PERCOLATION IN SUPERCONDUCTING CUPRATES: RESISTIVITY AND CRITICAL CURRENTS

Abstract

Weak links in metallic cuprates yield complicated percolative conduction networks enhancing the intrinsic resistivity ρi(T) = αiT + ρi0L to: [Formula: see text] with ρb describing the weak links and a geometrical factor p ≥ 1. In most cases "single crystals" and "epitaxial" films show insulating intragrain weak links yielding p ≈ 3 and Σρb ≪ ρi(100 K ), like good sintered YBa 2 Cu 3 O x. Rarely are the intrinsic values αi ≈ 0.5 μΩ cm/K and ρi(100 K ) ≈ 50 μΩ cm observed. In the superconducting state the intrinsic resistivity pi(T) vanishes and then the critical transport current density jc (B, T) is given by the superconducting aspects of conducting weak links: [Formula: see text] Here, jCJ (jCG) stands for Josephson junctions at intergrain (intragrain) weak links and jCP for pinning. ps(B, T) stands for current path systems changing with B-field and temperature. jCJ and jCG depend crucially on the care in preparation and alignment of the [Formula: see text]-planes, where the current flows, but values jCJ (0.77 K ) ≈ 103 A/cm 2 and jCG (0.77 K ) ≈ 105 A/cm 2 seem typical. Weak links are related to planar defects where large and small angle grain boundaries are dominant. jCP is shadowed by jCJ and jCG but jCP ≈ 106 A/cm 2 are inferred from experiment. The time-, temperature-, B-field- and voltage-dependencies of the current in — and in the transition to — the superconducting state are analyzed. Evidence is presented for arrays of microbridges being "electrically" the planar defects. These microbridges are typical superconductor-normal-conductor-superconductor (S-N-S) junctions, where pair tunneling is reduced by factors 10–103 as compared to single particle, normal tunneling. The reduced pair tunneling is due to normal conductors coating cuprates being related to O-disorder and the tetragonal phase, which also occur by radiation damage. The time and voltage dependence implies rapid flux motion or flux creep along the planar defects causing noise and rf residual losses.