Abstract
Single atomic adsorbates on ultrathin insulating films provide a promising route towards building bottom-up quantum architectures based on atomically identical yet individually addressable spin qubits on solid surfaces. A key challenge in engineering quantum-coherent spin nanostructures lies in understanding and controlling the spin state of individual adsorbates. In this work, we investigate single titanium (Ti) atoms adsorbed on MgO/Ag(100) surfaces using a combined scanning tunneling microscopy and electron spin resonance. Our measurements reveal two distinct spin states, S = 1/2 and S = 1, depending on the local adsorption site and the thickness of the MgO film. Density functional theory calculations suggest a Ti+ configuration for the Ti adsorbates with approximately 3 electrons in the 4s and 3d valence shells. Using multi-orbital magnetic multiplet calculations the site dependence of the spin can be rationalized as a charge redistribution between spin-polarizing and depolarizing orbitals. These findings underscore the potential of surface-supported single atoms as spin qubits with tunable spin and charge states, enabling atom-by-atom control in the realization of a versatile quantum platform on surfaces.
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Data availability
The data used and generated in this study have been deposited in the ZENODO repository47. The data are available at https://doi.org/10.5281/zenodo.17938903.
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Acknowledgements
S.P., H.T.B., W.S., Y.L., C.W., and A.J.H. acknowledge financial support from the Institute for Basic Science (IBS-R027-D1). R.R. and N.L. thank projects PID2021-127917NB-I00 by MCIN/AEI/10.13039/501100011033, IT-1527-22 by the Basque Government, 202260I187 by CSIC, ESiM project 101046364 by the EU, and computational resources by Finisterrae III (CESGA). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the EU. Neither the EU nor the granting authority can be held responsible for them. C.W. thanks Fernando Delgado and Susanne Baumann for insightful discussions.
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S.P. conceived the experiments. S.P., H.T.B., W.S., and Y.L. performed the experiments. N.L. and R.R. performed the VASP DFT calculations. C.W. and C.L. performed QE DFT and multiplet calculations. All authors discussed the results and prepared the manuscript.
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Phark, Sh., Bui, H.T., Seo, Wh. et al. Spin-state engineering of single titanium adsorbates on ultrathin magnesium oxide. Nat Commun (2026). https://doi.org/10.1038/s41467-026-68314-6
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DOI: https://doi.org/10.1038/s41467-026-68314-6
