Observation of Yu–Shiba–Rusinov States and Inelastic Tunneling Spectroscopy for Intramolecule Magnetic Exchange Interaction Energy of Terbium Phthalocyanine (TbPc) Species Adsorbed on Superconductor NbSe₂

We investigated the spin properties of the terbium phthalocyanine (TbPc) species adsorbed on the superconductor NbSe₂ surface using scanning tunneling microscopy and spectroscopy. TbPc₂ is a molecule in a class of single-molecule magnets (SMMs), and the use of superconductor electrodes attracts attention for the application to the devices using the spin degree of freedom. TbPc is a building block of TbPc₂ and can reveal the spin component's behavior. In the experiment, TbPc species were placed on the surface of the superconductor NbSe₂. We measured Yu-Shiba-Rusinov (YSR) states caused by the interaction between the superconducting state and magnetic impurity and inelastic tunneling spectroscopy (IETS) for the spin excitation, below 1 K. We also measured the Kondo state formed by the magnetic singlet formation. We detected the radical spin at the ligand position of the TbPc by the presence of the Kondo peak and demonstrated that the radical spin forms the YSR feature. In addition, the exchange interaction energy (E_ex) between the spins of the radical ligand (Pc) and the center 4f metal atom (Tb³⁺) is determined by using the IETS technique. E_ex is a critical parameter that determines the blocking temperature, below which the sample behaves as an SMM. IETS results show that the statistical distribution of E_ex has peaked at 1.3, 1.6, and 1.9 meV. The energy range is comparable to the recent theoretical calculation result. In addition, we show that the energy variation is correlated with the bonding configuration of TbPc.