Improved performance of tin variable-thickness superconducting microbridges

We have fabricated tin variable-thickness microbridges with which we have observed the ac Josephson effect to as high as 3.7 mV, corresponding to the 187th harmonic of X-band radiation. The dc I-V curves of the bridges show reduced hysteresis and strong subharmonic gap structure to very low temperatures. We use the gap structure as a local thermometer in order to demonstrate that the improved high-frequency performance correlates with the more efficient removal of Joule heat from these bridges.