Probing Coordination Environments in Buried Oxides of Aluminum Josephson Junctions by Resonant X-ray Reflectivity
cond-mat.mtrl-sci
/ Authors
Paul Corbae, Alex Abelson, Shivani Srivastava, Heemin Lee, Bevin Huang, Lyrik R-J Lee, Davis B. Rash, Cheng-Tai Kuo, Donghui Lu, Mihir Pendharkar
and 9 more authors
Loren D. Alegria, Tian T. Li, Keith G. Ray, Shannon P. Harvey, Apurva Mehta, David I. Schuster, Vincenzo Lordi, Paul B. Welander, Jun-Sik Lee
/ Abstract
Decoherence remains a critical obstacle to achieving high-fidelity, scalable superconducting qubits, with the tunnel barrier of Josephson junctions a key source of loss. Here we apply resonant X-ray reflectivity to non-destructively probe the electronic structure of buried layers in Al/AlO$_x$/Al Josephson junctions. At the Al $K$-edge, energy-dependent modulations in the reflectivity maps enable Kramers-Kronig-constrained extraction of the layer-resolved atomic scattering factors. The analysis reveals that the barrier coordination evolves from more tetrahedral toward predominantly octahedral character with increasing oxidation pressure. At the interfaces, the lower metal-oxide boundary is comparatively under-coordinated and disordered relative to the upper interface. Comparison with simulated X-ray absorption spectra identifies the dominant coordination motifs within the oxide and its interfaces, providing depth-resolved structural insight that constrains microscopic models of two-level system formation. These results link growth conditions, local coordination environments, and junction electronic properties, demonstrating resonant X-ray reflectivity as a powerful tool for probing the microscopic materials properties of Josephson junctions and providing a materials-level framework for mitigating decoherence in superconducting qubits.