A density-functional theory study of the Al/AlO_x/Al tunnel junction

The aluminum oxide Josephson tunnel junction (JJ) is a key component of superconducting quantum devices. The coherence time of superconducting qubits has seen five orders of magnitude increase over the last twenty years, however, further improvements in JJ quality and uniformity are needed to realize a scalable quantum computer. We used ab-initio calculations to study the atomistic features limiting JJ performance. We created realistic models of Al/AlOx/Al JJs and compared to experimental observations. The true thickness of the insulating part of the JJ can be different from what is visible under electron microscopes. We compute the JJ critical current by solving the Schrödinger equation for tunneling over the Kohn-Sham effective potential. Comparison between amorphous and crystalline cases highlights what is possible with better control over the interface microstructure. Further analysis of the interfaces revealed the amorphous nature of the oxide leads to local dipole formation, fostering disruptive two-level-systems. Our ab-initio atomistic analysis presents an approach to predict the performance of superconducting quantum tunnel junction and assess novel materials. Prepared by LLNL under Contract DE-AC52-07NA27344.