LANL, Los Alamos, New Mexico, USA
SLAC, Menlo Park, California, USA
KEK, Ibaraki, Japan
NIMS, Tsukuba, Ibaraki, Japan
STI, Santa Barbara, California, USA
ANL, Argonne, USA
TU, Philadelphia, USA
JLAB, Newport News, Virginia, USA
Demonstrating the idea of enhancing achievable surface magnetic field by coating multilayer thin film superconductors proposed by Gurevich is the main objective. DC magnetization measurements of 500 nm and 300 nm MgB2 films coated on Sapphire showed an increase in the lower critical magnetic field (Bc1) compared to that of bulk. Also, the Bc1 of a 300 nm film showed >200 mT at 4.5 K, which is >25 % higher than that of Nb (~145 mT). RF measurements using a 11.4 GHz pulsed Klystron and a TE013-like mode hemispherical copper cavity with a 2-inch (50.8 mm) diameter sample, however, have shown a low quenching field of 42 mT at 4 K. From detailed data analyses together with the data on Nb quench fields these quenches were found to be thermal, not magnetic, due to a high RF resistance caused by inter-diffusion of coated materials at the interfaces. Additionally, recent results of RF surface resistance at 7.5 GHz using a calorimetric technique at JLab will also be shown.
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).