ON EXTRINSIC EFFECTS IN THE SURFACE IMPEDANCE OF CUPRATE SUPERCONDUCTORS BY WEAK LINKS

The reported rf surface impedance of superconducting cuprates continues to drop with 2 years of improving material quality. As leading indicator for quality the weak-link critical current j(cJ)(T almost-equal-to 0, B almost-equal-to 0) has grown from 10(2) to 10(6) A/cm2 and thus the Josephson penetration depth lambda-j is-proportional-to 1/ square-root j(cJ) and the excess normal (leakage) tunnel current j(bl) are shrinking. This j(cJ) growth has now saturated, whereas the rf residual surface impedance Z(res) is still shrinking with material improvements. This shows clearly that Z(res) is an extrinsic property. Evidence that Z(res) is due to the large leakage current j(bl) and the small j(cJ) of weak links, where the latter destroys the intrinsic shielding from a lambda(I) thin-seam lambda(J) deep into the bulk, is presented. This causes rf residual losses R(res) almost-equal-to (omega-mu(0))(2)lambda(J)(3)sigma(bl)/2. R(res) stays finite at T almost-equal-to 0 by sigma(bl)(T --> 0) almost-equal-to sigma(bl) (is-porportional-to j(bl) being amplified by (lambda(J)/lambda(1))3 > 10(3) as a weighting factor. With slow crystal growth weak links are improved and their density is reduced so that R(res) values better than R(Nb)(4.2 K) are now obtainable. The j(cJ)(T,B) values explain lambda(res) and R(res) quantitatively and in temperature proportional to (a + T(m)); m almost-equal-to 1, T < T(c)/2, and proportional to (b + H(n)), n almost-equal-to 1; H > H(c1J) in field dependence. Here H(c1J) is the field where flux enters into weak links as Josephson fluxons having negligible viscous losses, but act by enhancing the penetration depth.