Comparison of microwave-induced constant-voltage steps in Pb and Sn Josephson junctions
The microwave-induced voltage steps at V=nhν2e in the current-voltage characteristics of lead and tin Josephson junctions have been compared using a superconducting-quantum-interference device (SQUID) as a null detector in a potentiometer with 2×10-13-V resolution. The junctions are individually biased and then connected to the voltmeter by means of a superconducting mechanical switch. The incident microwave power is provided by two phase-locked x-band klystrons whose variable difference frequency is stable to better than 1 part in 1012. The upper limit on the relative voltage difference between the n=15 (300-μV) voltage steps for a Pb-PbO and Sn-SnO junction is ΔVV≤5×10-9. An apparent systematic error associated with switching prevents further absolute accuracy. © 1972 The American Physical Society.