Residual microwave surface resistance of superconducting lead

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Abstract
An extensive experimental investigation aimed at minimizing the residual microwave surface resistance of superconducting lead is described. X-band cavities were made by electroplating lead on copper which had a residual Q as high as 3.7×1010, corresponding to an average residual surface resistance of 3.1×10−8 Ω. Electroplating techniques and surface preparation are discussed in detail. Residual resistance due to magnetic flux trapped through the surface was found to have frequency and flux dependence consistent with losses in normal metal exposed at the ends of the flux tubes. It was not necessary to consider flux motion. Loss identified with adsorbed material and surface protrusions was observed as well. Under the best conditions in cavities cooled in ambient magnetic field <10−4 Oe, residual resistance of 10−8–10−7 Ω remains which varies a factor of 2 or more on temperature cycling. A hypothesis involving thermoelectrically generated trapped magnetic flux is advanced to explain part of this loss, and it is argued that direct phonon generation as suggested by Halbritter and Passow cannot fully explain it.