Weak link behavior in polycrystalline BaPb0.75Bi0.25O3

A study has been made of the critical-current densities of polycrystalline samples of the perovskite oxide superconductor BaPb_0.75Bi_0.25O₃ (BPB). BPB does not have the complications of twin boundaries and a highly anisotropic unit cell as is found in the high-T_c oxide superconductors (HTSC), and the coherence length (ξ) is much longer than in HTSC materials (~7 nm as compared to ~0.5 to 2.5 nm). Thus some problems which have proved quite complex for HTSC materials (e.g. twin pinning or weak links) should be absent or have a different character in BPB. The authors have produced several samples of BPB. X-ray diffraction showed that the samples were single-phase, but an electron microprobe was used to identify about 1 vol.% of other phases present. The authors found that the transport critical-current density, as high as 2 A/cm² in zero field, drops markedly in fields of only 0.1 T. Flux penetration measurements place the magnetization critical-current density at about 10⁴ A/cm². This large discrepancy is attributed to weak links