Mgb2 Nonlinear Properties Investigated under Localized High RF Magnetic Field Excitation

TL;DR

This study investigates the nonlinear electromagnetic response of MgB2 superconducting films under localized high RF magnetic fields, revealing temperature-dependent mechanisms including vortex dynamics that affect its potential for RF cavity applications.

Contribution

It provides the first detailed analysis of MgB2's nonlinear response under localized RF excitation, highlighting mechanisms like vortex penetration and their temperature dependence.

Findings

MgB2 exhibits a strongly temperature-dependent third harmonic response.

Vortex nucleation and penetration are key nonlinear mechanisms.

Nonlinear response varies significantly with temperature.

Abstract

In order to increase the accelerating gradient of Superconducting Radio Frequency (SRF) cavities, Magnesium Diboride (MgB2) opens up hope because of its high transition temperature and potential for low surface resistance in the high RF field regime. However, due to the presence of the small superconducting gap in the {\pi} band, the nonlinear response of MgB2 is potentially quite large compared to a single gap s-wave superconductor (SC) such as Nb. Understanding the mechanisms of nonlinearity coming from the two-band structure of MgB2, as well as extrinsic sources, is an urgent requirement. A localized and strong RF magnetic field, created by a magnetic write head, is integrated into our nonlinear-Meissner-effect scanning microwave microscope [1]. MgB2 films with thickness 50 nm, fabricated by a hybrid physical-chemical vapor deposition technique on dielectric substrates, are measured at a fixed location and show a strongly temperature-dependent third harmonic response. We propose that at least two mechanisms are responsible for this nonlinear response, one of which involves vortex nucleation and penetration into the film. [1] T. M. Tai, X. X. Xi, C. G. Zhuang, D. I. Mircea, S. M. Anlage, "Nonlinear Near-Field Microwave Microscope for RF Defect Localization in Superconductors", IEEE Trans. Appl. Supercond. 21, 2615 (2011).