MgB2 nonlinear properties investigated under localized high rf magnetic field excitation-report 2

TL;DR

This study investigates the nonlinear electromagnetic response of MgB2 superconducting films under localized high rf magnetic fields, revealing temperature-dependent behaviors and potential mechanisms related to its two-band structure, relevant for RF cavity applications.

Contribution

It provides the first detailed analysis of MgB2's nonlinear properties under localized rf magnetic excitation, highlighting mechanisms linked to its two-band superconductivity.

Findings

MgB2 exhibits a strongly temperature-dependent third harmonic response.

Multiple mechanisms may contribute to the observed nonlinearity.

Localized high rf magnetic fields reveal complex nonlinear behavior.

Abstract

The high transition temperature and low surface resistance of MgB2 attracts interest in its potential application in superconducting radio frequency accelerating cavities. However, compared to traditional Nb cavities, the viability of MgB2 at high rf fields is still open to question. Our approach is to study the nonlinear electrodynamics of the material under localized rf magnetic fields. Because of the presence of the small superconducting gap in the $\pi$ band, the nonlinear response of MgB2 at low temperature is potentially complicated compared to a single-gap s-wave superconductor such as Nb. Understanding the mechanisms of nonlinearity coming from the two-band structure of MgB2, as well as extrinsic sources of nonlinearity, 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 [Tamin Tai, X. X. Xi, C. G. Zhuang, D. I. Mircea, and S. M. Anlage, IEEE Trans. Appl. Supercond. 21, 2615 (2011)]. 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 several possible mechanisms are responsible for this nonlinear response.