Magnetic flux distribution, quasiparticle spectroscopy, and quality factors in Nb films for superconducting qubits

TL;DR

This study compares epitaxial Nb films grown under different conditions, linking magnetic flux behavior and quasiparticle properties to their quality factors, thereby aiding optimization for superconducting qubits.

Contribution

It introduces a combined magneto-optical and quasiparticle spectroscopy approach to correlate material properties with device performance in Nb superconducting films.

Findings

Lower quality factor films exhibit poorer magnetic screening and irregular $\\lambda(T)$ behavior.

Higher quality factor films demonstrate better magnetic screening and consistent $\\lambda(T)$.

The methods provide an effective way to characterize and optimize superconducting films for quantum computing.

Abstract

Niobium is a practical material platform for superconducting microwave circuits; however, device-level performance can vary significantly depending on film growth and processing conditions. We compare three epitaxial Nb films grown on $c-$plane sapphire substrates under nominally identical conditions, except for the deposition temperature. To correlate internal quality factors, $Q_{\mathrm {i}}$, with material properties, we combine magneto-optical imaging of magnetic flux distribution with quasiparticle spectroscopy via measurements of the London penetration depth, $\lambda(T)$. In the low-$Q_{\mathrm i}$ film, there is a lesser ability to screen the magnetic field and an irregular temperature variation of $\lambda(T)$, implying the existence of localized in-gap states. High $Q_{\mathrm i}$ films show the opposite trend. We conclude that our measurements provide an efficient method for characterizing and optimizing superconducting films for quantum informatics applications.