Probing Coordination Environments in Buried Oxides of Aluminum Josephson Junctions by Resonant X-ray Reflectivity

TL;DR

This study uses resonant X-ray reflectivity to non-destructively analyze the atomic coordination and interface structure of buried aluminum oxide layers in Josephson junctions, linking growth conditions to electronic properties.

Contribution

It introduces a novel application of resonant X-ray reflectivity for depth-resolved structural analysis of Josephson junction barriers, revealing coordination changes and interface disorder.

Findings

Barrier coordination shifts from tetrahedral to octahedral with oxidation pressure

Lower metal-oxide boundary is under-coordinated and disordered

Resonant X-ray reflectivity constrains microscopic models of two-level systems

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.