Power dissipation at high fields in granular RF superconductivity

Simple qualitative arguments are given in this paper to confirm that the surface impedance of polycrystalline superconducting materials depends strongly on the electrical properties of the grain boundaries. The boundaries, considered here as weak links, dissipate energy when the superconducting current exceeds the critical current of the link. This gives rise to an RF field-dependent surface resistance, which turns out to depend strongly on the grain size. A method is proposed to obtain the distribution of the grain boundary specific resistance in superconducting materials. These ideas are also applied to explain the RF field limitations and residual surface resistance of low Tc superconductors in polycrystalline form.