Microscopic Mechanism of Coexisting Electron Spin Resonance and Kondo Resonance in a Single Iron Phthalocyanine Molecule

The multiplicity of orbitals in quantum systems significantly influences the competition between Kondo screening and local spin magnetization. The identification of orbital-specific processes is essential for advancing spintronic devices, as well as for enhancing the understanding of many-body quantum phenomena, but it remains a great challenge. Here, we use a combination of scanning tunneling microscopy/spectroscopy and electron spin resonance (ESR) spectroscopy to investigate single iron phthalocyanine (FePc) molecules on MgO/Ag(100). We observe the coexistence of ESR and Kondo resonance from the Fe(II) ion center at a temperature well below the Kondo temperature T_{K}, where the magnetic moments are determined to be 1 Bohr magneton (1μ_{B}), corresponding to a spin S=1/2 state in either of the ESR and Kondo resonance channels. On the basis of the observed in-plane twofold symmetry in the spatial distribution of ESR linewidths, in combination with density functional theory calculations, we attribute one spin S=1/2 state in the degeneracy-lifted d_{π} orbital to the ESR channel, and the other in the highly screenable d_{z^{2}} orbital to the Kondo resonance, respectively, within the framework of the S=1 two-channel Kondo model of an ordinary Fermi liquid.