Comparison of Microwave-Induced Constant-Voltage Steps in Pb and Sn Josephson Junctions

The microwave-induced voltage steps at $V=\frac{\mathrm{nh}\nu }{2e}$ 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×${10}^{-13\mathrm{}}$-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 ${10}^{12}$. The upper limit on the relative voltage difference between the $n=15\mathrm{}(300-\mu \mathrm{V})$ voltage steps for a Pb-PbO and Sn-SnO junction is $\frac{\Delta V}{V}\le 5\times {10}^{-9\mathrm{}}$. An apparent systematic error associated with switching prevents further absolute accuracy.