The precise tuning of local spin states in adsorbed organometallic molecules by a mechanically controlled scanning tunneling microscope (STM) tip has become a focus of recent experiments. However, the underlying mechanisms remain somewhat unclear. We investigate theoretically the STM tip control of local spin states in a single iron(II) porphyrin molecule adsorbed on the Pb(111) substrate. A combined density functional theory and hierarchical equations of motion approach is employed to simulate the tip tuning process in conjunction with the complete active space self-consistent field method for accurate computation of magnetic anisotropy. Our first-principles-based simulation accurately reproduces the tuning of magnetic anisotropy realized in experiment. Moreover, we elucidate the evolution of geometric and electronic structures of the composite junction and disclose the delicate competition between the Kondo resonance and local spin excitation. The understanding and insight provided by the first-principles-based simulation may help to realize more fascinating quantum state manipulations.

Adsorption of magnetic transition metal atoms on a metal surface leads to the formation of Kondo states at the atom/metal interfaces. However, the significant influence of surrounding environment presents challenges for potential applications. In this work, we realize a novel strategy to regularize the Kondo states by moving a CoPc molecular mold on an Au(111) surface to capture the dispersed Co adatoms. The symmetric and ordered structures of the atom-mold complexes, as well as the strong d π-π bonding between the Co adatoms and conjugated isoindole units, result in highly robust and uniform Kondo states at the Co/Au (111) interfaces. Even more remarkably, the CoPc further enables a fine tuning of Kondo states through the molecular-mold-mediated superexchange interactions between Co adatoms separated by more than 12 Å. Being highly precise, efficient and reproducible, the proposed molecular mold strategy may open a new horizon for the construction and control of nanosized quantum devices.

The precise control of local spin states in magnetic organometallic molecules by means of a mechanically controlled break junction (MCBJ) is a cutting edge frontier of single molecule science. For instance, a fine-tuning of the magnetic anisotropy of a Co­(tpy–SH)2 molecule has been achieved experimentally by sandwiching the molecule between two electrodes and gradually enlarging their separation. However, an accurate simulation of such a process has remained rather challenging, which is largely due to the complex structure of the composite junction as well as the static electron correlation originating from the nearly degenerate d-orbitals of the cobalt atom. In this work, by employing an embedding method which combines the high-level complete active space self-consistent field (CASSCF) method and a density functional theory (DFT) method, we carefully simulate the evolution of magnetic anisotropy of the Au–Co­(tpy–S)2–Au junction during the mechanical stretching process. It is found that the presence of the connecting electrodes has significant influence on the geometric and electronic structures of the sandwiched molecule, which further affects its magnetic anisotropy. The reasonable agreement between the calculation results and the experimental measurements highlights the potential usefulness of the embedding method for predicting the local magnetic properties of organometallic molecules in MCBJs.

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