Hard Materials Seminar - "4D-STEM Nanodiffraction Analysis of Amorphous AlOₓ in Josephson Junctions"

The Al/AlOx/Al Josephson junction (JJ) is a ubiquitous component across most superconducting qubit architectures for quantum technologies.1 In these structures, defects within the ultrathin (~2 nm) AlOx barrier are thought to generate two-level systems (TLSs) that couple strongly to the qubit, reducing its coherence time and reliability.2 Accordingly, understanding the atomic-scale origins of TLSs is a critical challenge for advancing quantum computing, but structural analysis remains difficult due to the amorphous nature of AlOx. In this work, we use four-dimensional scanning transmission electron microscopy (4D-STEM) nanodiffraction to establish an analysis framework for resolving structural variations in the AlOx barrier. Unlike real-space imaging methods, nanodiffraction is highly sensitive to changes in atomic ordering in amorphous materials.3 In particular, the cepstrum, obtained from the Fourier transform of the logarithm of the diffraction intensity, enables the detection of subtle periodic correlations within the amorphous material.4 By examining how cepstral features vary with scan position, we can identify spatial variations in the local structure of amorphous AlOx. Further analysis of these variations can provide insight into their relationship with local atomic pair correlations. Ultimately, this approach can help clarify the microscopic origins of defects in superconducting quantum devices.