CNAM, UMD, College Park, USA
UMD, College Park, Maryland, USA
TU, Philadelphia, USA
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 low surface resistance in the high RF field regime. However, due to the presence of the small superconducting gap in the p band, the nonlinear response of MgB2 is potentially quite large compared to a single gap s-wave superconductor such as Nb. Understanding the mechanisms of nonlinearity coming from the two band structure of MgB2 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]. Several MgB2 films (thickness 50 nm, 140 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] Tamin Tai, et al., “Nonlinear Near-Field Microwave Microscope For RF Defect Localization in Superconductors,” IEEE Trans. Appl. Supercond. 21, 2615-2618 (2011).
UMD, College Park, Maryland, USA
MgB2 has recently attracted much attention as a coating for Nb to enhance the RF critical field in Superconducting Radio Frequency (SRF) cavities. However the surface properties of MgB2, including its intrinsic nonlinearities as well as various types of defects, can limit the RF performance of such cavities. This work presents phase-sensitive nonlinear near-field microwave microscopy on MgB2 films. An intense localized RF magnetic field is induced on the surface of MgB2 films by means of a magnetic write head, and the amplitude and phase of the third harmonic voltage generated by the film are measured. Power and temperature dependence of the third harmonic response are studied to identify the dominant mechanism of nonlinearity in the film under different operating conditions, and a phenomenological model is developed to relate the nonlinear response to the local critical RF field. The method could be generalized by scanning the probe over the sample, and possibly over various defects, to generate a map of the critical RF field and enable classification of different types of defects in terms of their effect on the RF performance of MgB2.
