Tailoring the Electronic Configurations of YPc$_2$ on Cu(111): Decoupling Strategies for Molecular Spin Qubits Platforms

Among molecular spin qubit candidates, yttrium phthalocyanine double-decker (YPc$_2$) features a diamagnetic metal ion core that stabilizes the molecular structure, while its magnetic properties arise primarily from an unpaired electron (S = 1/2) delocalized over the phthalocyanine (Pc) ligand. Understanding its properties in the proximity of metal electrodes is crucial to assess its potential use in molecular spin qubits architectures. Here, we investigated the morphology and electronic structure of this molecule adsorbed on a metal Cu(111) surface using scanning tunneling microscopy. On that surface, YPc$_2$ adsorbs flat, with isolated molecules showing a preferred orientation along the (111) crystal axes. We observed two different types of self-assembly molecular packing when growing the molecular patches on Cu(111). For YPc$_2$ in direct contact with Cu(111), scanning tunneling spectroscopy revealed a widely separated highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO), suggesting the quenching of the unpaired spin. Conversely, when the YPc$_2$ is separated from the substrate by a few-layer thick diamagnetic ZnPc layer, we find the HOMO to split into singly occupied (SOMO) and singly unoccupied molecular orbitals (SUMO). We find that more than 2 layers of ZnPc are needed to avoid intermixing between the two molecules and spin quenching in the YPc$_2$. Density functional theory reveals the spin quenching to be due to the hybridization between YPc$_2$ and Cu(111) states, confirming the importance of using suitable decoupling layers to preserve the unpaired molecular spin. Our results suggest the potential of YPc$_2$/ZnPc heterostructures as a stable and effective molecular spin qubit platform and validates the possibility of integrating this molecular spin qubit candidate in future quantum logic devices.