Residual surface resistance of polycrystalline superconductors

By modeling a superconducting polycrystalline film as a network of superconducting grains coupled via Josephson junctions, various authors have shown that rf losses at the grain boundaries can be the main source of the residual surface resistance in high-${\mathit{T}}_{\mathit{c}}$ superconductors. The same model is extended here to include the effect of dc or rf applied fields and is then shown to be consistent with previous results on the surface impedance of low-${\mathit{T}}_{\mathit{c}}$ superconducting polycrystalline films. The model also provides a satisfactory, though qualitative, description of recent results on the quality factor of niobium thin-film-coated rf cavities for particle accelerators.