# Evaluation of the superconducting characteristics of multi-layer thin-film structures of NbN and SiO$_2$ on pure Nb substrate

## TL;DR

This study investigates the superconducting properties of multilayer NbN/SiO2 thin films on Nb substrates, revealing an optimal layer thickness that enhances the critical magnetic field by nearly 24%.

## Contribution

It provides experimental data on how multilayer thin-film structures affect the effective critical magnetic field in superconducting RF cavities.

## Key findings

- Optimal NbN layer thickness increases effective H_{C1} by 23.8%.
- Multilayer structures can improve superconducting cavity performance.
- Effective H_{C1} depends on film thickness and layering.

## Abstract

In recent years, it has been pointed out that the maximum accelerating gradient of a superconducting RF cavity can be increased by coating the inner surface of the cavity with a multilayer thin-film structure consisting of alternating insulating and superconducting layers. In this structure, the principal parameter that limits the performance of the cavity is the critical magnetic field or effective $H_{C1}$ at which vortices begin penetrating into the superconductor layer. This is predicted to depend on the combination of the film thickness. We made samples that have a NbN/SiO$_2$ thin-film structure on a pure Nb substrate with several layers of NbN film deposited using DC magnetron sputtering method. Here, we report the measurement results of effective $H_{C1}$ of NbN/SiO$_2$(30 nm)/Nb multilayer samples with thicknesses of NbN layers in the range from 50 nm to 800 nm by using the third-harmonic voltage method. Experimental results show that an optimum thickness exists, which increases the effective $H_{C1}$ by 23.8 %.