The role of 4f spin in Single Molecule Magnets (SMMs) in generating Kondo states and Yu-Shiba-Rusinov states: Insights from Bis(phthalocyaninato)terbium(III) (TbPc2) Adsorbed on Superconductor Pb(111) Surface

Spin impurities on superconductors have gained great interest since they can potentially form the Majorana particle, essential for constructing quantum computers. [1]. In this report, to understand the behaviour of magnetic molecules on superconductors, we investigate the Yu-Shiba-Rusinov (YSR) states, formed by the scattering of the Cooper pairs by a magnetic impurity, formed by a single molecule magnet (SMM) TbPc2 and superconductor Pb, as well as the intriguing Kondo peak split caused by inter-molecule exchange coupling between the magnetic moment from Tb3+ and π radical electron from Pc ligand.[2] In this work, a low-temperature ultra-high vacuum scanning tunnelling microscopy (LT-UHV-STM) experiment is performed for TbPc2 and YPc2 molecules on the Pb(111) surface to investigate the significance of 4f spin from the central metal. We first controlled the Cooper pair occupation of the Pb(111) surface by an outer magnetic field of 0.5T at 0.4K and observed the split Kondo peaks on the second layer of TbPc2 molecules, whereas only a single Kondo peak is obtained on YPc2 molecules whose center metal does not contribute to 4f spin. The Kondo resonance exhibits C2 symmetry over TbPc2, and we observed a significant difference in the exchange split energy between the molecular center and the lobes. In the absence of an external magnetic field, we observe two pairs of YSR peaks at the lobe positions of the TbPc2 molecule and two pairs at the Tb atom position. By comparing these findings with the YPc2 molecule, which consistently shows two pairs of YSR peaks homogeneously, we infer that the 4f spin of the Tb atom most likely contributes to the inner two pairs of YSR peaks in the TbPc2 case. This comparison highlights the significant role of the 4f spin in forming the YSR states for SMMs. Our work appears to be the first to elucidate the effect of 4f spin in SMM with intra-molecular spatial resolution, potentially transforming next-generation molecular spintronic applications.