The chemical versatility of carbon imparts manifold properties to organic compounds, where magnetism remains one of the most desirable but elusive1. Polycyclic aromatic hydrocarbons, also referred to as nanographenes, show a critical dependence of electronic structure on the topologies of the edges and the π-electron network, which makes them model systems with which to engineer unconventional properties including magnetism. In 1972, Erich Clar envisioned a bow-tie-shaped nanographene, C38H18 (refs. 2,3), where topological frustration in the π-electron network renders it impossible to assign a classical Kekulé structure without leaving unpaired electrons, driving the system into a magnetically non-trivial ground state4. Here, we report the experimental realization and in-depth characterization of this emblematic nanographene, known as Clar’s goblet. Scanning tunnelling microscopy and spin excitation spectroscopy of individual molecules on a gold surface reveal a robust antiferromagnetic order with an exchange-coupling strength of 23 meV, exceeding the Landauer limit of minimum energy dissipation at room temperature5. Through atomic manipulation, we realize switching of magnetic ground states in molecules with quenched spins. Our results provide direct evidence of carbon magnetism in a hitherto unrealized class of nanographenes6, and prove a long-predicted paradigm where topological frustration entails unconventional magnetism, with implications for room-temperature carbon-based spintronics7,8.
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The data that support the findings of this study are available from the corresponding authors on reasonable request.
The TB calculations were performed using a custom-made code on the WaveMetrics Igor Pro platform. Details of the TB code can be obtained from the corresponding authors on reasonable request.
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We thank M. N. Huda for assistance with STM measurements, T. Lübken for assistance with nuclear magnetic resonance measurements, and R. Ortiz, J. Fernández-Rossier and D. Passerone for discussions. This work was supported by the Swiss National Science Foundation (grant nos 200020-182015 and IZLCZ2-170184), the NCCR MARVEL funded by the Swiss National Science Foundation (grant no. 51NF40-182892), the European Union’s Horizon 2020 research and innovation programme under grant agreement nos 696656 and 785219 (Graphene Flagship Core 2), the Office of Naval Research (N00014-18-1-2708), ERC Consolidator grant (T2DCP, no. 819698), the German Research Foundation (DFG) within the Cluster of Excellence Center for Advancing Electronics Dresden (cfaed) and EnhanceNano (no. 391979941), and the European Social Fund and the Federal State of Saxony (ESF-Project GRAPHD, TU Dresden). We acknowledge computational support from the Swiss Supercomputing Center (CSCS) under project ID s904; S.K. and P.L. acknowledge funding from the Academy of Finland (grant nos 309975 and 318995) and the European Research Council (ERC-AdG no. 788185) and the facilities of the Aalto Nanomicroscopy Centre.
The authors declare no competing interests.
Peer review information Nature Nanotechnology thanks Manuel Melle-Franco and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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a, Schematic representations of 1 and di-1. b, Calculated singlet-triplet gaps (ΔES-T) of 1 (blue filled circles) and di-1 (red filled circles) as a function of the on-site Coulomb repulsion U, within the MFH model. Over a wide range of U/t (where t = 2.7 eV is the nearest-neighbour hopping parameter), ΔES-T, 1 is much larger than ΔES-T, di-1. c, Same as (b), with U/t ranging between 0.5 and 1.5, where it is evident that ΔES-T, di-1 values are essentially zero over the considered range of U, while ΔES-T, 1 shows a monotonic increase with increasing U. This is in support of the experimentally observed cases of spin excitations in 1 (finite ΔES-T) and Kondo resonances in di-1 (ΔES-T ≈ 0). ΔES-T values for both species are calculated as the mean-field energy difference between the converged electronic structures of the open-shell triplet (where n↑ = n↓ + 2) and open-shell singlet configurations (where n↑ = n↓).
Extended Data Fig. 2 Analyses of temperature-dependent characteristics of zero-bias peak at the unbound end of 1´.
a, HR STM image showing two 1´ molecules towards the top and bottom of the scan frame, along with di-1 and a dimer where the constituent molecules are fused in an off-centre configuration (V = −100 mV, I = 50 pA). The lower end of each 1´ molecule is bound to the elbow site, leading to a distinct topographic appearance of the molecules. b, Temperature evolution of the ZBP at the unbound end of the upper 1´ molecule with fit to the experimental data with the Frota function (V = −20 mV, I = 300 pA, Vrms = 400 μV). Tip position is marked by a filled circle in (a). c, Extracted HWHM of the ZBP as a function of temperature, with corresponding fit using the Fermi-liquid model. As seen from the fit data in (b) and (c), the ZBP exhibits the expected lineshape and resonance linewidth broadening characteristic of a Kondo resonance, with the Kondo temperature TK = 57 ± 2 K. Scale bar: 2 nm.
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Mishra, S., Beyer, D., Eimre, K. et al. Topological frustration induces unconventional magnetism in a nanographene. Nat. Nanotechnol. 15, 22–28 (2020). https://doi.org/10.1038/s41565-019-0577-9
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