On extrinsic effects in the surface impedance of cuprate superconductors

Presently, the rf surface impedance Z of cuprate superconductors is still shrinking with material improvements, which is shown clearly by Z = ZI + Zres still dominated by extrinsic properties summarized in Zres. In this paper, the authors present evidence that Zres is due to the large leakage current jbl and the small critical current jcl of weal links. The latter destroys the intrinsic shielding from a λ -thin seam λJ deep into the bulk. This causes rf residual losses Rres ∼(ωμ0)2 λ j3 σ bl/2. Rres stays finite at T ≅ 0 appropriate measure of weak links is the grain boundary resistance Rbn (Proportional to) p(0) enhancing λ j (Proportional to) Rbn and Rres (Proportional to) Rbn2. Thus, Zres is minimal for minimal extrapolated normal conducting resistivity p(T→ O). To identify the weak links as new entity the H-field dependence is most helpful, because at very low fields HclJ (Proportional to) 1?λ J Josephson fluxons penetrate into the weak links. These Josephson fluxon show negligible flux flow or flux creep, and enhance Zres by λ J(H, T) (Proportional to)1/ (ratio) jcj(H,T)