Variable-thickness-type microbridges made of thin Nb films

In a study of a Nb thin-film microbridge it is found that thinning a part of constriction of the short bridge is effective in localizing the weak region. Such variable-thickness-type bridges, typically thinned from 400 to 250 Å and 0.5-μ long and wide, have a normal resistance of 1–4 Ω and show the temperature variation of critical current of the type of Ginzburg-Landau depairing current. Microwave-induced current steps are observed up to 360 μV and down to T/Tc=0.56 in a low critical current bridge. The data of microwave response imply that heating is limiting the high-voltage operation.