$\textit{Ab initio}$ Identification of Hydrogen Tunneling as Two-Level Systems in Nb$_2$O$_5$ and Ta$_2$O$_5$

TL;DR

This study uses advanced simulations and ab initio validation to identify hydrogen tunneling as a key microscopic origin of two-level systems in Nb$_2$O$_5$ and Ta$_2$O$_5$, explaining their impact on superconducting devices.

Contribution

It combines machine learning interatomic potentials with ab initio methods to identify hydrogen tunneling as the microscopic source of TLS in amorphous niobium and tantalum oxides.

Findings

Hydrogen exhibits tunneling frequencies in the 0.1-10 GHz range.

Hydrogen TLS densities align with experimental loss measurements.

Hydrogen tunneling is a plausible microscopic TLS source in these oxides.

Abstract

Two-level systems (TLS) in native Nb and Ta oxides limit superconducting-qubit coherence and SRF-cavity quality factors in the microwave frequency range, yet their microscopic origin remains unclear. We combine MLIP-accelerated sampling of hydrogen configurations and diffusion pathways in amorphous Nb and Ta pentoxides with targeted $\textit{ab initio}$ validation. Hydrogen is the only light interstitial with barrier-distance combinations near the $\sim0.1-10$ GHz tunneling regime, and its ensemble statistics in amorphous oxides produce effective TLS densities and loss estimates consistent with the experimentally observed higher loss in Nb oxide than in Ta oxide. Our results point to H tunneling as a plausible microscopic TLS source in these materials.