Elucidating the local atomic and electronic structure of amorphous oxidized superconducting niobium films

TL;DR

This study characterizes the atomic and electronic structure of the native oxide layer on niobium superconducting films, revealing amorphous Nb₂O₅ and magnetic impurities that impact qubit coherence.

Contribution

It provides detailed atomistic and electronic insights into niobium oxide layers, linking structural features to potential decoherence sources in superconducting qubits.

Findings

Native oxide is amorphous Nb₂O₅.

Magnetic impurities are present in the oxide layer.

X-ray absorption can detect magnetic impurities.

Abstract

Qubits made from superconducting materials are a mature platform for quantum information science application such as quantum computing. However, materials-based losses are now a limiting factor in reaching the coherence times needed for applications. In particular, knowledge of the atomistic structure and properties of the circuit materials is needed to identify, understand, and mitigate materials-based decoherence channels. In this work we characterize the atomic structure of the native oxide film formed on Nb resonators by comparing fluctuation electron microscopy experiments to density functional theory calculations, finding that an amorphous layer consistent with an Nb$_2$O$_5$ stoichiometry. Comparing X-ray absorption measurements at the Oxygen K edge with first-principles calculations, we find evidence of d-type magnetic impurities in our sample, known to cause impedance in proximal superconductors. This work identifies the structural and chemical composition of the oxide layer grown on Nb superconductors, and shows that soft X-ray absorption can fingerprint magnetic impurities in these superconducting systems.