RF Characterization of an S-I-S’ Multilayer Sample

S-I-S’ multilayers promise to boost the performance of bulk superconductors in terms of maximum field and surface resistance. At HZB, RF-surface resistance measurements were performed with a Quadrupole Resonator (QPR) and an S-I-S’ sample (75 nm NbTiN on 15 nm AlN insulator on bulk Nb) prepared at JLab. Measurements were performed at 414, 845, and 1286 MHz at sample temperatures from 2 K up to well above the transition temperature of NbTiN of ~17.3 K. The Rs exhibits an unexpected temperature dependence: Rather than rising monotonically, as expected from BCS theory, a local maximum is observed. There is a temperature range where Rs decreases with increasing temperature. Such behavior indicates that an additional interaction between the superconducting layers may have to be included in the surface resistance model. Measurements of the baseline Nb sample prior to coating exhibited no such behavior; hence systematic measurement errors can be excluded as the explanation. The maximum field was limited by a hard magnetic quench near 20 mT, close to Hc1 of NbTiN, suggesting that the sample is limited by early flux penetration.
Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany