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Interpreting x-ray absorption spectra of vanadyl phthalocyanines spin qubit candidates using a machine learning assisted approach
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June 24, 2024
J. H. Lee, C. Urdaniz, S. Reale, K. J. Noh, D. Krylov, A. Doll, L. Colazzo, Y. J. Bae, C. Wolf, and F. Donati
Phys. Rev. B
Description
Vanadyl phthalocyanine (VOPc) shows promise as a candidate for molecular spin qubits in solid-state quantum computing. Understanding how interactions with supporting substrates and diamagnetic molecular layers affect their electronic structure is crucial for their integration into physical devices. Our study employs x-ray absorption spectroscopy and atomic multiplet calculations to investigate the orbital structure of VOPc on Ag(100) and on diamagnetic layers of titanyl phthalocyanine (TiOPc) on Ag(100). By analyzing characteristic X-ray absorption features using atomic multiplet calculations aided by Bayesian optimization, we reveal crucial properties such as orbital occupancy, splitting, and intra-atomic interactions. Our findings confirm that VOPc maintains its spin S=1/2 character across all analyzed film structures, while the energy separation and order of the V orbitals are sensitive to surface interactions and the presence of the TiOPc interlayer. Comparison with density functional theory (DFT) calculations enhances our understanding of the molecular system, reinforcing the potential of these molecules as promising candidates for spin qubits.
Abstract
The magnetic dilution of vanadyl phthalocyanine (VOPc) within the isostructural diamagnetic titanyl phthalocyanine (TiOPc) affords promising molecular spin qubit platforms for solid-state quantum computing. The development of quantitative methods for determining how the interactions with a supporting substrate impact the electronic structure of the system are fundamental to determine their potential integration in physical devices. In this work we propose a combined approach based on x-ray absorption spectroscopy (XAS), atomic multiplet calculations, and density functional theory (DFT) to investigate the 3𝑑 orbital level structure of VOPc on TiOPc/Ag(100). We characterize VOPc in different molecular environments realized by changing the thickness of the TiOPc interlayer and adsorption configuration on Ag(100). Depending on the molecular film structure, we find characteristic XAS features that we analyze using atomic multiplet calculations. We use a Bayesian optimization algorithm to accelerate the parameter search process in the multiplet calculations and identify the ground state properties, such as the 3𝑑 orbital occupancy and splitting, as well as intra-atomic interactions. Our analysis indicates that VOPc retains its spin 𝑆=
1
2
character in all configurations. Conversely, the energy separation and sequence of the unoccupied V 3𝑑 orbitals sensitively depend on the interaction with the surface and TiOPc interlayer. We validate the atomic orbital picture obtained from the multiplet model by comparison with DFT, which further allows us to understand the VOPc electronic properties using a molecular orbital description.