Before revision: J. Dolan, T. G. Phillips, and D. P. Woody, "Low‐noise 115‐GHz mixing in superconducting oxide‐barrier tunnel junctions," Applied Physics Letters 34, 347 (1979), Lead has a superconducting critical temperature of 7.2 K in bulk form, but lead oxide tends to develop defects (sometimes called pinhole defects) that short-circuit the tunnel barrier when the device is thermally cycled between cryogenic temperatures and room temperature, and as result lead is no longer widely used to make STJs.
After revision: J. Dolan, T. G. Phillips, and D. P. Woody, "Low‐noise 115‐GHz mixing in superconducting oxide‐barrier tunnel junctions," Applied Physics Letters 34, 347 (1979), Lead has a superconducting critical temperature of 7.2 K in bulk form, but lead oxide tends to develop defects (sometimes called pinhole defects) that short-circuit the tunnel barrier when the device is thermally cycled between cryogenic temperatures and room temperature, so lead is no longer widely used to make STJs.