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Spin excitations in 3d transition-metal adatoms on Pt(111): Observable with inelastic scanning tunneling spectroscopy or not?

Spin excitations in atomic-scale nanostructures have been investigated with inelastic scanning tunneling spectroscopy, sometimes with conflicting results. In this work we present a theoretical viewpoint on a recent experimental controversy regarding the spin excitations of Co adatoms on Pt(111). While one group [Balashov et al., Phys. Rev. Lett. \textbf{102}, 257203 (2009)] claims to have detected them, another group reported their observation only after the hydrogenation of the Co adatom [Dubout et al., Phys. Rev. Lett. \textbf{114}, 106807 (2015)]. Utilizing time-dependent density functional theory in combination with many-body perturbation theory we demonstrate that, although inelastic spin excitations are possible for Cr, Mn, Fe, and Co adatoms, their efficiency differs. While the excitation signature is less pronounced for Mn and Co adatoms, it is larger for Cr and Fe adatoms. We find that the tunneling matrix elements related to the nature and symmetry of the relevant electronic states are more favorable for triggering the spin excitations in Fe than in Co. An enhancement of the tunneling and of the inelastic spectra is possible by attaching hydrogen to the adatom at the appropriate position.