QUALITY OF HIGH-TEMPERATURE SUPERCONDUCTING POLYCRYSTALS AND SINGLE-CRYSTALS IN RELATION TO THEIR CRITICAL DYNAMIC FIELD

When discussing critical properties of inhomogeneous high temperature superconducting compounds it is preferable to compare experimental data collected from the greatest possible number of published results. However, such comparison is made difficult because of the wide variety of elaborate techniques, thermal treatments, characterization methods and sample morphologies involved in the experimental work reported in the literature. Here a phenomenological model, based on Clem's formulation and utilizing very low a.c. susceptibility measurements, is presented which demonstrates the existence of a critical field, hc*, corresponding to the appearance of flux pinning. The analysis allows the classification of a large variety of high temperature superconductors including bulk samples, thin films and single crystals; the strength of the critical dynamic field (varying over three orders of magnitude: 50 mOe < hc* < 50 Oe) is a measure of the quality (and also homogeneity) of the sample. The analysis applies to YBaCuO compounds as well as to LaBaCuO and BiSrCaCuO compounds or Chevrel phases. The best quality appears to be for YBaCuO single crystals, while sintered BiSrCaCuO exhibits peculiar behaviour with very low hc* or no hc* at all. © 1990.