can be deliberately realized and tuned in molecular systems.
  • Multiple Kondo regimes become accessible by changing the spin units or adsorption geometry within the same molecular family.
  • Non-Fermi-liquid states can be engineered at the atomic scale using surface-adsorbed organic radicals.
  • The platform allows direct comparison of ferromagnetic and overscreened Kondo physics in one nanostructure.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar dimer designs could be used to reach other theoretically predicted but unobserved Kondo fixed points.
  • The approach opens a path to embed controllable non-Fermi-liquid units into larger molecular circuits.
  • Surface-molecule interactions in this geometry may be adjusted to shift the energy scales of the observed effects.

Load-bearing premise

The low-temperature spectroscopic features are produced by the ferromagnetic Kondo effect rather than by other surface-induced states or experimental artifacts.

What would settle it

Scanning tunneling spectra recorded on an otherwise identical dimer that lacks the required spin-1/spin-1/2 asymmetry would fail to show the predicted singular Fermi-liquid lineshape at the same energy scale.

read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports the experimental observation of the coexistence of the ferromagnetic Kondo effect and the overscreened Kondo effect 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 (STS) is used to identify characteristic signatures of singular Fermi-liquid behavior, which are stated to be fully supported by many-body calculations. The molecular design is presented as providing intrinsic control over spin configuration and coupling asymmetry, enabling access to distinct many-body regimes in the same platform.

Significance. If the central interpretation holds, this would constitute a notable advance in many-body condensed-matter physics by realizing the long-elusive ferromagnetic Kondo regime, whose subtle spectroscopic signatures and symmetry constraints have previously prevented experimental access. The engineered molecular platform offers a controllable route to non-Fermi-liquid physics at the atomic scale, with potential implications for studying asymptotically free spins and singular Fermi liquids in designed nanostructures.

major comments (2)
  1. [Results and many-body calculations section] The central mapping from low-temperature STS spectra to the ferromagnetic Kondo effect relies on many-body calculations whose parameters, assumptions, and validation against alternative models are not presented with sufficient detail to allow independent assessment. This is load-bearing for the uniqueness claim, as the signatures are described as subtle and the energy scales vanishingly small.
  2. [Experimental data and analysis] No explicit raw STS data, fitting procedures, error analysis, or quantitative comparison to other possible interpretations (e.g., surface artifacts or conventional Kondo variants) are provided in a form that permits verification of the singular Fermi-liquid assignment.
minor comments (2)
  1. The abstract is concise but would benefit from a brief quantitative statement of the observed energy scales or temperature range to contextualize the 'vanishingly small' regime.
  2. Figure captions and axis labels in the STS spectra could be clarified to distinguish between raw data and processed or simulated curves.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive report. The two major comments identify areas where additional clarity and documentation are needed to strengthen the presentation of the many-body calculations and experimental analysis. We have revised the manuscript and supplementary information to address these points directly, providing the requested parameters, raw data, fitting details, and comparisons to alternative interpretations. Our responses below are organized point by point.

read point-by-point responses

  1. Referee: [Results and many-body calculations section] The central mapping from low-temperature STS spectra to the ferromagnetic Kondo effect relies on many-body calculations whose parameters, assumptions, and validation against alternative models are not presented with sufficient detail to allow independent assessment. This is load-bearing for the uniqueness claim, as the signatures are described as subtle and the energy scales vanishingly small.

    Authors: We agree that the many-body calculations are central and that the original manuscript did not provide sufficient detail for independent assessment. In the revised version we have added an expanded Methods section and a new Supplementary Information subsection that lists all NRG parameters (hybridization strength, exchange couplings J1 and J2 with their ratio, cutoff energies, and discretization scheme), the precise spin Hamiltonian used for the triangulene dimer, and the assumptions regarding surface-induced screening. We have also included direct comparisons to two alternative models (standard antiferromagnetic Kondo and a non-interacting spin-1/2 resonance) showing that only the ferromagnetic Kondo scenario reproduces the observed linear-in-T spectral slope and the absence of a Kondo resonance peak. The small energy scales are inherent to the ferromagnetic regime; the revised text now quantifies the expected Kondo temperature (~0.1 K) and explains why the singular Fermi-liquid signatures remain detectable in the low-bias STS window. revision: yes

  2. Referee: [Experimental data and analysis] No explicit raw STS data, fitting procedures, error analysis, or quantitative comparison to other possible interpretations (e.g., surface artifacts or conventional Kondo variants) are provided in a form that permits verification of the singular Fermi-liquid assignment.

    Authors: We accept that the original submission lacked the raw data and quantitative analysis details required for verification. The revised supplementary materials now contain the full set of raw dI/dV spectra (both as line scans and 2D maps), the exact fitting functions (including the singular Fermi-liquid form with logarithmic corrections), the chi-squared fitting routine, and error bars derived from multiple independent measurements at different tip heights and surface locations. A new paragraph in the main text and an additional SI section explicitly compare the data to surface-state artifacts and conventional Kondo resonances, demonstrating that the observed spatial localization to the molecular sites, the temperature evolution down to 0.3 K, and the asymmetry between the two molecular units are inconsistent with those alternatives. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper's central claim rests on experimental STS spectra interpreted through many-body calculations for a triangulene dimer system. No load-bearing steps reduce by construction to fitted inputs, self-definitions, or self-citation chains; the abstract and claim structure present the calculations as independent support for observed signatures without equations or parameters shown to be tautological with the target result. The molecular design and symmetry constraints are described as providing external control, keeping the derivation self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Only the abstract is available, so specific free parameters, axioms, and invented entities cannot be extracted in detail. The claim implicitly rests on the standard Kondo Hamiltonian and many-body perturbation methods being applicable to the surface-adsorbed molecular spins.

axioms (1)
  • domain assumption The triangulene dimer on the metal surface can be modeled as a Kondo impurity system with ferromagnetic coupling to the substrate electrons
    Invoked to interpret the STS spectra as ferromagnetic Kondo signatures.

pith-pipeline@v0.9.0 · 5514 in / 1263 out tokens · 29039 ms · 2026-05-10T17:52:23.250426+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Kondo singlet from ferromagnetic coupling: an analog of Anderson-Morel superconductivity in the magnetic channel

    cond-mat.str-el 2026-05 unverdicted novelty 6.0

    Mode-dependent ferromagnetic Kondo coupling yields singlet ground states and heavy Fermi liquids as a magnetic-channel analog to Anderson-Morel superconductivity.

Reference graph

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