Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-Tc superconductors

Abstract

A series of high-Tc superconductors have been prepared with the HgO addition/substitution. Significant improvement in the Tconset as well as Tc0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0.01-0.6 at % helps in constructing an entire spectrum: improvement of Tc0 up to 0.4 at % in the case of Bi-system and up to 0.03 at % in the case of Y123 system and slight drop in Tc thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10-100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energy EJ becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transport Jc approx. exp[-Δ Tc/Tc0] and the flux pinning energy U approx. Δ Tc, where Δ Tc is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving high Jc, high U yet low Δ Tc. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.