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Quantum dynamics of a single molecule magnet on superconducting Pb(111)

Abstract

Magnetic materials interfaced with superconductors may reveal new physical phenomena with potential for quantum technologies. The use of molecules as magnetic components has already shown great promise, but the diversity of properties offered by the molecular realm remains largely unexplored. Here we investigate a submonolayer of tetrairon(iii) propeller-shaped single molecule magnets deposited on a superconducting lead surface. This material combination reveals a strong influence of the superconductor on the spin dynamics of the single molecule magnet. It is shown that the superconducting transition to the condensate state switches the single molecule magnet from a blocked magnetization state to a resonant quantum tunnelling regime. Our results open perspectives to control single molecule magnetism via superconductors and to use single molecule magnets as local probes of the superconducting state.

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Fig. 1: Structure of Fe4SMe.
Fig. 2: STM and simulated STM images.
Fig. 3: XMCD and XNLD characterization.
Fig. 4: Magnetic behaviour of Fe4SMe SMMs across the superconducting phase transition.

Data availability

Experimental data used to draw Fig. 3a–d and Fig. 4a are included with the manuscript as source data. Additional data such as STM images, simulated hysteresis curves and DFT results are available from the authors upon request.

Code availability

The code to compute magnetic hysteresis using the kinetic Monte Carlo method is available from the ETH Zurich Data Archive, https://doi.org/10.3929/ethz-b-000393264.

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Acknowledgements

We acknowledge SOLEIL for provision of the synchrotron radiation facilities. We thank P. Ohresser, J. P. Kappler and L. Joly for the realization of the ULT-XMCD set-up and for assistance in using the DEIMOS beamline. The European COST Action CA15128 MOLSPIN, the Quantera ERA-NET Co-fund project SUMO and the FET Open Femtoterabyte project are acknowledged for financial support. Italian MIUR, through PRIN project QCNaMoS (2015-HYFSRT) and Progetto Dipartimenti di Eccellenza 2018-2022 (ref. no. B96C1700020008), and Fondazione Ente Cassa di Risparmio di Firenze are also acknowledged for financial support. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (ERC-2014-StG-633818-dasQ) and from French LabEx PALM (ANR-10-LABX-0039-PALM)

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Contributions

G.S., L.P., A.L.S., G.C. and L.M. performed the synchrotron experiments with the assistance of E.O., P.S. and M.M. L.P., G.S., B.C. and A.L.S. prepared and characterized the hybrid interface. M.B. and F.T. performed the DFT studies. F.P. and A.C. prepared and structurally characterized the Fe4SMe SMM. R.S., A.-L.B. and A.C. performed its bulk phase magnetic characterization. A.V. developed the kinetic Monte Carlo method and contributed with G.S. and R.S. to the simulation of the XMCD magnetic data. A.C., S.L., F.T., M.M. and R.S. supervised the activities of the project. All the authors contributed to the discussion and preparation of the manuscript.

Corresponding authors

Correspondence to Giulia Serrano or Roberta Sessoli.

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Extended data

Extended Data Fig. 1 Additional characterization of the Fe4SMe/Pb(111) surface.

(a) Magnification of the STM image of Fe4SMe/Pb(111) in Fig. 2a (main text). Orange and pink lines indicate the molecular easy axis projections on the surface. (b) Scheme of the 12 observed orientations of the easy axis projection with respect to the crystallographic directions of the Pb(111) surface. The projection of the easy axis lies along the main crystallographic directions of Pb(111), orange lines, or at 30° from them, pink lines (only \([\bar 110]\) is shown in the STM image for convenience). (c) Low Energy Electron Diffraction (LEED), 60 eV, of the clean Pb(111) crystal used to identify the main crystallographic directions of the substrate, x axis corresponding to the STM scanning direction.

Extended Data Fig. 2 STM characterization of Fe4SMe superstructures on Pb(111).

Magnification of the STM image of Fe4SMe/Pb(111) in Fig. 2b (main text). Single (1) and double (2) row arrangements of the triangular molecular Fe4SMe superstructures are highlighted.

Extended Data Fig. 3 DFT-computed structure of Fe4SMe/Pb(111).

Top view of three Fe4SMe molecules on Pb(111) calculated by DFT; pink lines indicate the projections of molecular C3 axes. This structure allows to explain the experimental STM topography of Fig. 2c.

Extended Data Fig. 4 Computed DOS and spin density map of Fe4SMe/Pb(111).

(a) TDOS for Fe4SMe/Pb(111) (purple curve), and pDOS contributions from Fe4SMe (blue curve) and Pb(111) surface (red curve). A full width at half-maximum (FWHM) σ=0.45 eV was used. (b) pDOS of atomic contributions in Fe4SMe calculated on Fe4SMe/Pb(111) (σ=0.45 eV). (c) Computed spin density for Fe4SMe on Pb(111). The surfaces are drawn for a value of 0.001 unpaired e bohr-3. Blue and pink colours correspond to spin up and spin down calculated spin densities, respectively. In the inset, arrows depict the arrangement of the spin vectors in the S = 5 ground state using the same colour code. We notice that a similarly negligible spin delocalization was observed for VOPc deposited on Pb with the vanadyl moiety pointing up, but the spin density increased significantly when the molecule was deformed by the STM tip (see Ref. 3). Here, a similar effect appears very unlikely due to the lack of spin density on atoms close to the Pb surface.

Extended Data Fig. 5 XAS and XMCD spectra recorded at θ =45° incidence.

XAS and XMCD signal measured at the Fe L2,3 edges on Fe4SMe/Pb(111) at θ = 45° (B = 3 T, T = 220 mK).

Extended Data Fig. 6 Hysteresis loops in the superconducting window Comparison of hysteresis loops on superconducting and normal substrates.

(a) Enlarged version of Fig. 4a (main text). (b) Magnetic hysteresis loop (grey spheres) of a monolayer of a Fe4 complex on a normal metal (here on a gold substrate, see Ref. 27) in the same field region as panel a. The dark grey solid line is the hysteresis loop simulated disregarding the effect of the superconductor.

Supplementary information

Supplementary Information

Supplementary Notes 1–8, Tables 1 and 2, and Figs. 1–12.

Source data

Source Data Fig. 3

ASCII file of XAS, XMCD and XNLD spectra and ASCII file of XMCD hysteresis loops measured at different temperatures and angles.

Source Data Fig. 4

ASCII file containing the XMCD versus magnetic field data.

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Serrano, G., Poggini, L., Briganti, M. et al. Quantum dynamics of a single molecule magnet on superconducting Pb(111). Nat. Mater. 19, 546–551 (2020). https://doi.org/10.1038/s41563-020-0608-9

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