8th International Workshop on Thin Films and New Ideas for Pushing the Limits of RF Superconductivity

First RF characterization of an SIS sample

9 Oct 2018, 10:40

25m

Villi Room (INFN - Laboratori Nazionali di Legnaro)

Other superconducting materials beyond niobium Other SC Materials beyond Niobium

Speaker

Mr Sebastian Keckert (Helmholtz-Zentrum Berlin)

Description

RF-surface resistance measurements have been performed on an S-I-S multilayer sample. The sample was prepared at JLab using DC-magnetron sputtering of a 75 nm NbTiN layer on a 15 nm AlN insulator layer on bulk niobium. The sample (and sputtered area) was a circular disc of 75 mm diameter, welded on an 85 mm high Nb pipe of the same outer diameter. This geometry allowed performing RF measurements of the film using the HZB quadrupole resonator. The sample was tested at two frequencies: 420 and 850 MHz. Sample temperatures ranged from 2 K up to the transition temperature of NbTiN of ~17.3 K. The measured surface resistance exhibits an unexpected temperature dependence: Rather than rising monotonically as expected from BCS theory, a distinct local maximum is observed. This means that there is a temperature range for this SIS system, where surface resistance is dropping with increasing temperature, which seems to contradict BCS at first glance. Measurements of the baseline Nb sample prior to coating exhibited no such behavior; hence, systematic errors from the experimental apparatus can be excluded as the explanation. Measurements were limited by hard magnetic quenches near 20 mT, suggesting that the sample is limited by early flux penetration. Measurement results will be presented and discussed in detail.