RF surface resistance tuning of superconducting niobium via thermal diffusion of native oxide

TL;DR

This paper investigates how thermal diffusion of native oxide and interstitial oxygen alloying improve the RF surface resistance of superconducting niobium cavities, leading to higher quality factors at lower heat treatment temperatures.

Contribution

It introduces a theory of oxide decomposition and oxygen diffusion to quantify the process, confirming surface resistance reduction at lower temperatures than previously known.

Findings

Native oxide diffusion enhances RF surface conductivity.

Lower temperature heat treatments achieve high Q factors.

Oxygen alloying is key to surface resistance reduction.

Abstract

Recently, Nb superconducting radio frequency cavities vacuum heat treated between 300-400 C for a few hours have exhibited very high quality factors (~5x10^10 at 2.0 K). New secondary ion mass spectrometry measurements of O, N and C show this enhancement in RF surface conductivity is primarily associated with interstitial O alloying via dissolution and diffusion of the native oxide. We use a theory of oxide decomposition and O diffusion to quantify previously unknown parameters crucial in modeling this process. RF measurements of a vacuum heat treated Nb superconducting radio frequency cavity confirm the minimized surface resistance (higher Q0) previously expected only from 800 C diffusive alloying with N.